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“Such is not the case with the SSTRAN AMT-3000 since it is designed to drive a 800 ohm load seen from the collector of the output transistor. It has been claimed that a series loading coil is the best approach for this unit since some of the coil inductance acting with the tuning cap. transforms the antenna system impedance to values acceptable for this unit.”
The optimum load for the AMT3000 output transistor is indeed 800 ohms as Neil stated. But, this is irrelevant when looking at the entire base-loaded antenna configuration. The impedance at the bottom of the loading coil where the transmitter connects is anywhere from maybe 10 ohms up to over 100 ohms resistive depending on the quality of the antenna’s ground system. The AMT3000 transforms this low impedance up to the desired 800 ohm transistor load by means of an L-net. The L-net consists of capacitance to ground at the antenna jack followed by a small amount of series inductance (about 20 uH) between the antenna jack and the bottom of the loading coil. The capacitance to ground is the parallel combination of capacitors C23 and the trimmer capacitor. Where is the series inductance? It comes from part of the loading coil! This sounds strange, but this is common practice with loading coils and it is proven by the fact that the inductance of two series inductors is equal to the sum of the two inductances. It doesn’t make any difference if you connect a 20 uH inductor in series with the loading coil or let the 20 uH come from the much larger inductance of the loading coil.
The loading coil is an absolute requirement for the AMT3000 to work properly. If you just connect a resistor load across the antenna jack without a loading coil, you will indeed find that the optimum load is 800 ohms because there is no longer an L-net transfrormer in the circuit. But, this is not a valid configuration for the AMT3000, so it may be of interest, but is irrelevant to normal operation of the AMT3000.
The L-net capacitance from the parallel combination of C23 and the trimmer ranges from 572 pf to 660 pf depending on the trimmer capacitor setting. This range may not be adequate to cover the extreme high and low range of a user’s actual ground resistance. Optimum loading is indicated when 13 VDC is measured across the meter terminals T1 and T2 with no modulation. If the trimmer adjustment does not result in the optimum voltage, then the ground resistance is outside the “typical” range. This can be corrected by installing more radials or changing the value of C23. It is preferable to have a better ground, but for those who can’t achieve a better ground, then changing the value of C23 is a second option to at least get the proper impedance match.