[VHFcontesting] DEM 432-38 Receive Problems Part 2

[Jimk8mr@aol.com]

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 at 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