[aprssig] Persistent EMI/RFI Problem Solved

Stephen H. Smith wa8lmf2 at aol.com
Sun Mar 13 15:53:53 CDT 2016


[I am recounting this story in the hope that it may be useful to others 
fighting degraded receive problems.]

For months I have been struggling with severely degraded receive for my ISS 
Satgate receiver.  On most passes I would copy nothing, even through I could 
hear packet bursts in the radio's speaker.

Further, my mobile D700 has been suffering severely degraded APRS receive. I 
would copy virtually nothing unless I was within 4-5 miles of a station or 
digipeater.

I tested both radios' sensitivity repeatedly with my IFR-1500 service monitor 
and both showed excellent sensitivity (better than .12 uV for 12 dB SINAD), 
-BUT- they just wouldn't receive in real life.


Just before departing on the road trip, 14 days ago, I was re-routing and 
tidying the cable tangles under the seats and in the trunk of my Jetta TDI.  I 
had the Kenwood D700 on monitoring a local QSO on .52 simplex.   As always, 
several of the stations were overlaid with the chronic hissy noise and popcorn 
racket I have been suffering from for months.  [I had been attributing this to 
a leak from the Comcast cable trunk running down the street.]

To tidy up the mass of red/black zipcord power leads plugged into an eight-hole 
PowerPole distribution strip under the passenger seat, I unplugged several 
cables.  This included the 12VDC input side of a 12VDC->19VDC converter that 
powers my Acer netbook. Suddenly the popcorn noise vanished and the QSO became 
crystal-clear.  At the same time, the left side of the D700 suddenly started 
copying digipeaters 40-50 miles away.

At the same time, the ISS satgate in the house started copying multiple 
stations on every pass.   (The satgate is connected to an Icom AH-7000 discone 
on a tripod on the car port roof.  The antenna is less than 12 feet above the 
parked car.)

The key to missing the source is that this kind of noise (increased noise floor 
of hiss and popcorn) is completely inaudible in an unsquelched FM receiver 
UNLESS A WEAK SIGNAL IS PRESENT.  (Hiss plus more hiss sounds exactly like 
unsquelched hiss alone!)

Listening to the power converter with my TS-2000 in *AM* mode, I could easily 
hear both an elevated continuous broadband white noise, and a constant snapping 
popping sound.  The white noise is to be expected from many switching power 
converters and supplies, but how the supply creates the random cracking popping 
noise still has me baffled.

I have three UIview receiver/igate/webservers running 24/7 at home. One is on 
30HF. One is the ISS satgate with it's antenna over the carport.   The third 
one, (the main 2M 144.39 APRS receive) is on an antenna at the other end of the 
house about 45' from the car port. It was far enough away that it didn't suffer 
the noise emanating from the switcher in the parked car.

I would have never discovered this problem if it hadn't been the coincidence of 
a weak QSO on 2M simplex while I was re-routing the wiring.......

_____________________________________________________________

Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
Home Page:          http://wa8lmf.net

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