Whenever we discussed about RF amplifiers, we were simultaneously referring to the importance of stage matching too. This is not simply a problem of stage loss where for ex. when a 10W signal is coupled to another stage, only 2 W reaches the other. Just like a ripple in a bowl returns back to the centre after touching the sides or like light being reflected from a mirror, we need to assume that the lost power returns back to the source. Here, the power that moves forward is called forward energy and the power that returns is called reflected wave energy. Also we need to know that full transfer is not at all possible even at the best optimum arrangements and care. Only when all the applied signal is fully absorbed by the following stage that absolute cancellation of reflected wave energy happens along with retention of stability in the stage output. At such situations the output impedance and the input impedance will be fully matched. The more mismatching is there the more will be the percentage of reflected wave.
Every component or part used in a matching configuration is determining in deciding the final impedance matching level. Suppose that the out put of a transistor is the source and the antenna is the load and even if we have calibrated the output and input impedances to perfect matching, the quality difference of the connector used in between is enough to spoil all the charm. At the same time the length of the transmission line (that connects the transistor out with the antenna) does not make much mismatch, provided it is not coiled anywhere. We are talking about a situation in which the out and input impedances are the same. Where there is reflected wave, a standing wave pattern is formed on the transmission line. It is the ratio between the lower value and the higher value of this standing wave voltage that we call VSWR (Voltage Standing Wave Ratio).
The SWR is usually thought of in terms of the maximum and minimum AC voltages along the transmission line, thus called the voltage standing wave ratio or VSWR. For example, the VSWR value 1.2:1 denotes an AC voltage due to standing waves along the transmission line reaching a peak value 1.2 times that of the minimum AC voltage along that line. The SWR can as well be defined as the ratio of the maximum amplitude to minimum amplitude of the transmission line's currents, electric field strength, or the magnetic field strength. Neglecting transmission line loss, these ratios are identical.
Standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmission line, and SWR is defined as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line (see fig. C-27/0A).
The SWR is usually thought of in terms of the maximum and minimum AC voltages along the transmission line, thus called the voltage standing wave ratio or VSWR. For example, the VSWR value 1.2:1 denotes an AC voltage due to standing waves along the transmission line reaching a peak value 1.2 times that of the minimum AC voltage along that line. The SWR can as well be defined as the ratio of the maximum amplitude to minimum amplitude of the transmission line's currents, electric field strength, or the magnetic field strength. Neglecting transmission line loss, these ratios are identical.If it is a transistor that is used in the final sage and if the output stage is mismatched, it won't be working in its maximum efficiency; at the same time the transistor may not also work long. Those who use transistors in the final can use SWR protection circuits that stops the stage function at critical situations. This is why it is said that at high power circuits a matching circuit is necessary. At the same time the resistance capacity of valves are very high; a valve may withstand such mismatching conditions for prolonged time. Load according to a transmitter is anything that is capable of absorbing RF power. Even though the commercial SWR meters show the exact standing wave ratio, SWR meters can be easily home brewed also, with which we can learn the intensity of the reflected wave. In fig. C-27/1 the circuit of such a gadget is shown.
The RF power from the transmitter is let through a 4" long coaxial cable having the same impedance as that of the out put and input. Since the shield of the cable works like electrostatic shield, only one side of it shall be grounded. The toroid used here can be any toroid that works in RF ranges. The coil should have 14 turns of 24 SWG wire. If the deflection seen in the meter is very high change coil connections.
Reading the SWR matching level is no protection; and it also does not mean that maximum power is radiated through the Antenna. What we want is an Antenna tuner that could reduce the SWR and make Source-Load matching possible.
Fig. C-27/2 gives the details of a simple Antenna Tuner (ATU) that can be used with low power transmitters. Another ATU circuit that had been successfully tried wit high power transmitters are given in fig. C-27/3
As shown in the picture, either the arrangement should be such that switching is possible to each turn of the coil or a soldering and testing at each turn pattern can be followed. In word sense what we require here is a variable roller inductor, which may be available from junk/flee markets. Another ATU circuit close to C-27/3 is shown in C-27/4.
The construction details of a simple SWR meter that shows standing wave strength at forward and reverse conditions is shown with details in fig.C-27/5. We have given here only a short account on VSWR and ATU.
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