[Moon-Net] QRM on 1296.000

Leif Asbrink leif at sm5bsz.com
Thu Nov 10 15:05:10 CET 2016


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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