On 1/17/2016 00:50, Jim Brown wrote:
On Sat,1/16/2016 10:30 AM, Franki ON5ZO wrote:
Question 1 is: would I benefit from a pre-amp right at the feedpoint
of such a flag or single K9AY loop antenna? Coax to the shack is
about 50-60m max.
This fundamental rule works for ANY antenna. Connect it to your radio.
If the noise level from the antenna is at least 10 dB higher than the
noise level with no antenna and a 50 ohm resistor at the antenna
input, you don't need a preamp. Do this test under the quietest actual
conditions -- if on 160, between an hour before sunset and an hour
after sunset, on a day when static is low.
One case in which a preamp is useful, even if signal to noise is
adequate without, is if weaker signals don't reach the AGC threshold of
the receiver. When this happens, the S-meter doesn't move and the
operator has to turn up the audio gain to hear weak signals. If a strong
signal then appears, said operator's ears get hammered. I find that very
unpleasant, and it can cause hearing damage. A preamp may get these
signals above the AGC threshold, and minimize this problem. (It's not
the ideal solution, but it doesn't require modifying the receiver.) Of
course, it may sometimes be necessary to switch out the preamp when
there are a lot of strong signals about. I think mainly older receivers
have this problem. A modern one with programmable AGC could certainly
be set up to behave this way, but that could probably be fixed without a
preamp, except with very lossy receiving antennas.
I like to operate with the noise level just moving the S-meter, so I
know I'll get AGC action on all signals. It's more comfortable, and
protects my hearing. I can always reduce the RF gain to distinguish
between signals of different strengths. I know many operators prefer
the higher AGC threshold, so they can easily hear the difference in
signal strengths, but that comes at a price. It's a personal
preference, so let's not have an argument about it!
73,
Scott K9MA
--
Scott Ellington K9MA
Madison, Wisconsin, USA
k9ma@sdellington.us
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