Elecraft] RF gain sweet spot

[Edward R Cole]

Ed (K7WIA):

OK, so it looks like we have a very similar set ups.  Your numbers
let me think about this a bit more.

Your ambient noise is maybe 1-2 s-units higher than mine:
P50VDG:  you=S7, mine=S4.5-5
PRE (K3): you=S3, mine=S3  [interesting - I'll come back to this]
K3 with all off:  you=S2, mine=0.5-1

All I can think of is that you are reducing noise input when you
lower your RFG to match the internal noise produced by your radio
plus preamps (if on).  And that explains the drop in received noise
to a point where the gain drop faster.

Receiving sensitivity on HF/MW/LF is mainly determined by receiver
gain, bandwidth, and detector threshold.  If you have enough gain you
maximize sensitivity.  Noise Figure (and its counterpart noise
temperature) are not significant compared to the huge level of sky
noise present <30-MHz.  Sky noise temperature is something like
15-20,000 Kelvin (K)

OK, at 1296-MHz sky temperature is in the range of 5-10K, so
obtaining a very low receiver noise temp (noise figure) is extremely
important for good receiver sensitivity.  Current Gas-Fet technology
provides us with 18-30K low-noise preamps.

At 50-MHz we are half like HF and half like VHF/UHF as sky noise runs
about 2000K.  The ARR P50VDG and the K3 PR6 are about 0.5 dB noise
figure (or 35.4K).  If we add Tr=35.4K to Tsky=2000K and maybe add
Tant=100K the result is 2135K.  As you can see lowering receive noise
temp (noise figure) will not make much improvement in sensitivity due
to the sky noise background.

In urban environments you add local noise, Tn to Tsky and that might
be several additional hundred's K of noise temperature.

So at 6m a receiver's signal sensitivity is mainly a function of
having sufficient gain to drive the detector in the radio
properly.  It looks like this is reached somewhere between the gain
of the PRE and either P50VDG and PR6.  I will hazard a guess this is
about 18-dB gain.  Both preamps produce about 24 dB gain so that is
why the s-meter reads higher than with the PRE.  The PRE is set in
gain just about at the level for best reception thus we see little
change when RFG is lowered.

In K7WIA's case he lowers the gain to reach this point.  In my case I
have less external noise so lowering gain does not appear to have any
effect on received noise.

What would be a better test is with a weak signal looking at detected
SNR.  Also being sure both radio parameters like bandwidth and mode
are the same.  One of the reasons to narrow bandwidth at VHF is to
reduce the amount of noise power being received.  You reach optimum
at about the point the bandwidth matches the width of the signal
being received.  For CW this about 5-15 Hz (depending on speed)
filters ring when that narrow so the human mind is the final CW
filter with 50-Hz about the practical minimum bandwidth that is usable.

Digital modes like WSJT (JT-65) are 4.7 Hz wide so DSP filters of
that width are used to dig out the signal way below the level of a
signal that can be heard with human ears (about 10-dB weaker than CW).

Anyway I digressed from the main topic.

I would be interested if other K3 owners looked at their 6m noise
indication when RFG is backed off slightly.

73, Ed - KLUW
------------------------------

Message: 12
Date: Thu, 21 Jul 2011 13:00:27 -0700 (PDT)
From: K7WIA <[hidden email]>
Subject: Re: [Elecraft] RF gain sweet spot
To: [hidden email]
Message-ID: <[hidden email]>
Content-Type: text/plain; charset=us-ascii

Yes I am using the K3
I was using the preamp that is in the K3,    I also have the ARR P50VD
preamp on the back of the K3 but using it just raises the background noise
to S7.
Remember I live in the city.  I can turn off the preamp that is in the K3,
disconnect the antenna listen to the hiss of the K3, and then hook the
antenna back and hear the noise increase.. i am going to try it with out
using any preamp at all...I can still detect the weak beacon that are in my
area.  and still hear a increase in noise when the antenna is connected. but
just to reduce the background noise has helped my 73 yo ears make the weak
ones stand out better.
even without the preamp on I still get the 2nd bar on the S meter to
flicker,  and  i can reduce the Rf gain just slightly ( not much) to dip the
S meter.
just the joys of living in a big city,  powerline, and wireless headphones
on 6 meters

Ed   K7WIA




73, Ed - KL7UW, WD2XSH/45
======================================
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 50-1.1kw?, 144-1.4kw, 432-100w, 1296-60w, 3400-?
DUBUS Magazine USA Rep [hidden email]
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