Topband: low angle reception using active antennas

[Lee K7TJR]

  Yes there is a difference in putting an amplifier in the middle of a shortened Hi-Z element rather than at the end using high impedance buffers. The radiation pattern remains virtually the same. You have to remember that if your base element source capacitance was 22 pf for a certain length wire referenced to ground, then putting the amplifier in the middle causes your element on each side to be shorter by half and then it has a much smaller source capacitance (almost half) to feed its side of the amplifier as a result. Even though you may have the extra gain of a differential amplifier it will likely have the same input capacitance on each side as it did with the single input. The common mode input capacitance of your differential amplifier becomes very important here as well. Whatever you may seem to gain by differential signals will be easily lost by the larger capacitive voltage division at the input to your differential amplifier. My opinion is that you would be better off using a single ended amp at the base of your element. I have made field tests on the phase shift of received signals in both transformer and Hi-Z amp coupled elements and there is a real measurable effect of having cables,wires and any other local capacitances near the elements, Even radials. This effect is much worse with transformer coupled elements. Of course, the shorter you make the elements, the greater the error effect. Therefore I do not think you would want any cables or amplifiers any where near the center of the element in order to maintain your phasing with environmental changes. As far as input protection for static on your amplifier, you might consider biasing up your clipping diodes with a zener diode. You might also try the amp circuit at www.k7tjr.com in the 8 element array section under Lees radios. It has input clipping both at ground and whatever your supply voltage is. Typically 13.8 volts. I have had 8 of them running for three years now and have not lost one to static or anything else; yet. I have not taken the time to measure the intermodulation products of these amps so if there is any one that can do that I will send them an amp to test. I know they are pretty high because I can measure the 1Db output clipping point at a bit over 2Vrms into 75 ohms. Which, in this design is over 4 Vrms at the input.

 Lee K7TJR Oregon