From: MarkusVester@aol.com
Message-ID: <bf.11e89de.25e32663@aol.com>
Date: Mon, 21 Feb 2000 18:38:11 EST
Subject: LF: 136.547 kHz tx intermodulation
To: rsgb_lf_group@blacksheep.org
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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 kHz 
(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 cause 
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