Antenna Pre-amp Power Sequencer Protection Circuit

By John, KD7DO

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 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 relays. I call this strategy my 'cave-man mentality design approach' although using relays for timing applications has been around for a long time, they are fairly plentiful, relatively inexpensive and reliable. However, one must take into account they can generate killer-electrical spikes if not suppressed with diodes (bad for the poor-little pre-amp). Also relay contact bounce must be accounted for if used for more critical timed switching applications.

With all this rational used in the design-approach behind us, lets review the schematic of the finished 'power sequencer'. This design was tailored for my equipment, which integrated the HF rig (In this case my TS-440 HF transceiver) a multi antenna systems, with a single mic shared between several transmitters. If you don't need these amenities, they may be eliminated for your specific requirements to simplify my gadget further. One last note . . . when using the add-on brick power amps, most of these devices can extend SSB hang/hold-on periods for 2-3 seconds after the mic key is released. You should adjust this for less than a half second maximum hold delay, or else you will defeat the power-sequencer pre-amp protection scheme.

After assembling your unit, recheck, recheck and recheck again to ensure that the relay sequence periods and pattern is correct for your particular application before applying transmitter power to the system. I used a dipole driven element for my yagi. If yours is anything other than that, use an additional d.c. blocking capacitor so the 12 volt power source isn't shorted, if warranted I found this project to significantly enhance signal to noise ratio, compared to the pre-amp located in the 'brick' high power amplifier.

With that, good luck on your project. I've rechecked the schematic several times to ensure that all the changes/corrections have been added. Hopefully I caught everything. However, if I missed something or you want to discuss this further, email me at vanaglash@msn.com .

73 John, KD7DO

Here's a reduced size version of the schematic.  Click here for the full size version.

small schematic