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Re: LF: 136.547 kHz tx intermodulation

To: [email protected]
Subject: Re: LF: 136.547 kHz tx intermodulation
From: Väinö Lehtoranta <[email protected]>
Date: Tue, 22 Feb 2000 10:50:50 +0200
In-reply-to: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Hello all,

I have heard weak FSK on 136.6 kHz during morning
hours only, can not use Spegram etc on LF until
get my Rx --> PC interface QRM somehow cleaned.

Will continue daily Field strength measurements until at least the 3rd week of March. Please if
possible make 3-5 s long 'tests' with key down.
In propagation studies, only signal strength -
preferably estimated field strength is of value.

Thank you for your cooperation   de Vaino, OH2LX

PS  CFH 137.0 was strong last nite with strength
   having a broad peak 2120-2150, max -123 dBmW

At 18:38 21.2.2000 EST, you wrote:
Hello Alan, hello friends,

I also see a weak continuous trace 136.547kHz also another exactly 100Hz
higher, I did wonder whether they were the mark and space frequencies of a
100Hz shift FSK station. The lower frequency is weaker here. Anybody know
this one??

73 de Alan G3NYK   JO02PB
yeah, sure know this one. Took a while to figure things out...

On 136.75, there is nothing here. However, the other carrier you see appears to be a tx-intermodulation product generated in the output stages of two strong transmitters in Mainflingen near Frankfurt, 165 km from my qth JN59NK.
One of these is called DCF42 (I think), a bigger brother of our now-famous
DBF39 (138.83/139.17 kHz). It operates on 128.93/129.27 (86 dBuV/m here) and
carries the same fsk telegrams for remote-power-control purposes. There is a
time lag of approx. 60 ms between the master (DCF42) and the slave (DBF39).
The other tx sends 1190 baud data (DGPS?) centered on 123.69 kHz (74 dBuV/m)
and has a broad, noise-like spectrum. It is accompanied by an auxiliary
carrier on 122.50 (59 dBuV/m), presumably to aid synchronization to the data
rate. And there is a weaker harmonic of the auxiliary 2380 Hz below center
(121.31 kHz, 26 dBuV/m).
The upper IM product (2*f1-f2) is again a heap of noise from 133 to 135.4
(34 dBuV/m, ie. only 40 dB down), with the auxiliaries appearing on 135.36 (29 dBuV/m) and 136.55 (10 dBuV/m), close to the ropex qrg.
These IM-products also carry the DCF49 modulation with the shift doubled to
+680 Hz. In this part of Germany, we thus see periodic interference during
the telegrams. Also, the higher order DGPS-modulation sidebands seem to
a significant continuous noise increase up to 137 kHz, limiting the rx-sensitivity of stations in the Frankfurt area.
To get the ERP (=EIRP/3) estimates in dBm, subtract 5.2 dB from the dBuV/m
figures for 165km. Thus DCF42 should have 120 kW, DGPS is aired with 8 kW,
and the im carriers are .25 W on 135.36 and 3 mW on 136.55 ... congrats on
your good ears Alan!
73, all the best

Markus, DF6NM

V.K.Lehtoranta, OH2LX, POBox 50, FIN-05401 Jokela, Finland
------ Tel: +358-9-4173965 ---- Fax: +358-9-4173961 ------
E-mail: [email protected] - alias: [email protected] & [email protected]

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