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Message-ID: <32CE3C3B52964CEE91B90B8E66E3033A@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <1245582735.4291.6.camel@gerhard-laptop> <33D6A4990B7B45149663D84D52C052CA@JimPC> <1245595125.3782.14.camel@gerhard-laptop>
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Date: Sun, 21 Jun 2009 17:12:38 +0100
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Subject: Re: LF: Re: 137khz RX-Test
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Dear Gerhard, Nicolas, Jean-Pierre,

Thanks for the reports - the beacon is now QRT at 1530utc. I will try again 
later when it begins to get dark at around 2000utc for a few hours - I 
expect signal levels will be higher after dark, but so will be QRN, of 
course.

OE3GHB wrote:
>
> I'm using my shortwave-antenna, a 55m long endfed wire in about 12m
> height as antenna for lf. it's connected to the unsymmetrical input of
> the Selective Level Meter. I think using this antenna is the first
> problem. Another one ist that I don't have a preamp yet.

The signal level from that antenna will be much higher if it is tuned to 
resonance, which could be done with an adjustable series inductance of a few 
mH, or, for RX-only, with a fixed inductance in series with the RX input and 
a variable capacitor connected between antenna and ground to make a simple L 
matching network. A long-wave ferrite rod antenna would work for the 
inductor in this case. This will give very sharp selectivity, and an 
increase in signal level of 20 - 30dB, so a preamp should not be neccessary.

>The 2nd antenna
> is a resonant frame antenna with about 1m diameter. Unfortunately I
> haven't managed to bring it outside somehow and so I'm using it in the
> shack which gives me a lot of QRM, especially from the tube-monitor
> (n*75Hz vert. frequenzy).

Usually, such an antenna must be distant from mains electrical wiring to 
achieve a low QRM level, so usually has to be outside to work well. But, if 
you can find a good position, it will achieve 30dB or more rejection of 
unwanted signals if the null direction is adjusted carefully. This would 
help with the QRM from DCF39, if directions are favourable.

> The Level Meter only has a 22Hz and a 3.1khz filter. I just can use the
> 3.1 filter which gaves me lot of QRM from DCF39. I have tuned to 137.15
> becasue I found that this qrg is quiet (-107dbm) compared to some 100Hz
> up and down where the noise level is up to -92dbm. I think this noise is
> produced by the carrier of DCF39 somehow.
>
The DCF39 sidebands have maxima and minima across the band , due to the 
spectrum of the FSK modulation used. I guess in OE, similar sidebands from 
HGA22 could also be a problem. I can transmit on 137.15k, or another 
frequency with a low noise level if you prefer - let me know.

Cheers, Jim Moritz
73 de M0BMU