I have been building preamps for the past three years for a variety of
Beverage antennas. The first ones were from an article in QST by the late
W1FB-they used a transistor followed by an IC. These worked well, but were
located at the operating position. I did not experience the severe overload
problems identified by some builders (good notes from KM1H on this subject),
but I decided to change things around and put a preamp out in the field for
each Beverage. These are all fed via underground cable, and so far I have
not had any problems, even though the closest one is only about 30 ft from
my TX vertical.
The circuit I used is similar to the one in June, 1978 issue of 7s
Magazine-it uses a pair of FETs, one as an amplifier and the second as a
follower. I reconfigured the input to use an FT50-75 toroid with 30T of #26
for the primary and 20T of #24 for the secondary (though the latest of these
uses a larger, less lossy toroid core-FT114-62-as per KM1H again, I used a
quadrifiliar winding-see ON4UN's book for the details). I omitted the
back-to-back diodes and substituted a 12V "wheat germ" bulb in series with
the antenna lead. I also did not employ a variable resistor in the source
of the first FET but used 10 ohms fixed. So far, no problems from Thor.
How do these simple preamps work? I'm not much for measurement on things I
have built, other than what my ears tell me. From experience over the last
year, I think this setup with an 880 ft Beverage east, a 550 ft west and a
pair of 330 ft northwest and southeast hears quite well. I can usually hear
what the big guns are working if the band is open to my corner of the world.
I have read a good bit about commercially-produced preamp/transformer units,
and I am sure they work very well. But for the ham who likes to build on a
limited budget, homebrew preamps do produce good results. As for the FETs,
I have used MPF102, 2N5486 (hot) and 2N4416 (hotter). Keep the feed voltage
down to about 12 volts and the FETs will happily produce good signals for
your Beverage, ewe, or what have you.
Bill, VE3CSK
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