A week ago I posted a query about intermodulation problems with the Downeast
Microwave DEM 432-28 transverter. Today I was able to borrow a spectrum
analyzer to provide some more details of the problem signals.
The TV carrier on 525.26 (channel 23 video) is about -45 to -40 dbm for most
beam headings, with peaks at about -35 dbm. Due to this being well off the
design frequency of the 432 yagi, the lobes and (deeper) nulls are in rather
random directions.
The FM station on 93.1 ranges between -55 dbm and -45 dbm in most
directions. Again, the variations are not related to nominal beam headings,
and there
are nulls down to about -30 below the peak directions.
I was surprised that the well reputed DEM stuff would have these problems,
but perhaps this out of band intermod was not a design consideration. There
appears to be no particular bandpass filtering on the input, with the GaAsFet
preamp and one MMIC before the internal helical resonators.
The bad frequency pairs, giving an image center frequency on 432.15, are TV
Channel 23 (525.25MHz) and FM 93.1 (my problem); Channel 24 (531.25) and FM
99.1; and Channel 25 (537.25) and FM 105.1. With the 200 KHz spacing for FM
stations, the next lower FM channel could cause problems at the very low end
of the 432 band, but probably would not be noticed up in the normally used
areas of the band.
For me it could have been worse: the nearby FM station on 95.5 generates an
image on 429.76 that is about 25 db stronger than the one on 432.16.
I'd also be concerned with digital TV stations on channels 22 through 25
(518-542 MHz) mixing with FM stations. I suspect you'd probably never know
what
was hitting you, just that your noise floor has gone up in a lot of random
directions.
But in a way all is now well for me. I bought a DCI 435-10 bandpass filter,
about $140 from AES, and the band is now clean. With that I hope I will be
able to work lots of you this weekend from here in EN91!
73 - Jim K8MR
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a message dated 1/8/2006 1:24:53 P.M. Eastern Standard Time,
Jimk8mr@aol.com writes:
For most, if not all, of the many years I've had my DEM 432-28 transverter
I've been bothered by a swath of crud covering much of the useful 432 band.
Being about 2 miles from a tower loaded with pagers, two FM broadcast
stations, and who knows what else, I kind of figured I just had to live
with it.
Recently, however, I've taken another look and determined that the
offending
stuff was the mixing product of a TV channel 23 visual carrier (525.26MHz)
and an FM station at 93.1 MHz creating the FM signal at 432.160. The
amazing
thing was that the TV station is 30 miles away and the FM station 10 miles
away.
Yesterday I was able to check things with a FT-847, which was clean, and
also to add a DCI 435-10 bandpass filter in front of the DEM transverter,
which
also cleaned up things.
This seems like very poor front end performance in the DEM transverter.
Have other people experienced this problem? Any suggestion as to what
might be
going wrong with mine if this is not a normal condition? The transverter
otherwise seems to be working... the several birdies I could hear on both
rigs
seemed even stronger on the DEM than on the FT-847 (and went away on both
with
the antenna disconnected).
73 - Jim K8MR
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