Hi Mike I would agree with that. When I started trying to record stations in
the pre-waterfall period (DCF, SXV, CFH etc near 136 I had a thick band on
the plot around 8 to 10dB wide. I experimented taking 6 digitisations at 10
second intervals and thowing away all but the lowest ones. This generated a
clean single line plot showing small variations in level, completely
different from averaging which gave the 'mean' of the static. Wolf later
introduced a function into SpecLab that did it with just two samples only
recording the lowest one. This idea might be useful for investigating
aerial noise. It obviously worked best on a wind bandwidth where the static
did not ring receiver filters, stretching the pulse, and should work well on
SDRs using the paramerters of SL to select the measurement frequency. You
wont find this useful on signals :-(( but it might help to understand better
the local receiving environment.
Note to me...must try it with an SDR sometime :-))
Alan
G3NYK
----- Original Message -----
From: "Mike Dennison" <[email protected]>
To: <[email protected]>
Sent: Saturday, March 17, 2018 2:19 PM
Subject: Re: LF: Pre amp *after* BPF??
I would go along with Rik in principle, but beware of high static
levels giving a misleading high figure of "antenna noise". This test
only works on low static days. I would, in any case, suggest a
"threshold" of several dB to ensure you are really looking at the
band noise rather than static peaks. Perhaps ensure the "antenna
noise" is at least 3dB more than the receiver noise. I have seen
posts in the past suggesting that this figure should be as much as
10dB.
Mike, G3XDV
===========
Keep in mind that you only need a pre amp if the "antenna noise" is
below the receiver noise.
Most receivers have reduced sensitivity at LF / MF, but the
noiselevels at these frequencies is high compared to HF.
Easy to check: Listen (with connected antenna) on a quiet frequency
(only noise, no signal). Next disconnect the antenna. If the noise
level decreases you do not need a pre amp (in fact a pre amp will only
deteriorate the IMD behavior and thus create more ghost signals).
73, Rik ON7YD - OR7T
________________________________
Van: [email protected]
<[email protected]> namens Andy Talbot
<[email protected]> Verzonden: vrijdag 16 maart 2018 12:54 Aan:
LineOne CC: Chris Wilson Onderwerp: Re: LF: Pre amp *after* BPF??
After, definitely
Before is applicable to VHF and up where equipment noise figure
dominates sensitivity and a filter in the antenna side wold add loss
and degrade overall NF
At HF and certainly LF, atmos noise dominates to noise figure is
unimportant. That way you get the benefit of rejecting strong OOB
signals that may stress even the preamplifier
'jnt
On 16 March 2018 at 11:36, Chris Wilson
<[email protected]<mailto:[email protected]>> wrote:
Hello LF'ers,
Been Googling and see a mixed response to should a receive pre-amp go
before or after a BPF. I have mine before the pre-amp, does the panel
concur with this being correct? Thanks!
Aerial, isolation transformer for ground loops, BPF, pre-amp, Red
Pitaya as the LF receiver.
--
Best regards,
Chris
mailto:[email protected]<mailto:[email protected]>
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