I had a similar experience here with trying to DF a 120 hz noise source
on my TB Beverage antennas selector falsely believe the antennas would
give some indications where the noise was in relation to the shack. The
location was right at the Palomar Pre-amp which allows my Beverages to
perform better on higher frequencies. However it was not generated by
the pre-amp at all but rather by a switching supply use to power it. (My
other model of the same unit has it own internal DC supply and it was
clean.) By changing to a small linear DC supply solved the problem.
Later by using a small hand help AM radio I was able to zero right down
to the unit itself which was ironically clean except under load.
Some the tests we worry about about first on noise soures should be the
ones we carry out first like powering the RX from a battery and pulling
the power to *all* items internal in your house. IMHO ersatz switching
DC power supplies are probably the worst low frequency offenders and
ironically the easiest to cure. Yes I know....I used to carry a sledge
hammer around in the trunk of the car while listening to 1690 AM until
one day in the Minnesota winter night a concerned neighbor called the
police to report some "lunatic" was beating on power poles on his street.
73,
Herb Schoenbohm, KV4FZ
On 12/26/2013 1:22 PM, Jim Brown wrote:
On 12/26/2013 6:22 AM, Shoppa, Tim wrote:
If anyone has any insight for how 120Hz impulse noise can just
disappear below a certain frequency, that might help me find it.
Several logical reasons that can happen. 1) The antenna radiating it
is more effective at higher frequencies. 2) The directivity of your RX
antennas with respect to the source is such that they reject the noise
on those lower bands. 3) It's not broadband, because it's
electronically generated. This is true of virtually all switching
power supplies, and many electronic sources. The noise from my SteppIR
controller and its switching PSU wipe out some bands and not others.
The PSU is worst on 12M, bad on the bands around it, but not so bad
lower in frequency. Now that I've replaced it with a linear supply, I
hear the controller on the160M that's 25 ft away, so I have to turn it
off when I want to work 160M. 4) The source you're hearing on 40M
might not have been active when you were on 160 last night.
73, Jim K9YC
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