[Elecraft] K3 and Preamp

[Edward R Cole]

Ted,

Observes that his K3 noise level drops on 10m if he disconnects the 
antenna.  That indicates that sky noise (plus any man-made noise in 
his locale) is the dominant source of noise.  But then he states that 
the PR6 improves weak-signal copy.  That is not exactly 
contradictory.  Noise power, Pn:
Pn = Psky + Prx + Plocal

So all these sources add up.  The PR6 lowers the internal receiver 
generated noise so Prx get smaller and that helps lower the N part of 
S+N/N (or SNR as some write it).  If Psky were many times larger than 
Prx+Plocal then adding a preamp to lower Prx will have little effect 
(probably not noticeable)...typically what exists on lower HF bands.

Also, it you live in a noisy urban electronic environment the Plocal 
noise factor may rule what you can hear and the preamp will not be 
much help.  Noise level of your locale will certainly have an effect.

The point raised about receiving weak-signals with adjacent high 
level signals is also true.  High IP3 devices should be used in a 
preamp used under these conditions.  Typical low-noise devices are 
sensitive to any RF energy in their passband which usually pretty 
wide.  So in conditions often seen on HF, a preamp may not work 
well.  The dynamic range of the K3 may be more important in that 
situation.  Operating on 6m during a contest would even be an example 
of that in many locations with high ham population.

If you live in a quiet neighborhood and have no local QRO stations, 
then a low-noise preamp will usually help get those weak ones (on 
10m+).  This is much more apparent at 2m and above.

On 2m noise on the horizon is about one s-unit higher than looking up 
into the sky.  On 1296 I can only find about two birdies in the first 
100 KHz of the band; my antenna noise is dictated by what I am 
looking at with my dish.  I see the noise rise as I lower it to the 
horizon where I pick up thermal noise of the earth (rises about 6-dB 
over cold sky.  If I look at the sun my noise rises 16-dB!

73, Ed - KL7UW
-------------------
From: "Dauer, Edward" <edauer at law.du.edu>

No argument with the theory here, though my personal experience is not
consistent with the earlier observation that a preamp won?t do much if the
noise level drops perceptibly when the antenna is disconnected.  On my K3
on 10M the noise level drops when no antenna is connected (by 10dB?  Maybe
not); AND the PR6-10 preamp improves the readability of very weak signals
when it is in the circuit compared to when it is not.  If that?s just a
placebo effect, I?ll take it.

I received a couple of off-list comments that might be of interest to
those who are following this thread.  One was that narrowing the BW also
improves the S/N.  That is quite true - indeed, BW narrowing in the K3 is
generally better, in my circumstances, than the K3?s on-board NB and NR
facilities are (though I confess I have much to learn about those.)
==snip

73, Ed - KL7UW
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