Preamp Protection

Posted on Feb 6, 2014

While working two-meter SSB from the Benson area, I decided that local terrain and the limited height of my home-made six-element yagi, necessitate I incorporate a high gain antenna pre-amplifier at it`s terminations to enhance receiver signal strength. This approach would give that little extra advantage to help dig the signals out of the noise levels. I knew

Preamp Protection
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that this method would produce superior results, compared to a pre-amp located close to the transmitter at the other end of the transmission line. Unfortunately, this approach requires that the pre-amp be protected from the high power being fed from the finals in the transmitter to the yagi. In my case it was a 160 watt output `brick` driven from a Kenwood TR-9000 all mode transceiver. Consequently, the pre-amp needs to be switched out of the line during SSB transmission. There are a number of schemes/approaches for doing this task. The most common uses an rf switch (senses minute amounts of rf during initiation of the transmission phases and activates a relay to place the pre-amp off line). Unfortunately, there are many examples of failure using this technique to really protect the pre-amps reliably. Case in point. many `brick-amp/pre-amp` combos available to the ham community suffer from blown pre-amps. There are some expensive sophisticated power management/switching devices available, but again cost can be a factor incorporating them into the average `Joe`s` ham shack budget. I conducted a detailed search on the net for a low cost approach, and couldn`t find a suitable project, thus the unit which I came-up for my needs is presented herein. As with all prudent projects, the old scrap/salvage box becomes a key player in the design, and this endeavor is no exception. My approach to make a simple timing circuit uses...

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