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.
