[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
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
In a message dated 11/10/2016 7:13:03 A.M. Central Standard Time,
leif at sm5bsz.com writes:
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.
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
> 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
> 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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