Hi Ford,
> 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.
Anything switching or arcing can cause a problem like that. Most
likely it is something arcing, or switching very hard at a very low
frequency (like a light dimmer, battery charger, or gas vapor lighting
of some kind).
It is almost 100% certain not a BC station mixing problem.
> 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.
What is the slope of sensitivity of the antenna you are using?
Remember the antenna's sensitivity will change the noise level.
> 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.
It's really rare for a transmitter to make a noise like that at any
distance.
> 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?
No.
> 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.
I would look at the power lines, and what is connected to them.
The amount of noise-generating junk today is unbelievable. I had an
Igloo cooler that generated a noise like you describe in a switching
12 volt supply. My Panasonic FAX machine is readable at 1000
feet from my mobile!
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/topband
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|