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TopBand: Re: Rx amps/noiseless feedback

To: <topband@contesting.com>
Subject: TopBand: Re: Rx amps/noiseless feedback
From: w8jitom@postoffice.worldnet.att.net (w8jitom@postoffice.worldnet.att.net)
Date: Mon, 6 Oct 1997 22:39:19 +0000
N7CL wrote:

 >Peter,
 >
 >It is true that on 160 or generally on the low bands for that
 >matter, the background noise level is moderate to high.  And full
 >size beverage antennas although not very efficient, are efficient
 >enough that an extremely low noise figure in the preamp is
 >probably not necessary.

Here's some measured data Eric.

At the quietest location I have ever operated from, where a 1 wl
Beverage was over a mile from the nearest power line and during the
middle of the winter day (the quietest time), the 250 Hz bandwidth 
noise level at the Beverage's transformed 50 ohm terminal was -122 
dBm, or .18 uV.

That noise level equates to a required receiver noise figure of 18.45 
dB for a 10 dB S+N/N ratio with 250 Hz bandwidth. 

With that in mind, I agree even three dB noise figure is overkill!!! 
An old 6SK7 tube would be just as good as a GASFET for 160, so far as 
S/N ratio on a Beverage is concerned.

With a Beverage the main concern should be head room, not noise 
figure.

>>However, physically small wideband
> >(untuned) loop based antennas are very much les efficient than
> >even a short beverage.  So these types of antennas (including the
> >EWE if sized to preserve pattern up to 7 - 10 MHz) really do
> >require a very low noise figure for the preamp that they require.
> >The Anzac circuit (or a push pull version of it for balanced
> >designs) should be near ideal for use with small wideband loop
> >receiving antennas for topband.  Generally, the preamp needs to
> >be quiet enough so that you can hear the band noise with the
> >antenna.

I don't have exact terminal voltage data for this system, but with a 
directional array using four untuned ~14 ft diameter loops my system 
was over five dB out of the background noise during mid-day on a 
quiet winter day at noon using a 1.5 dB NF amplifier AFTER the 
combiner and phasing system. I would suspect (but I have never 
confirmed) a single loop would only require about 10 dB NF. 

73, Tom W8JI
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