Greetings!
I have been plagued this winter with excessive wide band noise at my
location. In my quest to find its source, I have made several interesting
observations I want to share and get some help interpreting...
Originally, my IC746 was most suspect. Day and night, I have a noise floor
that measures S7 to S9. During the summer, skywave QRN peaks dominated the
noise. Now, virtually no "spikes" can be observed. I used an HP394
variable attenuator ahead of the receiver to check out overload issues.
There are no observable overload thresholds. Substituting an old Kenwood
R-599 at the antenna port and I get the same result (although it measures
S5). My conclusion? It's not the receiver in the IC746.
So what is it? I placed my HP8553B spectrum analyser on the antenna. Wow!
The receiver is just picking up noise that really is at -70dbm (70
microvolts into 50 ohms). There is no "point" source. It appears
everywhere. I've tried the MFJ version of the ANC4 noise canceller and was
completely frustrated because that type of device will work great on single
point noise sources.
When I set the analyser to look at 500kHz to 4mHz or so, the noise inside
the AM band is unbelievable! I have an AM station at 1600 about 20 miles
away that is at 0dbm! That's 200 millivolts to an S meter--60 over on the
IC746! Another station at 1500 is at -20 dbm and is over 60 miles away!
Inside the AM band, the apparent noise floor (the best I can tell) is up
around -30dbm or so. As you look up the band, the noise drops significantly
by 1750. At 1830, the -70dbm is very real at the antenna. I can hardly
fault a receiver to being sensitive to signals! The noise floor tapers off
to an S3 level by the time you get to the bottom of 80 meters.
I am struggling to interpret the information. Are these stations spewing
wideband garbage? Looking at the second harmonic of the 60 over 9, 1600kHz
AM station, the spur at 3200 is about 20 over 9! That's ugly but hardly a
reason for "wideband" noise.
I speculate that the AM transmitters may be picking up noise fed back into
their final tank circuits and "retransmitting" the noise back out. Is this
reasonable?
I also speculate that the nearby power lines can be absorbing wideband RF
and re-radiating it to my antenna. It is not insulator noise. I am
familiar with that type of noise and can locate a bad insulator (single
point source) quickly.
Perhaps one of the techno weanies on Topband can help me identify
experiments I can execute to narrow down the problem.
Ford-(soon to be N0FP--perhaps the FCC will let me know today!)
ford@cmgate.com
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