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
Click here to download the full size of the above Circuit.

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...




Leave Comment

characters left:

New Circuits

.

 


Popular Circuits

VHF FM transmitter
Proximity-sensor
Power-Saving Intermittent Converter
Space Age Sound Machine Circuit
Computer Microphone
GPS reference clock NTP server with Gentoo
555 audio oscillator Analog Integrated Circuits
Analog IO Protection
RF amplifier and filter for 2.5MHz
Exploring a digital I2C/SPI accelerometer (MMA7456L) with Bus Pirate
A8902 Typical application wiring diagram



Top