James Moritz schrieb:
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
Hi Jim,
above ur observation. my is as follows:
I plotted the LF band with RegTP (the german FCC) equipment.
result (inter alia):
DDH48 has two unintended emissions: 137,4kHz es 157,2kHz;
i.e. 147,3kHz +/- 9,9kHz. the 9,9kHz is a suspicious frequency
(IF mirror) but the receiver used has 75MHz and 455kHz IFs.
the emissions are still ZOA if the DCFs are closedown.
DDH48 location is abt 30km off mine.
the attenuations of the unintended emissions are with`in the rules
of radioregulations - unfortunately!
regards
Uwe/dj8wx
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