[CQ-Contest] to pre-amp or not to pre-amp

Scott Ellington k9ma at sdellington.us
Sun Jan 17 11:47:07 EST 2016

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!


Scott  K9MA

Scott Ellington  K9MA
Madison, Wisconsin, USA

k9ma at sdellington.us

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