[Moon-Net] QRM on 1296.000

TexasRF at aol.com TexasRF at aol.com
Thu Nov 10 23:21:51 CET 2016


Demi transverters use a fairly high gain and bandwith MMIC in the i.f.  
output circuit. There can be strong local oscillator tones present of various  
frequencies related to the method used to  generate the l.o. signal. This is 
very noticeable when an A32 board is used for  l.o. generation. There is no 
filtering of the 144 MHz i.f. used so these  spurious high level tones will 
be routed directly to the 144 MHz receiver  connected to the transverter 
i.f. output. If the problem is present, a power  meter such as HP432, 436. etc 
will show the presence of high level tones.
 
This scenario was observed recently in a 6cm transverter and I am only  
suggesting that it could possibly be present in other DEMI transverters as  
well.
 
It seems feasible that a simple 144 MHz bandpass filter could be added  
between the transverter and receiver to attenuate the spurious signals but I  
have not tried that. The problem in my 6cm transverter was eliminated  by 
adjusting levels between the A32 board and the transverter.
 
If you are not using a DEMI transverter then none of this is relevant of  
course.
 
73,
Gerald K5GW
 
 
 
 
 In a message dated 11/10/2016 7:13:03 A.M. Central Standard  Time, 
leif at sm5bsz.com writes:
 
Hello Courtney,

Sounds like  oscillations in the LNA or transverter. I suggest
you slowly turn down the  supply voltage to one of them, then to 
the other while monitoring the  signal level of a real signal, 
a beacon perhaps.

You should see the  gain go down and maybe rhe frequency 
change a little when the transverter  gets a lower supply voltage.

If the interference disappears you have  located the problem.

"As I move the dish around, the signal level and  other properties 
changes" I think the key is "other properties." ONLY the  level
should change if it is an external interference. If the frequency  
changes as you move the dish, particularly if you elevate it
the  problem is likely the LNA and not the transverter front end.
Oscillations  might be on a sub-harmonic to 1296 since the frequency
moves as expected  when you shift the IF from 144 to  146.


73

Leif




Courtney Duncan  <courtney.duncan.n5bf at gmail.com> wrote:

> I made a first real  attempt today to find the source of serious QRM on 
1296.000 at my  location.  Fortunately, it is confined to the band 1295.990 - 
1296.010 so  I can do most normal EME operations except for looking for 
ON0EME.  I’ve  made CW QSOs on .020.
> 
> I have ruled out the IF rig by using  two different IF rigs, FT-817 and 
DSP-10.  Same thing on each.  Same  place.
> 
> I have ruled out a specific IF frequency by switching  the transverter 
from 144 MHz to 146 MHz output.
> 
> The problem  is actually at 1296.000 and sounds like some electronic 
equipment  oscillator.  But, very stable, always in the range quoted above.
>  
> First I turned off power to the house at the mains outside.  The  IF rig 
continued to operate on its own internal battery, the transverter and  LNA 
on an external battery in the shack.  Problem still there.
>  
> Disconnected everything on the shack battery except the transverter  and 
LNA.  Problem still there.
> 
> Turned off the computer  and everything powered from it (dongles, mouse, 
anything on a USB, speakers,  monitor etc.) and disconnected from charging 
power (which was off anyway since  the mains to the house were off).  Problem 
still there.
> 
>  Went around the house removing the batteries from everything — clocks,  
wireless thermometers, cordless phones, smoke detectors.  Problem still  
there.  (It was time to change batteries in some of these  anyway.)
> 
> Turned all cellphones completely off.
>  
> Then spent half an hour putting everything back together and setting  all 
clocks that don’t set themselves and turning things back on.
>  
> As I move the dish around, the signal level and other properties  
changes, meaning it has some directivity to it, but there is no direction I  can 
point, including straight up, that doesn’t see it, at about the same  levels - 
> 
> I can send anyone who is interested a screen shot  of the waterfall of 
this noise.  It doesn’t decode, of course, but is  quite audible, on the level 
of a local beacon that decodes at about -3, on the  order of 20 KHz wide.
> 
> Listening to this QRM, sometimes it  sounds like it has modulation like a 
paging transmitter.  Sometimes it  just squeals like a wall wart.
> 
> Seems likely that this is  something in a neighbor’s house, hospital, or 
commercial transmission  site.  Neighbors are tens of meters away, the 
nearest hospital about a  kilometer.  I don’t know of any transmitters except on 
the hospital  itself.  I do receive other QRM when I point directly at the  
hospital.
> 
> Has anyone experienced any QRM that was this stable  and been able to 
determined what it was?
> 
> Thanks and 73,  Courtney, N5BF
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