Adjustment Of Gamma Match By Use Of Antennascope And Griddip Meter
o o cess of tuning the array is made a substantially separate process as just described. After the tuning operation is complete, the resonant frequency of the driven element of the antenna should be checked, directly at the center of the driven element if practicable, with a grid-dip meter. It is important that the resonant frequency of the antenna be at the center of the frequency band to be covered. If the resonant frequency is found to be much different from the desired frequency, the length of the driven element of the array should be altered until this condition exists. A relatively small change in the length of the driven element will have only a second order effect on the tuning of the parasitic elements of the array. Hence, a moderate change in the length of the driven element may be made without repeating the tuning process for the parasitic elements.
When the resonant frequency of the antenna system is correct, the antenna transmission line, with impedance-matching device or network between the line and antenna feed point, is then attached to the array and coupled to a low-power exciter unit or transmitter. Then, preferably, a standing-wave meter is connected in series with the antenna transmission line at a point relatively much more close to the transmitter than to the antenna. However, for best indication there should be 10 to 15 feet of line between the transmitter and the standing-wave meter. If a standing-wave meter is not available the standing-wave ratio may be checked approximately by means of a neon lamp or a short fluorescent tube if twin transmission line is being used, or it may be checked with a thermomilliammeter and a loop, a neon lamp, or an r-f ammeter and a pair of clips spaced a fixed distance for clipping onto one wire of a two-wire open line.
If the standing-wave ratio is below 1.5 to 1
it is satisfactory to leave the installation as it is. If the ratio is greater than this range it will be best when twin line or coaxial line is being used, and advisable with open-wire line, to attempt to decrease the s.w.r.
It must be remembered that no adjustments made at the transmitter end of the transmission line will alter the SWR on the line. All adjustments to better the SWR must be made at the antenna endof the line and to the device which performs the impedance transformation necessary to match the characteristic impedance of the antenna to that of the transmission line.
Before any adjustments to the matching system are made, the resonant frequency of the driven element must be ascertained, as explained previously. If all adjustments to correct impedance mismatch are made at this frequency, the problem of reactance termination of the transmission line is eliminated, greatly simplifying the problem. The following steps should be taken to adjust the impedance transformation:
1. The output impedance of the matching device should be measured. An Antenna-scope and a grid-dip oscillator are required for this step. The Antennascope is connected to the output terminals of the matching device. If the driven element is a folded dipole, the Antennascope connects directly to the split section of the dipole. If a gamma match or T-match are used, the Antennascope connects to the transmission-line end of the device. If a Q-section is used, the Antennascope connects to the bottom end of the section. The grid-dip oscillator is coupled to the input terminals of the Antennascope as shown in figure 18.
2. The grid-dip oscillator is tuned to the resonant frequency of the antenna, which
Figure 19
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