[VHFcontesting] DEM 432-38 Receive Problems Part 2

[Shawn Tayler]

This is not surprising Jim,

Steve put out a mod for all DEM transverters that raised the 1dB
compression point of the GaAsFET by 10-15dB.  Its in the tech note library
on the DEM Website.  The way to tell if you have the mod is to look
at the voltage regulator that feeds the GaAsFET.  If its a 7805 5V
regulator then it is the older design and more prone to the problem you
are seeing.

Shawn
N7LQ


On Mon, 16 Jan 2006 19:49:14 EST Jimk8mr at aol.com exclaimed:

>  
> 
> 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
> 
> 
> 
> 
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