Sunday, March 18, 2007
Advanced Receiver Preamp Modification
I have used both 2 meter and 440MHZ GaAsFET versions. If you read the comments on eham, you'll likely be confused as to whether it is a good product. I can assure you they are. The reason the older bipolar ones don't seem to have a back eye is simple, they are less sensitive.
However I've found that if you add a pair of back to back silicon diodes (1N914) to the GaAsFET preamp's input this will protect it from overloading. That's almost an essential mod on these GASFET preamps as they are very static sensitive. Even more so for a repeater site where strong field strengths (or lightening prone areas) are present. However, at repeater sites, ideally your preamp should have a preselector.
You add a pair of back to back 1N914 diodes to the preamp's input to protect it from overloading. As you can see the two I drew-in are in parallel, in back to back, opposing directions (anode to cathode). The pair is soldered onto the center feed antenna input to ground. The idea is to always clamp the input signal to always a safe level and protect the preamp's FET from overload.