Yes, sometimes with interstation interference the problem is not coming from
the feedline or antennas.
I ran into something like this at my station recently. One of my 20m yagis
generated much worse interference on other bands compared to other antennas.
The CW note on the interfering bands also had a rough "growly" quality. At
first I thought maybe there was a bad connection rectifying the signal. I
checked the coax connectors and replaced the balun for that antenna. No change
at all.
I finally figured out what was happening: the yagi in question is pointed
(fixed) towards the house. When I turned the other 20m yagi on that tower
towards the house I got a similar bad interstation interference. My shack is on
the second floor. What was happening was that when the antenna was pointed at
the house, the house wiring was picking up the signal and it was getting into
my station from the AC line. Putting a large type-33 toroid on the AC feed
reduced the interference quite a bit.
I still need to verify by running on battery power if there is some device
plugged in somewhere in the house that is causing the problem, or if it was RF
directly getting into the station power supply (Astron RS35) from the AC line.
I also haven't compared a differential filter yet either. But a common-mode
choke on the AC line did get rid of most of the problem.
TorN4OGW
On Thursday, September 10, 2015 5:14 AM, Jim Brown
<k9yc@audiosystemsgroup.com> wrote:
On Tue,9/8/2015 9:24 PM, donovanf@starpower.net wrote:
> That "growly" signals you're hearing are harmonics of your transmitted
> signal generated by unintentional radiators, especially switching power
> supplies. This type of RFI -- sometimes called "flooding" -- is caused
> by solid state devices flooded by intense RF fields (your transmitted signal)
> in which they were not intended to operate.
Great info! I've often heard this stuff, and wondered about the
mechanism. It makes perfect sense. Thanks, Frank. Some questions.
1) Have you run into this with linear power supplies?
2) Do common mode chokes on wiring connected to these sources provide
suppression? (In the case of rotator controllers, this would, of course,
include the control cable to the rotator, and for all devices, the AC
line cable).
3) Are conventional AC line filters effective for the power line
component? What I'm really asking here is, "is the coupling to the noise
source common mode or differential mode or both?"
73, Jim K9YC
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