Message-ID: <A4A0BDF1A5954E3E9A169C0C7FB6EF0E@gnat>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <772364530.20180316113613@gmail.com>, <CAA8k23SNvwo3aGTizLE059FLZf79p=zam3u2+MaqVAGe7TD7eQ@mail.gmail.com>, <1521203464226.5324@kuleuven.be> <5AAD23F1.29953.B1CA31C@mike.dennison.ntlworld.com>
Date: Sat, 17 Mar 2018 16:06:44 -0000
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Subject: Re: LF: Pre amp *after* BPF??
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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" <mike.dennison@ntlworld.com>
To: <rsgb_lf_group@blacksheep.org>
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: owner-rsgb_lf_group@blacksheep.org
>> <owner-rsgb_lf_group@blacksheep.org> namens Andy Talbot
>> <andy.g4jnt@gmail.com> 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
>> <dead.fets@gmail.com<mailto:dead.fets@gmail.com>> 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:dead.fets@gmail.com<mailto:dead.fets@gmail.com>
>>
>>
>>
>>
>
>
>