Dear Alan, LF Group,
At 01:09 02/10/2002 +0100, you wrote:
There is something there I have seen for some time
(months) which looks like a very weak 100Hz RTTY shift signal, but this
could be a more distant TV or PC, but this is down at the level of the Loran
sidebands ( which are fairly weak here).
Cheers de Alan G3NYK
[email protected]
A while ago, I had a problem with a weak FSK signal around 137.4kHz - it
turned out to be a 3rd order intermod between the "DCF" stations on 138.83
and 128.93, and DDH47 on 147.3, so:
147.3 - 138.83 + 128.93 = 137.4
The result was FSK, just about audible, with gaps when the data bursts
occurred on the DCF signals. The problem was due to the rather wide 1st IF
filter in the RA1792 (about 20kHz), in combination with a fairly wide-band
antenna - when tuned to 137.4kHz, all 3 signals reach the early IF stages
and 2nd mixer with little attenuation, resulting in the intermodulation,
allthough they are removed by the main IF filters. The solution was to add
preselection with a BW of a few kHz - if the antenna is sharply resonant,
this is enough to do the trick. If you are using an antenna that has
relatively low Q, there are several possibilities for this type of
intermodulation to appear.
Cheers, Jim Moritz
73 de M0BMU
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