These features are defined by the fact that high and especially high and ultra high frequencies dramatically increase the energy loss in lamps, oscillating circuits and various dielectrics. Normal bulbs that work well at low and high frequencies (30 MHz) at high frequencies are poorly or even not at work.
Such dielectrics as a paraffin, a phenolic resin, Karbolit, plated hardened paper, cardboard, rubber, cause in contours so great losses, their use in the VHF equipment should be considered totally unacceptable.
For this and other reasons (which are discussed below) novice ultramarathoner should never make testing a particular design, without the help of so-called "volatile" montages that fans often calls trial. As a rule, almost any VHF equipment assembled at a very good scheme, but hastily, sloppily, with a messy arrangement of parts, long and confusing circuit wires in substandard insulation, using low-quality dielectrics always gives poor results or not work at all.
That is why, before proceeding to the construction of the planned design, it is recommended to read the following comments and tips that can be very useful for everybody.
In the oscillatory circuits of the VHF equipment have to deal with coils of very low inductance and capacitors minor capacity.
The higher the frequency, which is calculated receivers or transmitters, the less workers inductance and capacitance. So. at frequencies of 40, 144 and, especially, 420 MHz these values are comparable with the interelectrode capacitances of the lamp, the inductance of the lead wires, the parasitic capacitances of the installation and the inductance of the connecting wires. So you should always strive to ensure that the capacity of the installation of the high frequency circuit was minimal, and the connecting wire is straight and short as possible. On these frequencies the conductor length of 5-10 cm is the inductance of the same order of magnitude as the inductance of the contour CA of the carcass. But if this conduit is curved, i.e., has the form palowitch, its inductance will be more. The irregularity VHF installation leads. first, to a sharp change in the frequency of natural oscillations, the deviation from the calculated value, and secondly, to the deterioration of the q-loop to increase the damping in it. From this point of view the rational location on the chassis lamps and high-frequency parts is crucial for a good performance of VHF equipment.
When selecting a location for placement of parts and lamps and their relative positions should be guided by the following rules:
(a) Contour coil should be placed near the lamps to which they relate.
b) the Lamp of amplification of high frequencies, local oscillator and mixer positioned near the block of variable capacitors.
in Lamp amplification stages of the intermediate frequency oscillations to be placed near the intermediate frequency transformers.
Designer VHF equipment should also be borne in mind. that with increasing operating frequency, the gain is ordinary, not special lamps quickly Nada, approaching the unit is already at frequencies around 80 MHz. In this case, improving the quality of the oscillatory circuit, the use of high quality silver and ceramics do not give any positive result. For this reason, the designer should always strive to apply special wedge base lamp having a small interelectrode capacity and is designed for use in УSW range. Such lamps all lamps are of the type "acorn", lamp NP, SP, SP, SP, RP, ESP, JP, SU-32. SU-29, etc.
But even special lamps have ultra-high frequencies low input impedance. The main reason that causes the lowering of the input impedance of the lamp depending on the growth of the operating frequency, is the inertia of the electrons. The inertia of the electron flow causes the grid current. which means the emergence of an active component of the input admittance. (At the same time the grid current increases the noise floor.) The inductance of the pin conductors of the lamp also reduces the input impedance of the lamp. As a result of the fact that the coil inductance at high frequencies is small, and the losses in the lamp is large, the resonance impedance of the circuit is small (1500 ohms or less).
Given this, for УSW generators should be applied circuits with high quality factor. To reduce losses in the circuit, you should always avoid the use of a large number of dielectrics. Dielectrics should be used only of high quality, designed to operate at high frequencies. The plated hardened paper, Karbolit, the PCB at frequencies above 30 MHz should not be used because of excessive losses in them.
Best coil for the circuits of the generator is a coil constituting a frame of the high-frequency ceramics, a spiral groove which is coated with a layer of silver. This coil has a low-loss, rugged and provides almost constant value of inductance in a wide temperature range. The use of such coils in the transmitter with an excitation ensures adequate frequency stability.
Minor when warm-up frequency drift caused by the variation of the geometric dimensions of the connecting conductors can be easily compensated for using in the circuits the capacitors with a negative temperature coefficient.
In Amateur conditions such coils to produce almost impossible. However, the coil with enhanced stability, necessary first of all to the master oscillator, can be wound from copper (preferably silver coated) wire, preheated to a temperature of 100-120° C, stacking it with some tension in the grooves of the ceramic frame. It is clear that in the doublers and the output stage, in which there is generation frequency, it is possible to apply more than a simple, frameless coil. However, in all cases, we must strive to ensure that the contours were mechanically strong.
Very often Amateurs, wishing to improve the factor of the circuit, make the coil too large diameter generators it leads to large radiation losses. Should be advised of the coil with a diameter of 15-20 mm, the output stage is 30-35 mm.
Post of the coil should away from masses of metal to avoid losses by eddy currents. The minimum distance of the coil from large metal surfaces shall be not less than the diameter of the coil.
At frequencies in the 400-450 MHz and above is convenient to use resonant circuits, in the form of short-circuited quarter-wave lines. If the quality factor of conventional circuits is a few tens of units, the quality factor of the circuit lines can be increased to several thousand. As described in this collection of transmission structures intended for operation in the range of 420-425 MHz, is applied instead of a conventional coil line consisting of a copper-plated tubes.
Special attention constructor should be paid to the quality of the capacitors, on the reliability of the rubbing contact in it. In all cases, when possible, the rotor of the capacitor must be "grounded, i.e. connected to the chassis. This will eliminate the influence of the operator's hand in tuning the circuit.
In the transmitters is best to build the agent on the scheme with electronic communication. This facilitates the mounting of the condenser and removes the impact of hands on the frequency of the generated oscillations. Usually the anode circuit such exciter tuned to the second harmonic, and thus, using one lamp, exercise frequency doubling. Lowering the frequency of the master oscillator increases its stability. The advantage of this scheme is that the generator with two lamps will have options not worse muehlenweg generator.
Down the transmitter, the designer must bear in mind that each resonant circuit in a multistage transmitter must have a body configuration (handle AC capacitor). Constant adjustment in the anode circuits of the doubler and the output stage to the middle frequency range leads to a substantial decrease in output to the antenna oscillating power in the restructuring of the transmitter at frequencies that are different from the average.
When establishing the generators never remove the lamp subsequent stages; the bulb should be left in the socket, and in order that they is not out of order, it is necessary to release from the anode voltage. If the designer during the operation of the master oscillator and the setting of the desired frequency range will take out the lamp of the doubler, and then after the configuration of the doubler will re-insert it in its place, thanks to the capacitive coupling between these cascades master oscillator will be so upset that in the doubler circuit will not be detected oscillations. For the same reason you can't. for example, to select a particular harmonic in the anode circuit of the doubler disabled when the coupling capacitor.
When designing УSW receivers all the efforts of the designer should be aimed at obtaining higher sensitivity, which is possible only under the condition of applying a high-frequency amplifier with minimum noise. Best for this purpose to use a triode, but the scheme included "a grounded cathode, grounded grid.
As already mentioned, on VHF input and output resistance of the lamps is greatly reduced. Therefore, the loss of vibrational energy in the lamp significantly Outweigh the losses in the circuit; furthermore, the lamp sharply shunt circuit reducing its quality factor. In order to reduce the shunting effect of the lamp should be connected to the tube grid, not the entire path, but only a part of it. To this end, the communication circuit of the amplifier to the grid of the subsequent lamp is a must autotransformer. This reduces the attenuation introduced by the lamp in the circuit, and produces the greatest gain stage. In decoupling circuits and circuits of the cathodes УSW receivers cannot apply the big capacitors, as they have an appreciable inductance, the value of which at high frequencies cannot ignore,
If the circuit has a capacitor of large capacity, for example, electrolytic, possessing, as we know, visible-inductance, in this case, in parallel to this capacitor to attach a mica condenser of small capacity, which has a small inductance. Thus, simultaneously filtering will be implemented as ultra-high and lower frequencies.
It is clear that the long connecting wires and a common ground wire in the high frequency paths create a noticeable parasitic inductance and capacitance. So we should use straight and short about connecting with the water and without any insulation, as a dielectric will cause additional energy loss. Grounding each point of the scheme should run a separate wire, and all the ground conductors belonging to one lamp and cascade, it is necessary to attach to the chassis at one point.
Structurally, an Amateur station may be decorated differently. The undoubted advantages of modular design, in which the modulator and generator are independent units, inmates in the General frame of the transmitter. Modular design facilitates adjustment, repair and replacement in case of malfunction.
Receiver for many reasons must be done separately without linking it firmly with the transmitter. This expands the possibilities of experimentation in cases where the receiver should be removed from the transmitter.
The rectifier is recommended to fulfill in the form of an independent unit associated with the transmitter hose power. Useful the output of the rectifier, in the form of chips, duplicate the socket with clips. Use duplicate clips very convenient when connected to a rectifier of any other structures that require power and having chips or connectors of a different type than those used for connection of the rectifier with the transmitter.
In this brief introduction does not discuss other issues of interest to the radio Amateur-ultramarathoning. However, in many of them he will find the answers directly in the descriptions of individual structures.
Literature:
Publication: N. Bolshakov, rf.atnn.ru