Message-ID: <004301cb0544$56965980$0201a8c0@Clemens04>
From: "Clemens  Paul" <cpaul@gmx.net>
To: <rsgb_lf_group@blacksheep.org>
References: <004801cb04d6$e9740b80$0401a8c0@xphd97xgq27nyf> <CD190EC05BA84C1685A9A2F4A56DD284@JimPC>
Date: Sun, 6 Jun 2010 08:48:55 +0200
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Subject: LF: Re: Re: 9 kcs
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Jim,

>Since the RX or preamp noise level can be
> strongly dependent on the source impedance,

I can not confirm this with my RX,a Perseus SDR.
When I switch on 9kHz or any other qrg between 50 Ohm, open and short,
I always get the same reading within a tenth of dB,e.g -118,4dBm RMS=
 at a BW of
732Hz (arbitrarily chosen)
The reading also stays constant between the three sourcees
when I do the same test with preamp ot attenuator on.
Maybe your observation is true for RX/preamp inputs with
considerable reactance in the input impedance.

73
Clemens
DL4RAJ

----- Original Message -----=20
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, June 06, 2010 12:29 AM
Subject: LF: Re: 9 kcs


> Dear Mal, LF Group,
>
> G3KEV wrote:
> ...>Information is also scarce in the UK especially about receiving
> capabilities, what is being heard especially alpha signals, strength=
,
> audibility and general band noise.
>
> If you read previous posts to this reflector on the subject, you wil=
l know
> that the receive side of things is fairly straightforward. What is=
 being
> heard on 9kHz is QRN and QRM, except for very short range "induction=
" tests,
> where the signals can be audible. If you cannot receive the "Alpha"=
 beacons
> in the UK, it probably means they are not transmitting, or your RX=
 or
> antenna have stopped working. But, although they may be interesting=
 in
> themselves, they are largely irrelevant as a measure of 9kHz receive
> performance, since they are on a different frequency, are vastly mor=
e
> powerful than amateur signals, and for the DX tests that have been=
 done,
> have a much wider bandwidth / shorter duration compared to the amate=
ur
> signals.
>
> A better test of receiver performance at 9kHz is to compare the nois=
e level
> with and without the antenna. Since the RX or preamp noise level can=
 be
> strongly dependent on the source impedance, it is best to replace th=
e actual
> antenna with a "dummy antenna" with similar impedance when making th=
is
> comparison. For example, for my loop antenna, I have a change-over=
 switch
> that connects the preamp input to a small choke with about the same
> inductance and resistance as the actual antenna. This enables quick
> comparisons to be made between noise levels at different times and
> locations, and is also a good check on the local QRM level. Obviousl=
y, you
> want the antenna noise level to be at least several dB greater than=
 that
> with the dummy antenna, which is usually easily achieved.
>
> The general band signal and noise levels are being regularly monitor=
ed at
> several locations - you can see the info at http://abelian.org/. But=
 you
> already know that...
>
> Cheers, Jim Moritz
> 73 de M0BMU
>


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