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"It
is vain to do with more
what can be done with less"
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William of Occum 1290-1350
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WEEKEND PROJECTS FOR QRPERS
By George Dobbs G3RJV
First offered as part of the Four Days In May Symposium
at the 1999 Dayton Hamvention.
An Active Antenna
It occurred to me that it might be useful to have a small
general purpose active aerial for times when I might have a receiver and
be without an aerial. The requirements seem to be for a small whip antenna,
or similar, to feed a radio frequency amplifier capable of around 20 to
30 dB of gain. Too much gain and local noise or even internal amplifier
noise becomes a problem and too little gain and it will serve little purpose.
The short antenna will offer a high input impedance and the output will
probably need to be low impedance to suit most receiver inputs.
The Active Antenna is a two-stage amplifier using a FET followed by a
bipolar transistor. The FET stage offers a high impedance to the small
antenna. I chose not to use the usual telescopic whip but mounted a socket
for a short wire antenna. Apart from saving the cost of a telescopic whip,
this has several advantages. The length of wire can be varied according
to the desired amount of signal pick up, the wire can be moved around
for best reception and the unit is smaller.
TR1 is an MPF102 but the common 2N3819 would do the job. Be careful about
the pin placement, they are not the same. TR1 is biased by choice of the
source resistor and a 1mH RF Choke provides a RF load at the drain. Although
no large signals are involved here, it is wise practice with these FET
RF amplifiers to keep the input physically away from the output.
The bipolar RF amplifier, TR2, uses shunt feedback between the base and
collector and the un-bypassed emitter resistor also provides some degenerative
feedback. Most common transistors will serve for TR1 although it should
have a fairly high fT.
A small RF Transformer matches the output of TR2 to the typically low
impedance found in receiver input circuits. I opted for a very simple
choice, a transformer wound on a small "pig nose" ferrite former
- they are common items in the surplus market.
Fig2. shows the winding of the transformer. The primary is 12 turns of
30 swg (about 28 awg) enamelled copper wire and the secondary is 2 turns
wound from the opposite end.
Fig.2. also shows how the connections are made to the transformer. More
sophisticated constructors might like to make a trifilar wound 4:1 RF
transformer but the simple arrangement I used appears to work well.
This simple little circuit appears to be at home with a
variety of receivers and it a useful extra item to have for casual listening
with resort to a larger antenna.
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