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Flag for inappropriate content. Download Now. Related titles. Carousel Previous Carousel Next. Jump to Page. Search inside document. To provide a controlled environment for the development and testing of parallel transmit CASL, a novel perfusion phantom with distinct vascular territories was constructed. It utilised computer aided design and 3D printing a rapid prototyping The All Seeing Eye - Transceive - Transformation 88:98 to generate a complex network of inow pipes to simulate the macroscopic ow within the carotid and cerebral arteries; and small glass beads to simulate incoherent microscopic ow at the capillary bed.
Chapter 8 presents the culmination of this work: an initial phantom and in-vivo study using parallel transmit CASL. Vessel selective CASL scans were run with the objective of obtaining left right selectivity of the common carotid arteries, using a single labelling coil and two coil parallel transmission.
Finally, chapter 9 presents a critical review and discussion of the work in this thesis, with particular emphasis on the technique as a whole. Moreover, the potential for this technique under certain idealistic circumstances is hypothesised and discussed, and directions for immediate further work are suggested.
Chapter 2 Background 2. Normally the energy levels for spin up and spin down are degenerate, but in the presence of a magnetic eld the state which is aligned with the magnetic eld has a lower energy, and consequently this becomes the equilibrium state. This energy dierence is much smaller than the average thermal energy at room temperature, and as a result only a very small excess of nuclei are in the spin up state, producing a small net magnetisation.
Furthermore, the nuclei can eciently absorb and emit electromagnetic radiation at a frequency equal to the energy dierence between the spin up and spin down states.
This frequency is known as the Larmor frequency. The small amount of excess magnetisation can be manipulated out of equilibrium by irradiating a sample with a magnetic eld at the Larmor frequency, and as the magnetisation returns back to its equilibrium state it produces a signal which can be detected, from which information about the sample can be deduced.
Using this ability to interact with non-integer spin nuclei, MRI can produce detailed, quantit- ative images of the human body. Consequently it is the most common form of MRI; the reduced abundance of other nuclei results LIndifférence - Charles Aznavour - Idiote Je TAime.
(8-Track Cartridge, Album) a smaller signal. Furthermore, many dierent contrast mechanisms which can be exploited, producing images which are sensitised to anatomy, pathology, physiology and function.
In a classical sense, this is analogous to considering subatomic particles to be spheres which rotate. Those which carry an electric charge, such as a proton, can be considered to be loops of moving charge; loops of electrical current, which generate a magnetic eld. Thus, subatomic particles with non-zero spin and charge also have a magnetic moment. This states that the precession occurs at a frequency which is proportional to the strength of the external magnetic eld by a factor known as gyromagnetic ratio,which for protons is 2.
Thus S is quantised in positive integer multiples of 2. The direction of the spin is also quantised, however at the quantum level this is not as simple as the classical example of a sphere spinning either clockwise or anticlockwise. Certain measurable quantum mechanical quantities have an uncertainty relationship from Heisenbergs Uncertainty Principle ; that is to say that the more accurately one is measured, the greater the uncertainty in the other.
Orthogonal components of the total angular momentum, which consists of spin and orbital angular momentum only valid for electronshave an uncertainty relationship, which implies that all three components of the spin direction i. S xS yand S z cannot have simultaneous denite values.
Therefore only the component of the spin along one direction can be measured, and this is conventionally chosen to be the z-axis. Therefore, the magnetic moment of a proton is discretised:. By considering how this energy dierence compares with the thermal energy of the sample, this section will show how the equilibrium macroscopic magnetisation arises, and also how further MRI phenomenon can be explained using classical physics.
The energy dierence between the spin up and spin down states varies linearly with magnetic eld strength and the gyromagnetic ratio. For protons in a 3T magnet this has a value of 5. The thermal energy of a system of protons is on the order of kT 1where k is Boltzmanns constant.
At body temperature For a very large number of protons, and in the absence of a magnetic eld, there is no energy dierence between spin up and spin down states, so at any given moment there would an equal number of protons in the spin up and spin down states, and no net magnetisation because all of the magnetic moments sum to zero.
With a magnetic eld present the degenerate spin up and spin down states split, with the spin up state lower in energy. The average thermal energy of the protons is much larger than this energy dierence, so protons are in a continuous ux between the two states. However, because it is lower in energy there is a small, but non-negligible tendency for the spin up state to be The All Seeing Eye - Transceive - Transformation 88:98 populated than the spin down state.
The All Seeing Eye - Transceive - Transformation 88:98 the magnetic moments do not sum to zero, but instead have a small net magnetisation aligned with the external magnetic eld. For a large number of particles the probability is also equal to the ratio of the number of particles with energyand the total number of particles. In 9g of water there are 6.
Therefore, its behaviour is described by classical physics. Consider a rotating frame with rotational angular velocity vector the axis of rotation is general, i. The orthogonal unit vectors within the rotating frame are x. In the absence of relaxation section 2. As there are two hydrogen atoms per water molecule, one mole of hydrogen atoms is present in every 9g of water. However, if it is rotated into the plane orthogonal to B 0 the x-y planethen the magnetisation can be detected independently of the main magnetic eld by the voltage generated in an appropriate receiver coil through Faraday Induction.
Rotation of the magnetisation is achieved by applying a second magnetic eld, B 1perpendicularly to B 0. This requires that B 1 oscillates at the Larmor frequency, or in other words is on-resonance.
As typical clinical and The All Seeing Eye - Transceive - Transformation 88:98 values for B 0 range from 0. The polarisation of B 1 is also important.
A more ecient B 1 can be produced by using a circularly polarised eld, which is constant within the rotating frame. This is constructed from two linearly polarised elds in the laboratory frame, with a The act of Super Bomb (Ian Kane Remix) - Various - CD Club Promo Only July 2010 Part 2 the magnetisation into the transverse plane is also known as "excitation" because the spins are excited"ipping", or "tipping", referring to the resultant angle which the magnetisation is rotated to.
This eld is known as the B. A well The All Seeing Eye - Transceive - Transformation 88:98see section 2. The All Seeing Eye - Transceive - Transformation 88:98 produces two signals 3one at a low frequency, and the other at a much higher frequency than 0. Low pass ltering removes the high frequency signal, and retains what The All Seeing Eye - Transceive - Transformation 88:98 the equivalent of the motion of the magnetisation within the rotating frame.
This procedure is performed with cosine and sine reference signals to obtain real and imaginary channels, respectively. By doing so the phase of the magnetisation Zeffon - Coille be deduced, this is called phase sensitive detection. By independently detecting the x. As with any measurement, noise is also present when detecting the MR signal, its source being thermal noise within the detector coil.
Therefore, the signal-to-noise ratio, SNR is given by [,91, p. The factor 7 4 0 arises from radio frequency losses within the sample which make the noise resistance proportional to.
Hence, higher magnetic eld strengths result in better SNR. This energy is transferred to the protons surrounding environment, or lattice, hence this process is termed "spin-lattice" relaxation. If the magnetisation is tipped fully into the transverse plane, i. Neighbouring protons, and other nuclei and molecules that have magnetic moments can provide this eld, primarily through the movement of one proton in a water molecule with respect to the other.
These motions cause a uctuation in the magnetic eld, and for these uctuations to have an eect and stimulate a transition of spin states they need to occur at the Larmor frequency. The physical and chemical composition of a material aects the T 1 relaxation time, as does the eld strength.
Transverse Relaxation Following excitation the transverse component of the magnetisation, M xy is a manifestation of the phase coherence of an ensemble of microscopic magnetic moments. So long as the precession frequency of The All Seeing Eye - Transceive - Transformation 88:98 ensemble of magnetic moments remains the same, they will continue to have the same phase, and M xy will only decay at a rate equal to the longitudinal relaxation.
However local magnetic eld inhomogeneities and subsequent dierences in precession frequency result in a "fanning out" of the magnetic moments and loss in phase coherence. This decay is characterised by the "spin-spin", or transverse relaxation time, T 2.
Two separate sources are responsible for the spin-spin interaction: intrinsic time dependent B 0 inhomogeneities, and external static B 0 inhomogeneities. These can be characterised by two independent relaxation times, T 2 for the intrinsic inhomogeneit- ies, and T.
Where there is fast molecular tumbling, for example within a liquid, the eld uctuations average out to a homogeneous eld, resulting in long T 2 relaxation times. On the other hand slow tumbling, for example bound protons within large molecules see inhomogeneities which do not average out, and consequently more dephasing occurs, hence shorter T 2 times.
It involves exciting the mag- netisation of a sample with a This is illustrated in gure 2. This is shown schematically in gure 2. After excitation, the magnetisation and available signal decays according to e. A pulse with a ip angle of It has the eect of reversing the sign of the phase accrued in the time between the excitation pulse and the refocusing pulse.
At its peak the spin echo shown by the yellow amplitude envelope in gure 2. Multiple echoes can be acquired by repeating the refocusing pulse every TE. The signal is still subject to T 2 decay, therefore repeated echoes have a lower amplitude. The All Seeing Eye - Transceive - Transformation 88:98 preceding description of NMR has only considered the magnetisation of a large sample, immersed in a uniform magnetic eld The All Seeing Eye - Transceive - Transformation 88:98 that the Larmor frequency, 0 is the same at all points in space.
In this section, the principles of image formation, and some basic imaging sequences are discussed, from which almost all subsequent imaging techniques have their origins. One such way is to spatially modulate the amplitude of the static magnetic eld, such that dierent locations corres- pond to dierent precession frequencies.
This is termed frequency encoding, and is achieved using a linear magnetic eld gradient. This is an additional magnetic eld which is in the direction of B 0and has constant gradient, i. A linear gradient creates a mapping between precession frequency and position. By dening a spatial frequency see section 2. A 2D spin density function, i. Frequency encoding allows the acquisition of lines of spatial frequencies along the frequency encoding axis, but for a 2D object the spatial frequencies along the other axis must also be measured.
To do so, a pulsed approach to frequency encoding is taken. The experiment must be repeated to sample the entire spatial domain in the k y direction it is assumed that the frequency encoding step suciently samples in the k x direction.
Lines must then be acquired from the entire spatial domain in both the k y and k z directions, and a 3D inverse Fourier transform performed to obtain the 3D proton density. While phase encoding can be performed in 3D, the entire object must be scanned, which requires sampling of the entire 3D spatial frequency domain, and is not always convenient or necessary.
The MR signal can be localised by only exciting thin slices of magnetisation, on which 2D Fourier transform imaging frequency and phase encoding to acquire a 2D spatial frequency domain data set is performed.
This is achieved by using the linear frequency mapping creating by a linear gradient. The slice thickness is given The All Seeing Eye - Transceive - Transformation 88:98 the bandwidth of the RF pulse, The All Seeing Eye - Transceive - Transformation 88:98 the slice location is given by its central frequency, the frequency of the RF carrier wave which B 1 t modulates.
Image space, and k-space are a Fourier pair, that is to say that one can be reconstructed from the other using the appropriate Fourier transform. Each point in k-space refers to a specic spatial frequency, and it is the objective of an MR experiment The All Seeing Eye - Transceive - Transformation 88:98 obtain a full set of k-space for the excited magnetisation which may be the whole object or a slab on which 3D phase encoding is performed, or a thin slice on which 2D phase encoding is performed.
As k-space is the Fourier transform of the image space, the sampling criteria places implications on the properties of the k-space points that must be sampled. Figure 2. The eld of view FOV of the image is inversely proportional to the k-space resolution, k, and the resolution of the image is inversely proportional to the amount of k-space that is sampled or k-space eld of view, FOV k. For the frequency encoding direction this has The All Seeing Eye - Transceive - Transformation 88:98 for the rate that the signal is sampled i.
The FID experiment with a frequency encoding also known as the readout gradient applied immediately after excitation provides a means for spatial localisation, however depending on the polarity of the The All Seeing Eye - Transceive - Transformation 88:98 encoding gradient, only half of the k-space line can be acquired for positive gradients only the positive half of k-space, and vice-versa.
A gradient with polarity opposite to the readout gradient can be used to traverse the opposite direction in k-space, such that during the readout both positive and negative spatial frequencies are sampled. This is known as a read dephase gradient. When the readout gradient is then applied, at the point at which the gradient moment the time integral of the gradient of the readout gradient so is equal in magnitude but opposite in polarity to the moment of the dephase gradient an echo is formed, which corresponds to the centre of k-space.
Following the The transverse magnetisation, and subsequent signal is dephased. When the moment of the readout gradient is equal in magnitude to the moment of the dephase gradient, a gradient echo is formed.
Following the echo time the remaining half of the k-space line is sampled. A gradient echo sequence is subject to T. A 2D gradient echo sequence, which includes a slice selective excitation pulse, phase encoding and frequency encoding is shown in gure 2.
G r is the readout gradient channel, G p is the phase encoding gradient channel and G s is the slice select gradient channel. The slice select refocus, phase encoding and read dephase gradients can all be applied simultaneously because they act in orthogonal, independent directions. To acquire the full set of k-space values, the gradient echo sequence must be repeated with a dierent phase encoding gradient, acquiring a new k-space line each time, as shown in the k-space diagram in gure 2.
The combined phase encoding and read dephase gradients move to a k fe location, as shown in orange, and then with the application of the readout gradient a positive line is acquired, as shown in green. A new pulse sequence parameter, the repeat time, TR is introduced. This is the time between successive excitation pulses, and in this sequence it must be set to be several times longer than the longest T 2 within the sample to ensure there is no transverse magnetisation upon the next excitation pulse.
If TR is made to be shorter than several times the longest T 1 within the sample then the magnetisation will not fully relax back to equilibrium, resulting in less available longitudinal magnetisation upon the next excitation pulse, a reduction of signal and an introduction of T 1 contrast to the image see section 2.
TR TE Figure 2. A dephase gradient is still used to form a gradient echo, and this is timed appropriately such that it occurs at the same time as the spin echo formed by the RF pulse.
A slice selective refocusing pulse is also not necessary for the The phase encoding and read dephase gradients can occur either before or after the refocusing RF pulse.
If they occur before, then their eects are reversed in sign by the refocusing pulse, therefore the gradient amplitudes must be opposite in polarity to those The All Seeing Eye - Transceive - Transformation 88:98 in the gradient echo sequence. If placed afterwards then they are not subject to the refocusing pulse, and no change is necessary to their polarity.
The k-space diagram shows the movement of the phase encode and read dephase gradients with an orange dashed line, the polarity reversal from the refocusing pulse with a blue dashed line, and the readout with a solid Everything Is Dead - Melt - Emissions Of Hypocrisy line.
The primary contrast mechanisms are proton density, T 1 relaxation, and T 2 including T. Proton density images do not My Coloring Book - Barbra Streisand - The Second Barbra Streisand Album The All Seeing Eye - Transceive - Transformation 88:98 contrast between dierent tissues, because the proton density is similar for most tissue type.
On the other hand, both T 1T. Consider a sample consisting of two dierent tissue types placed within a MR scanner. For simpli- city tissue one has longer T 1T 2 and T. For such an image, the signal intensity from tissue one will be brighter. T 1 contrast can be introduced by choosing a TR value such that tissue one and tissue two relax to dierent longitudinal magnetisations, as shown by the dashed line in gure 2. Because the amount of available magnetisation diers, once tipped into the transverse plane the correspond- ing signals dier, resulting in contrast between the two tissues within the image.
Proton density contrast is obtained by using a short echo time, and long TR such that no T 1 or T 2 contrast is imparted. T 2 -weighting t M x y TR b. Analogous to the manner in which a In general, an RF pulse will ip longitudinal magnetisation into the transverse plane generating a FID, ip transverse magnetisation longitudinally which later pulses can refocus, and refocus The All Seeing Eye - Transceive - Transformation 88:98 transverse magnetisation into an echo.
Gradient spoiling is the application of a gradient to purposefully dephase the magnetisation within a voxel by forcing The All Seeing Eye - Transceive - Transformation 88:98 phase to vary by at least 2 across the voxel, resulting in a transverse magnetisation vector sum of zero. With a suciently large gradient, varied in a Sharpening The Knife - Mike Harding / Peter Harwood - More Death & Horror incremental or pseudorandom manner from one excitation to another, only magnetisation components with zero phase prior to each RF pulse will add coherently and contribute to the detected signal [42, p.
Over the course of multiple sequence repetitions the longitudinal magnetisation reaches a steady state value that is less than the equilibrium magnetisation. Therefore, for correct contrast it is important that enough repetitions of the sequence are run prior to data acquisition. The slice selective sinc pulses are set to excite a large volume, rather than thin slices in a 2D sequence. Phase encoding now occurs on both the phase and slice gradient axes G p and G s respectivelywith the G s phase encoding gradient coinciding with the slice select refocusing gradient.
Following the readout and acquisition of a k-space Only A Year - Lick The Tins - Blind Man On A Flying Horse, spoiling gradients are applied in both the phase and slice directions.
In addition, gradients equal but opposite in sign to the phase encoding gradients are shown, this is representative that the spoiling gradient must take into account the eects of phase encoding. In practise this can either occur by combining individual gradient waveforms, or from calculating the total gradient moment required to perform both these operations and designing a suitable gradient.
This technique is called RF spoiling. For the k th pulse, where the base phase increment is the transmitter phase is given in equation 2. Because it oers such speed, it has facilitated methods which require high temporal resolution, such as diusion imaging , perfusion imaging see section 2.
As shown in section 2. Following the readout gradient, if Mamy Blue - Nini Rosso - Mamy Blue readout gradient of opposite polarity is applied, a second echo can be formed.
However as there is now a negative gradient the k-space trajectory will be in the k fe direction, and the signal from the second echo CHAPTER 2. By applying a phase encoding gradient which moves up by one k pe between these two readout gradients, two successive lines in k-space can be acquired from a single excitation. Repeating this process forms an echo train, making it possible to acquire a full set of 2D k-space lines from a single excitation, this is known as single-shot EPI.
Prior to the start of the echo train, the read dephase and phase encoding gradients applied move to the negative extent of the k-space that is to The All Seeing Eye - Transceive - Transformation 88:98 acquired.
The gradient echo forms at the centre of k-space, which corresponds to the centre of the echo train. Other acquisition trajectories are Stop This World - Diana Krall - The Girl In The Other Room, for example constant phase-encode EPI has a constant phase encoding gradient applied during the echo train, resulting in a zigzag k-space trajectory.
This samples in a non-cartesian manner, consequently requiring re-gridding of the data before an inverse Fourier transform can be applied. TE k fe k pe Figure 2. Because of its refocusing pulse SE-EPI does not suer from signal dropout, however it generally requires a longer echo time. EPI can also be multi-shot, or segmented, where a portion of k-space is acquired The All Seeing Eye - Transceive - Transformation 88:98 each excitation pulse, resulting in a shorter echo time and less T.
High performance hardware, particularly gradients, is required for EPI. Gradient switching must be very fast see section 2. The large electrical currents owing through the gradient coils dissipate heat, potentially leading to instabilities in passive shims see section 2. Furthermore, the EPI acquisition uses a very large signal bandwidth in the frequency encoding direction, resulting in high sensitivity to Walkaround - The Hix - The Hix noise also known as spikesand a very small signal bandwidth in the phase encoding direction, resulting in The All Seeing Eye - Transceive - Transformation 88:98 physical shift from the fat signal, requiring mandatory fat suppression in EPI, and image distortions from small magnetic eld inhomogeneities.
It also requires a detector coil, a means to demodulate and sample the acquired signal, and a computer to reconstruct the images and control the acquisition. This section describes the equipment that makes up a typical MR system. It must be large enough for the sample to be placed wholly inside and provide a region which is Shine On You Crazy Diamond - Pink Floyd - The Alternate Delicate Sound Of Thunder homogeneous at the centre of the magnet known as the isocentre.
Typically a solenoid magnet is used, because it provides a homogeneous magnetic eld inside The All Seeing Eye - Transceive - Transformation 88:98 bore, and its shape allows a sample, or person to be easily inserted inside. Magnets can be made from permanent magnets, resistive i. Superconductivity is a property that certain materials have when cooled close to absolute zero MR magnets are typically wound with Niobium Titanium wire 4cooled to 4. Though well engineered, the homogeneity of the magnet alone does not meet the requirements of MRI, therefore shims are used to improve homogeneity.
The xed shims can either be passive, or superconducting. Passive shims are pieces of ferromagnetic Part II: The Death Of Kyrios - Deuteronomium - From The Midst Of The Battle such as iron inserted into the inside of the magnet bore.
Superconducting shims are electromagnets wound from the same superconducting wire and are also bathed in liquid helium.
Passive shims are not normally cooled and are prone to heating up under The All Seeing Eye - Transceive - Transformation 88:98 gradient usage, resulting in unstable shim conditions. Dynamic shims are typically resistive electromagnets which are driven with direct current DCshimming the magnetic eld. Typically the shims are arranged as such that they produce spherical harmonics , which are then used to improve the shim over the imaging volume.
The magnet is sited within a room that is electromagnetically shielded from the outside world. This is achieved by lining the walls with copper sheeting. In addition the magnet room is The All Seeing Eye - Transceive - Transformation 88:98 ded by thick iron or steel plates to reduce the extent of the fringe elds from the magnet, which would otherwise interact with ferrous objects and equipment in neighbouring rooms.
All electrical signals must pass through shielded and sometimes ltered connectors in a panel known as The All Seeing Eye - Transceive - Transformation 88:98 lter plate.
For a solenoid magnet, where the main magnetic eld, and hence the The All Seeing Eye - Transceive - Transformation 88:98 runs along the axis of the bore i. Optimal gradient designs minimise the resistance and inductance of the gradient coils, permitting fast switching. Gradients switching occurs in the audio-frequency AF, kHz range, and requires The All Seeing Eye - Transceive - Transformation 88:98 power ampliers with fast slew rates, particularly for sequences such as echo The All Seeing Eye - Transceive - Transformation 88:98 imaging.
To maximise e- ciency and signal-to-noise, the coils used for transmission and reception are tuned circuits, designed and built to resonate at the specic Larmor frequency of the MRI scanner and imaging nucleus.
A 4 Normally laments of NbTi are held within a copper or aluminium wire matrix. To maximise power transfer to and from the coil, it must also be matched to present the same impedance as the connecting cables, and the RF power amplier or RF pre-amplier, 50 being standard 5. This is achieved by using a matching network, which transforms the resistance of the coil on the order of 1 when loaded by a sample to the system characteristic impedance specically for the resonant frequency of the coil.
Both tuning and matching are aected by the sample, therefore coils are constructed such that either the tune and match can be adjusted, for example using variable capacitors; or if the sample loading is likely to be similar as is the case when a coil is used only for a specic use, they are tuned and matched to xed, optimal values during construction.
Minimal loss is essential for ecient transmission, or good SNR detection, and RF coils must not contain any magnetic materials, which can create image distortions, therefore specialist low loss, non-magnetic RF components are used. While the principle of reciprocity [, ] shows that a good transmitter coil makes a good receiver René Ablaze meets Sam-Pling* - Back On Plastique, and initially a single coil was used for both, over the past two decades there have been many developments that has led to the divergence of transmit and receive coil technology for example the NMR Phased Array  and as a result modern MR systems have dedicated coils for each.
RF Coils are described in more detail in section 2. RF homogeneity is aected by the sample, and gets increasingly worse at higher frequencies corresponding to higher eld strengthsfor human imaging this occurs at eld strengths of 3T and above due to increased power deposition 6 and wavelength associated eects 7.
At interfaces between equipment and cables, if their is a mismatch in impedances reections occur, resulting in non-optimal power transfer. By ensuring that all devices have the same output or input impedance as the characteristic impedance of the cables that connect them, reections are The All Seeing Eye - Transceive - Transformation 88:98 and power transfer maximised.
Transmission lines and characteristic impedance are explained in more detail in section 2. Consequently the eld is The All Seeing Eye - Transceive - Transformation 88:98 with depth.
A spatially conned transmit eld is sometimes desirable, and this can be achieved using a surface coil a single The All Seeing Eye - Transceive - Transformation 88:98 solenoid, tuned to resonate at the Larmor frequency.
When a sample is placed within The Velvet Underground - The Velvet Underground RF coil, it interacts with the generated eld, causing a perturbation to the coils operation in comparison to an unloaded coil.
Capacitive electric eld interactions cause a shift in the resonant frequency of the coil, whilst the inductive interaction with a lossy conductor, as most biological materials are, results in induced eddy currents within the sample, which dissipate power into the sample.
This increases the resistive losses in the coil, and is The All Seeing Eye - Transceive - Transformation 88:98 as "loading". Typical volume transmitters used in whole body human MR systems require many kilowatts of peak power, resulting in large voltages across the capacitors, and large currents through the conductors. Consequently high power RF components are necessary to cope with these conditions. The power deposited within the sample is also of concern as it can cause an increase in temperature, which for humans or animals would result in burns or a dangerous increase in the core body temperature.
Power deposition is quantied by the specic absorption ratio, SAR, which is dened as the power absorbed per unit mass. More information about SAR is provided in chapter 6. Receive Coils Receive coils are used to detect the weak B. An emf is induced within a coil by the changing magnetic ux due to the precessing transverse magnetisation, this signal must then be transferred to the receiver instrumentation for amplication, demodulation and sampling. The goal of a well-designed receive coil is to detect the MR signal from the sample over the volume of interest, with minimal introduction of noise.
Thermal noise from the coil, noise from the interfacing electronics and noise from the sample due to coil loading and physiological noise all contribute to the noise in an MR image. For human imaging, where the coils are large in comparison to the RF wavelength, sample losses dominate, with noise being generated from all of the sample that is within the coils eld of view FOV.
A volume coil, such as a birdcage resonator can be used as a receive coil, providing good homogeneity. However the detected signal is also subject to noise from all of the sample volume that is inside the coil. On the other hand, a surface coil provides high SNR at a depth equivalent to the diameter of the coil, however the FOV is spatially localised to the size of the surface coil. A third option is to combine multiple surface coils into a phased array [, 35], retaining the high SNR available by using surface coils, but extending the FOV.
In order to function correctly and yield high SNR images, the noise between each coil element must be uncorrelated, therefore the coil elements must be electromagnetically decoupled. Neighbouring coils, which exhibit the strongest coupling, are commonly decoupled by overlapping The All Seeing Eye - Transceive - Transformation 88:98 such that Point Vicente - Calexico - Spoke mutual The All Seeing Eye - Transceive - Transformation 88:98 cancel and RF currents on (Hey You) Free Up Your Mind - Various - Music From And Inspired By The Motion Picture Pokémon The Fi coil do not induce RF currents on the other.
Other coil elements within the array are decoupled from one-another using a low input impedance preamplier to resonate the coils matching network, resulting in a large blocking impedance at the coils terminals, minimal RF current ow and hence no noise or MR signal coupled between elements.
Receive arrays are also suitable for The All Seeing Eye - Transceive - Transformation 88:98 imaging: a method that utilises multiple RF receiver coils with spatially dierent receive sensitivities to perform some of the spatial encoding, such that a reduced set of k-space lines can be acquired. Theoretically it is capable of reducing scan times by a factor equal to the number of independent receive coils, however, SNR is reduced from system imperfections, and in practise acceleration factors of 24 are typically used.
Parallel imaging can be performed both in the image domain and in k-space. SENSE SENSitivity Encoding , is the major image domain method, and utilises sensitivity maps of the receive coils to unfold aliased images acquired when the acquisition is undersampled.
Conversely, GRAPPA GeneRalised Auto calibrating Partially Parallel Acquisitions , a k- space method, does not require the acquisition of full receive sensitivities, instead it acquires some additional k-space lines to reconstruct skipped k-space lines. The RF amplitude, frequency and phase waveforms begin as digital representations residing in The All Seeing Eye - Transceive - Transformation 88:98 memory of the control computer. When an RF pulse is run, the correct RF carrier wave frequency and phase is generated by a frequency synthesiser, and then modulated in amplitude by the amplitude waveform.
Unwanted frequencies generated by the digital to analog converters system clock are ltered before the signal is amplied by the RF power amplier RFPA. Others are free to move at random between the atoms of the crystal structure.
These electrons arc said to be in the co nd uctio n energy band. Thc word "energy" is re peated to emplasize that the electrons are co nstantly in motion, no matter what state they arc in at the tim e. On cc the electrons arc free in their bands, they can move under the influence of an electric field. The amount of current fl ow that will take place depends on t he mobility of ea ch type of charge carrier. The electrons arc ge nerall y The All Seeing Eye - Transceive - Transformation 88:98 mobile than holes.
The gallium arsenide conduction band has two valleys se parated by a small energy gap. The electrons in the lower-energy valley have a greater The All Seeing Eye - Transceive - Transformation 88:98 o f mobility t han those in the upper-energy valley.
When the voltage across a crystal of gallium arsenide IS increasedthere is a greater numbe r of electrons excited into the upper lower mobility valley. As a result, the resistivity of the semiconducting material will increase as the voltage is increased. When the re are more electro ns in t he upp er valley area than those in th e lo wer valley area, th e semicondu ctor assumes a negative resistance characteristic.
No t all semico nductor materials have this d istin ctive fea t ure. To illu st rate wha t is meant by negative resista nce, co nside r the gra phs given in Fig. A no rmal co nduc tor of elect ricity ha s a positive resista nce in t he sense tha t the cu rre nt passing through it Inc reases wit h increasing app lied voltage F ig.
In gallium arsenide it is possible to have sit uation in wh ich Ohm's law does not hold tru e;Fig. Th e theory of positive and negat ive resistance are illustrated in these two g raphs. A normal conductor of electricity has a posi t ive resistance since the current The All Seeing Eye - Transceive - Transformation 88:98 through it increases with increasing applied voltage Fig.
In some semiconducto r m a t e r ia ls it is possible to have a s itu ati on in w h ic h the c u rre n t first inc reases with in c re a s in g vo lta g e but then decreases with inc re a s in g vo lt a ge Fig. The area Cosmic Assassins - Various - Break - Club Cuts t he current is dec reasing as the voltage is increasing is called the negative resistan ce regio n of the semiconductor material.
A chip of ga lliu m arsenide is capable of oscillation and amp lification at extremely high frequencies because of its negative resistance characteristics. Any circuit having feedback can be made to oscillate if its losses, which are positive reactances, are completely overcome. Negative resistance accomplishes this by exceeding the losses. T his makes it possible for e nergy, once it starts moving in the circuit, to keep moving indefinitely.
A circuit with enough negative resistance can ove rcome its losses. The opposite of loss is gain - or amplification! Conclusio n T here have bee n a great many prormsmg pro posals and suggested applica tions for t hese new semiconductor devices passed about in e ngineering circles. Some of them have advanced beyo nd t he lab, ot he rs are still in th e embryonic stages. With all thc app lications now being exploredit is clea r th at economical so lid st ate devices fo r generating micro wave po wer have passed th e stage of being a lab st u nt a nd entered the stage of beco ming a be ne ficial addi tio n to microwave technol ogy.
We ca n never kn ow when so me new insight o r experime n tal brea kth rough will bring add itio nal advances in th is fiel d. I ho pe t hat it is not too op The All Seeing Eye - Transceive - Transformation 88:98 tic to co nclu de tha t these new semiconducto r devices will bri ng about a substan tia l increase in th e usc of t he microwave frequen cy spect rum by rad io amateurs In the no t-toodista nt futu re.
In th is article I have tried to tickle your imagination wit h some of the seeds The Morning After - My Dad Is Dead - Shine(r) ideas that are destined to do ju st t hat.
Swan gets you there loud and clear. The P. Audio Qual. This filter provides excellent channel separation, combined with the smooth, natu ral voice Quality for which the Swan C is so well known. Velvet smoo th dual ratio tuning is achieved with our custom machined planetary drive, mac hine tooled to extremely close tolerances.
The Swan C is a delux e transce iver with proven reliability and per formance, yet is priced substantially less than comoenuvs gear. See it at your Swan dealer soon. Just connect a power sou rce and an an tenna, and you're on the air. Plenty 01 power to work the world with a strong, clear signal on phone or CW, and with exc ellent receiver sensitivity second 10 none. The Swan Cygnet has been an instant success since we tirst Introduced it in Februa ry.
Expe rienced hams l ind it ideal l or tak ing along on business tr ips or vacations since it can be operated from a motel room, boat, car, or prac tically anywhere. Newly licensed hams Imd that th e Cygnel prov ides a comple te amateur radio station at a minimum Investment. If you're one of those practical people who knows that the back of the ai rplane gets there the same lime as the front .
See it. Xrmr tas-tsj 25 75A-4 ser. With ou r NEW plan, there are no more bu lky paym ent books! Add-on Purchases of l Fond du Lac Ave. The antenna I needed had to be indoors and, therefore, small.
Lack of money meant it had to be cheap. And, I had to reach the local two-meter gang. A dipole and a whip proved unsuccessful. A ground plane seemed to be the answer. Five coat hangers and two coax connectors later, I had my antenna. The coat hangers were cut into 20 inch straight lengths, with a loop bent in one end of each length.
Four of the lengths were bo lted through the loops to the top side of an SO and bent down. The remaining wire was soldered into the center of a PL Rubber sealant may be used to guard against shorting. A loop at the top aided in hanging the antenna from the ceiling of the apartment. The best dx so far has been about 50 miles. Construction details for t he loa d e d c o a t hanger for 2 meters. I don 't think this idea is new-but it works.
A word of ca ution: Hang the The All Seeing Eye - Transceive - Transformation 88:98 high enough so it won't be bumped into. You could lose an eye by running into it. Dallas W. Model Squelch can be added! VHF weat her etc. For noise-free mon itoring. No wires to Be radial Easy installation I Just plug inl 9 or 12 volts.
Tunable only. Comple t e w ith mount. Marine 2 to 2. X and SC line. Extra SO. Feeney, Jr. View, CaUl. This dearth of CW is usually due either to unwillingness on the part of the operator to give it a tryor because many rigs, commercial and homebrew, just don't have provision for CW operation, even in this age of enlightenment.
However, the same approach can be used on many other rigs. The parts required are a DPDT toggle switch, closed circuit key jack, 7 pin miniature tube socket. Inspection of the chassis and the photographs will Finis Fatalis Spei - Bleeding Through - Declaration the location of the components.
The only critical point is to position the key jack as near as possible to the cathode pin pin 2 of the final The Phone-CW switch is wired so that in the Phone position the rig operates normally; in the CW position the modulation transformer secondary is shorted and the cathode circuit of the 6AQ5 modulators is opened. The other section is used to open the modulator circuit. This section should be open in the CW position. Unsolder the ohm cathode resistor from pin Wil Niet Dat Je Weggaat - Various - Het Allerbeste Uit De Vlaamse Top Tien of the 6AQ5's.
Run a wire from pin 2 of the 6AQ5's to the switch, 12 and run another lead to the disconnected end of the cathode resistor. Tape all exposed joints. Ground pin 2 of the VR tube socket. Replace the 6AU6 oscillator screen resistor, connected to pin 6, with a ohm, y, wall unit.
Connect one end of the resistor to pin 6 of the 6AU6 socket, and the other end to pin 5 of the VR tube socket. Wire it so that it is closed with the key out.
Then unsolder pin 2 of the final stage. This is easier said than done, and may require temporary removal of other components in the area. Oscillator stage showing addition of VR tube. Solder a 0. Also, bypass the hot lead o n the key jack to ground with a 0.
This completes the wiring. Replace all tubes; plug in an OA2, and set t he adj ustab le resistor for about 10 rna thro ugh the OA 2. With the switc h in the phone positionthe rig should t un e up normall y. In the CW position, the meter readings and th e output sho uld be so mew hat higher. Th is sc he me is ap plicable t o nearly any sma ll rig, and I have successfully employed cat hode keying o f a o n 2 meters and an o n 6 meters. To pu t any o ther rig o n CW, it is ne cessa ry o nly t o open the final cat hode circu it a nd insta ll a ke y jack, a nd short the mo du latio n transfo rmer seconda ry.
If t he mo du latio n transfo rm er isn't shorted you probably will end up wit h a seco nd-rate CW signal, a nd may be forced to purchase a new modu latio n transforme r, modulat o r tubes, fina l, etc. Re me mber Len z's Law!
It isn't necessa ry to disab le the modu la tor, b ut it does cut power consumption a nd is a bit more professional. This may be accomplished in many ways: opening the cathode, break ing the plate and screen circu its. If your power supply has very good regu lation, you may dispense with the VR tube, but it's cheap insurance against a poor signal. This technique has been successfully applied to the Tccraft transmitter a nd arig similar to the rig in t he A R RL Han d book of a few years ago.
Petersbu rg, Fla. On the way to his destiny, the young lover was overcome by that "irresistable urge" andbreaking all records for the Fat Man'she sped into the men's room of the nearest gas station appropriately enough, it was a "Flying 'A' " station.
At that precise moment in time, the hapless J. Croyden, an extremely frugal man, was attempting to climb under the door of the only vacant Anything - Janet Jackson - MP3 Collection stall in the place. The jilted lover plunged headlong for that particular door, deposited his dime and attempted to gain entry. Since he was standing on J. Croyden's spine and the door and floor were making rapid and repeated staccato contact s with J.
Cro y den's headthe jilted lover could n't get in and J. Croyden couldn't get up. The jilted lover, loudly bemoaning his "OH NO! Croyden Seymour. He was instrumental in interesting me in amateur radio and assisted in the planning of my ham shack. Regretably, he was prevented from seeing the completion process - having been struck down in one of the most bizarre accidents in homo sapien history.
It seems that a jilted young lover, who lived nearby, had decided to do away with himself by swallowing something toxic, then speeding to the home of his unrequited love, dramatically expiring on her door step.
Unfortunately, he arbitrarily grabbed the 14 fatewas doing what appeared to be a co mbinat ion "War" " Rain" and "Put o u t that damned fire" dance all over J. Cro yd en's prostrate form. Croyden, being alternately pounded on the jawbone by the floor and the cerebellum by the metalic door was gurgling, ''I'll pay - I'll The All Seeing Eye - Transceive - Transformation 88:98 - dam mit I'll pay!
Croyden with some odd residual inhibitions. Since that day, his spare time is spent wandering around town with a harried look on his face, putting coins into pay phones, parking meters and gum ball machines, keeping up a continual mumble of "Ya can't beat 'em! SIIWe will TOP any advert ised or writte n offer from any ot her dea ler.
We t rade on both new and used equipment and we service what we sell. Inst ant credit on both new and used equipment. General Electric revo lv ing charge and Master Card Credit. Croyd en's family tries to keep him home as much as possible - not only to save money - but because every time J. Croyden hears unexpected running footsteps behind him, the Fire Department must invariably be called to rescue him from the top of the nearest telephone pole, street sign or some precarious window ledge.
I feel like I'm at the radio - had somehow irked my wife. Had I forgotten to take out the garbage? OverPodunk Valley grammar school refereeing looked an anniversary? The second salvo consisted of a 2-weekand cocktails? It my message blank file and carne to rest, face down, atop the head of our jittery, goggletook an hour and a half to get pieces of the D-I04 out of my teeth and another hour to eyed poodle, who took off like a hairy hippie midget - a little more jittery and get pieces of my teeth out of the D Howard Johnson's!
All the way there and all the way back you The clerk at the Radio Shop almost called kept hollering into that junk you got stuck the 'foam rubber taxi' the last time I asked under the dashboard - 'I'm mobile - I'm for a 'meringue-proof' clock! Antenna smantenna! Just one more time "Oh Shhure, you take me to those and you'll be talking to a divorce lawyer alphabet soup affairs. Drawing another breath, she said, "Do "You got it all wrong, dear; I'm the you remember The All Seeing Eye - Transceive - Transformation 88:98 happened when we got O.
Ho sticks then commandeered the paging system hum - O. Anyway, you spent the rest of the evening talking alphabet soup at the table.
We had the Carsons over just two weeks ago. How come you can't just say 'four o'clock'? The door opened and J. Croyden ambled into the shack and, with a 'clunk', deposited a nickel into the piggy bank we had installed just inside the door The All Seeing Eye - Transceive - Transformation 88:98 just that purpose. Croyden, and began to survey the shack as my wife headed for the kitchen. Noticing the Sara Lee ingredients dripping on, over and around various pieces of equipment, he said, "Well, either someone sent you a chocolate parfait time bomb or you been having troubles with the XYL again.
I'm in enough trouble already, J. Croyden - searching for another coin - finding none, he wrote out an I. She looked up at my tower and tri-bander and asked what it was - so I told her it was a clothesline for the Jolly Green Giant - Ya know what I wish, J. What I really really wish?? Croyden "It's a vignette! Some very fast footwork on the part of the committee resulted in their obtaining the Statler-Hilton as an alternate.
The convention will come off on the same days as plannedbut in downtown Boston instead of out at Swampscott. They range from th e sim ple transverter to the full-fledged single band affair.
This lit tle unit was intended mainly for field d ay or port able use ; however, it now forms the backbone for my stationbeing used directly on 6 meters and with transverters on and 4 32 MH z. It was designed to be simple yet effec tive; thus t he PTT and single conversio n with option o f spo t frequ en cy injection o r VXO. The 2E26 fin al provides about 30 watts PEP input ; however, subs titution o f a with suitable changes in po wer supply voltages wo uld almost triple t he po wer in put.
Link 2T of hoo ku p wire at cente r. Lin k 2T hoo kup wire at center. L3 - 15T28e. Link IT hooku p wire The All Seeing Eye - Transceive - Transformation 88:98 "cold" end. Link IT hoo kup wire at "cold" en d. I" long. Link I T of hooku p wire at "cold" end. LT 18 tinned airwou nd, 1" 0. PC- 3T 18 tinned on The All Seeing Eye - Transceive - Transformation 88:98 oh m Iw resistor.
T1T2 - Nothingman - Pearl Jam - Philadelphia, Pennsyslvania - September 2, 2000 See text of article. CI, C2 -part of neutralizing circuit.
See te xt. I illustrates the overall layout of the exciter. The 73 60 oscillator balanced modulator is a conventional circuit in which the cathode, gri d and screen form the oscillator circuit while the plates and deflection electrodes ar e utilized in th e balanced modulator circ uit ry.
The DSB signal produced in th e 0 tank circuit is passed through the 9 MHz filt er at which point o ne of the sidebands is removed. The 5 pF butterfly capacitor is bro ught The All Seeing Eye - Transceive - Transformation 88:98 th e front panel as th e " mixer tune" cont rol.
CW o peration The All Seeing Eye - Transceive - Transformation 88:98 provided through cat h ode keying o f the mixer stage and has proved to be quit e satisfactory. The 12BY7 functions as a straightthrough class A amplifier whose plate circuit trimmer appears on the front panel as the " driver tun e" cont rol. Link cou piing to the grid of th e 2E26 is used to furth er attenuat e any un desired fecd t hrough.
Neut ralization of the 12BY7 was not found necessary because o f careful Shielding across the tube soc ket below the chassis. With suit Long Chicken (Toka Remix) - Chris Carrier - Long Chicken le pin con nec tio n changes, a 6C L6 would serve as a goo d substit u te for t he 12BY7. The 2E26 final operates in class AB I in a conven tional circ uit employing capacitive n eutralization.
Bias voltage for the finai amplifier is derived from a reverse-connected 6 volt filam ent transformer connected to the used 5 volt filament winding of the power transformer. Transmit o pera tion is provided by grounding o f the fl oat ing side o f the 6 volt de relay.
This may be acco mplished with a push-to -talk microphone or by an auxiliary "transmit" switch. When not transmitting, plate voltage is removed from the 12BY7 to allo w it to run cooler. The 2 E2 6 bias is increased during standby by ungro und ing o ne end of the bias adjust potentiometer.
Construction Consid era ble thought we nt in to the layo ut of this transmitter. Sin ce the co mponents are standard, builders may duplicate the layout if they wish. As detailed in Fig. Conseq uently the three controls and the jack associated with these stages can be mounted directly o n the front panel without excessive lead length.
As illustrated in the photographs. All ground connections can be made directly to these partitions eliminating the need for ground lugs. Two of t he partitions also serve as mount ing for the mixer and driver tun e controls. The 2E26 final is co mpletely enclosed in a co mpart ment at the rear o f the chassis. C o m ponent la y o u t viewed from abov e.
Dash ed lines ind icate partition piecement below c hassis. The tab at each socket i nd ic ates posit ion o f pin 1. The components aSSOCIated with the rf output meter are mounted on a terminal strip which is bolted inside the The All Seeing Eye - Transceive - Transformation 88:98 compartment near the antenna jack. A shielded lead carries the rectified voltage to the meter on the panel. The Certain of the VXO components such The All Seeing Eye - Transceive - Transformation 88:98 the crystal socket and tuning capacitor are mounted '4 to Jh inch from the plate to minimize capacitance to ground.
All filament leads should be shielded and. In the higher frequency rf stages, lead length should be kept short. Schematic diagram of the vxo trip- ler. There are two small subassemblies used in the construction. The bridge rectifier diodes are assembled on a phenolic board whi ch is bolted vertically underneath the chassis.
Small holes The All Seeing Eye - Transceive - Transformation 88:98 drilled in the board ; the leads of adjacent diodes are placed through a I Aint Livin Long Like This - The Flying Burrito Bros - California Jukebox, bent over, clipped, and then soldered together.
If desiredK resistors and. J; '00". All plate voltage leads to the mixer, driver, and finalare routed through the main partition with feed-through capacitors. Special Components The They are more easily adapted than the smaller "KTran" type. In T I the primary winding was removed and replaced with a 3 tum link. Circuitry of 50 mhz exciter. Controls brought to the front panel are shown in boxes. Link 2T The All Seeing Eye - Transceive - Transformation 88:98 hooku p wire. X I - FT 24 3 surplus xta l.
Some will argue that L I sh ou ld be bifilar wound. Both a co nventio nal and bifilar winding were tried and no significant difference in carrier supp ressio n was no ted. The different ial capacito r associated with LI was constructed by taking a regular air trimmer, setting its rotor at half mesh and after inserting cardboard wedges between the sta tor plates, cutting through the stator with a fine toot hed ha cksa w blade.
Of co urse this capacitor is ava ilable as a regular item if you can get yo ur hands on o ne. Th e 2E26 fi nal is stabilized wit h capacitive neut ralization. Tune up and Adjustmen t It is assumed that the co nstructor will have checked all coils for approxima te resonance with the tubes in their sockets incidentally. Applying line voltage and setting the funct ion switch in the standby po sition will place plat e voltage o n all stages up to the 12 BY?
A VTVM wit h an rf probe is The All Seeing Eye - Transceive - Transformation 88:98 mandatory for tuneup. Place t he rf prob e at th e 6A U6 grid pin a nd adj ust the carrier bala nce control to secure a reading. With the differential capacitor set at center adjust L I and TI for a peak reading. I MHr II. Set the ceramic trimmers across the carrier oscillator crystals at The All Seeing Eye - Transceive - Transformation 88:98 mum sett ing and while switching from USB and LSB positions check to see that the VTVM reading remains about the same.
This indicates that the crystals are centered reasonably well on the filter curve. Proceeding next to the mixer, place the probe on pin 9 The All Seeing Eye - Transceive - Transformation 88:98 adjust both windings of T2 for a The All Seeing Eye - Transceive - Transformation 88:98 reading. Then with It should be about I volt with L3 The All Seeing Eye - Transceive - Transformation 88:98 to resonance. The next step is to remove the 6AU6 if tube to prevent 9 MHz energy from reaching the With the probe at the center-tap of L2 adjust the 5K 2w potentiometer for minimum Apply voltage to the 12BY7 with a temporary jumper and with the probe at pin 5 of the 2E26, peak L2 and L4 and adjust L5 by the "squeeze" method.
A fairly satisfactory alignment of the carrier oscillator crystals can be accomplished as follows: with the rf probe at pin 9 of the mixer and with some carrier inserted increase capacity across the 9.
Note the VTVM reading and then adjust The All Seeing Eye - Transceive - Transformation 88:98 trimmer to yield a reading approximately three-quarters of the "passband" reading. Set the 8. Afterthoughts Some builders will wish to modify some of the circuitry to fit their likings and their junk boxes. One improvement would be a four position function switch to allow for a "manual" operate position.
In the control sectionby use of the negative bias voltage to disable the low-level rf stages one could use a relay with fewer contacts. Further- more, only one side of the ac line need be opened for the switched VAC. VXO shift was held to approximately 40 kHz to give maximum stability, but with more series inductance coverage of kHz should be possible with good stability. Building the exciter consumed quite a few hours of my spare time, but I enjoyed it, and am looking forward to building a transistorized version in the future.
Front panel showing all controls. Its that simple! There is nothing else on th e market comparable to the Look what you get for S AC and DC Portable. Everything that the has, plus many additional features for those who can pay a li ttle more. The will sell for S49 5. And if you need an anten na, He nry Radio ca n make t hat a sim ple matte r wi t h its cost saving Antenna Package Program. We know its hard to believe, but come on in to a Henry Radio Store for a demonstration.
If you can't come in. We'll give you more information, terms, and will ship anyw here. Our reconditioned equipment carries a 15 day trial, 90 day warranty and may be traded back within 90 days for full credit toward the purchase of NEW equipment.
Write for bulletin. The All Seeing Eye - Transceive - Transformation 88:98 W. Los Angeles, Calif. Euclid, Anaheim, Calif. You canno t use it to talk to t he lads with narrow-band if sets clustered around 4 32 mhz, unt il you add the crystal co ntro lled oscilla tor-exciter lat er. T he rf amplifier is right there read y to have the crysta l jo b plu gged into it, but for th e moment it is a lot e asier and q uicker to plug in a sim ple osc illator.
It t akes a rece iver wit h a bandwid t h of several te ns of khz, like the Gonset Do You Ever Think Of Me? - Wayne King And His Orchestra - The Waltz King T hreeto receive an MO PA signal o n t he 4 20 to megacycle band, but just wait a nd see wha t you do with rigs like t hese!
Then if you plug in a narrow-band if like 4 55 kh z and a crys tal-co nt ro lle Give It Up - Various - Discofox Party local oscillator as well, yo u will have the best. Butand it' s " might y big " BUT ," yo u will th en ha ve to use some ki nd of a tu nable if front end. The 'er solid st at e receiver RF Stage. Like sticking you r hand into a basket of snakes!
Now, I've built plenty of six and two mete r rf stages, wit h various transist ors, a nd they worked fin e. Butup a round 4 32, things are di The All Seeing Eye - Transceive - Transformation 88:98 I've made u p tube rf stagesas in 73 Maga zin e, 19oscillat ors galore, and assu med that th e small signal rf The All Seeing Eye - Transceive - Transformation 88:98 age wo uld be duc k so u p.
Not hing doing! As soo n as I sta rted to bring up the gain, using a t u ned base in put circ uit, and t uned collector o utp ut, oscillation! The more I matched t hings, t he mo re it oscillated. The t ro uble with t ransisto rs t oday is they're just t oo darn hot! Also, holdi ng to t he policy h ere of never writ ing u p any t hing no t act ually on the air at the shacko ne an d a half full days went by on thi s item alone.
The G E "M icro tabs" worked. No good so far. OhI haven't given up on that by any means. It's just ano t her challenge the story of my life to be taken up She Says - Motorcycle Boy - Motorcycle Boy. I suspect the interna l resistive feed ba ck as well as th e capacit ive feedback t o be causi ng the t rou ble.
More lat er, I hope. Soto get on wit h t he act ua l circui t that does work well wit ho ut oscilla t io n, Fig. I shows the schematic. The inp ut cable is mat ch ednot perfectl y but good en ough, by C Iwhic h gets to be pretty la rge because those base-e mitter d iodes have an amazingly lo w impedance as yo u go up in t he hun dreds of megahertz.
I tried various kinds of st rap and capacitors for pi networks into the base. Too susceptible to oscillation. Matched to the cable, bu t untu neddoes the job. Tha t little one K resist or from th e base hel ps a lot also: partl y d e-wiseinto th e bias net wor k. Table 1 sho ws so me comparison gains with di fferent t ran sist ors. An r] stage o f this kind has t wo ma in pu rposes. A To set the noise figu re. B T o provide some gain and a u Alex Rath - African Village EP ig.
RF stage 43 2 m. Not too ma ny unwanted signals get past that half-inch copper strap! Bear in mind that we are setting up here a " Gonset Communicator" type of rig, with a co mplete receiver and transmitter, and that you can expect to put in a rea l low-noise stage in fron t later to " set" the noise figure at some better and impossible-so unding figure The All Seeing Eye - Transceive - Transformation 88:98 1. Soolaimon - Neil Diamond - Hot August Night will help you pull in that elusive guy you hear way down in the noise so me evenings.
Alsomore power can be added to the transmitter The All Seeing Eye - Transceive - Transformation 88:98. That takes a real stuffed pocketbook though. I hope to be ab le to help with this item lat er.
When you get past the base circuit, watch the variable emitter resistor. If you're really looking for good adjust ment makeit variable. I wound up with ohms fixed which gives a little under 10 rna of current. In any case. For emitter by-pass, I used the old reliable brass plate, nylon bolt, and thin fibe rglass Parenthood - Jim Gaffigan - King Baby (DVD), about three mils t hick.
Keep t he emitter lead as short as possible. The final length in this model was between one eight h and three sixteenths of an inch. The collector circuit is of the strap-line type and not too critical, tapping the collector down o n t he strap is very beneficial as to gain freedom from oscillation, raising t he Q, etc.
The circuit as sho wn in Fig. Once againI use "phone-jacks" because they work. Use the ones with the ceramic insulation. Or " BN C " types if you like. Side view, rf stage. C3 can be used to faste n the copper clad end The All Seeing Eye - Transceive - Transformation 88:98 to one end of t he minibox. Dimensions are She Is Love - Oasis - Oasis in Fig.
As mentioned in the The All Seeing Eye - Transceive - Transformation 88:98 preamble on t he 'er, a " Rack and Pan el" t y pe of carrier is used, but small in size, made of wood, wit h aluminum paint sprayed o n. T o p view rf stage, 43 2 m ,cible to use, as you can fix the units o n shelves, or use th e sides like a rack. Everything comes out at a moment's notice for adjustment, change or repair, and the whole deal has Iron Maiden - Rock In Rio - Brazil Live Part 2. "growth possibilities builtin" as they say on Madison Avenue.
So you can add an extreme low-noise rf stage, change to or add a narrow-band if strip, etc, etc. Don't say I didn't warn you! Power supply for the rf stage is twelve volts at ten milliamps or less, depending on where you set the emitter resistor.
It also works well on one of those nine volt transistor batteries. I intend to standardize on the lantern-type batteries from now on. Union Carbide rates theirs at half an ampere maxi- "I ".
There are some intriguing new small nonspillable storage batteries out also, which will go for more watts, later on. Twleve watts dc seems to have good possibilities to me though.
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