Hi Ford,
Using active means to terminate the small loops can be made
to work, but for the cost of a small transformer (or four) may not
be worthwhile. A standard 4:1 ratio transformer (MiniCircuits,
or hand-knitted) is an almost perfect fit (800ohm), and cheap.
Having been down the active-feedpoint route with 160/80 Ewes
in the past and LF receive antennas nowadays, there are a few
pitfalls. Nothing major, just added complexity and considerations.
All the excellent advice given here on the reflector about feeding
Beverages and such is relevant to this, in addition to things here
specific to going active.
The good news and the bad news is that the output from these
loops, Ewe, Pennant etc., is quite low, which means that the
follower (active terminator) might not have to be capable of driving
huge signals down the 50 ohm coax to the shack ('real' signal levels
demand things more akin to small transmitters as line-amps to
have sufficient dynamic range). A simple follower does give one
effectively 6dB of 'free' gain (12dB from no transformer step-down,
but add line termination loss), or a slightly more involved
scheme the full 12dB at the expense of headroom, but one should
bear in mind this dynamic range issue should one get adventurous
and fall for the natural temptation to add 'real' gain up front, too.
Secondly, the transistor must be protected from your transmitter;
this can be as simple as a little bit of series resistance and back-
diodes to the supply rail and ground. When you transmit, though,
the active stage *will* assuredly saturate; the huge signal
screaming up the feedline has been known to cause some radios
to 'lock up', stick in transmit; a disconnect relay may be
necessary.
Thirdly, there may well be problems with power-line buzz/fizz being
picked up on the feedline (not-so-uncommon-mode) or being induced
in a ground-loop should you be deliberately grounding at both there
and the shack. Conventionally, the antenna decoupling afforded by a
transformer is a start at getting rid of these and preserving the
antenna's directionality (feedline radiation in reverse), with the
recommended addition of common-mode line filters (at least one
at the feedpoint) going a lot further. (At 136kHz I find decoupling/
filtering at the station end necessary, too.) For an active system,
the common-mode filters will be essential, not an option, lacking
the transformer.
However the preamp/follower is powered, up the coax or through
a separate wire, the same common-mode filtering needs to be
applied to it. too. One should also be very wary of the possibility
of power-line noise being introduced by the supply. Even though
you may be able to regulate it and filter it to look a bit like
DC, a considerable amount of 'fizz' can be introduced, again,
common-mode across the stellar transformer in the wall-wart.
A good bet, should you choose to accept this mission, is a 2N5179
CATV transistor as a follower, pulling say 50mA to get it linear.
Good luck, 73,
Steve W3EEE
|