Hi Chris,
It is possible the RF currents have destroyed some mains filter components.
Does your LF antenna has a separate RF earth near the antenna feed
point? I would recommend it. If you have a separate LF earth there can
still be RF currents in the house electricity, maybe destroying a common
mode choke in a small EMI filter.
You could try to measure common mode currents on your coax to confirm
your assumption.
It is one reason why many LF OPs recommend to use an isolating
transformer for the LF RF near the feed point. A LF antenna with a
separate ground, then a big ferrite core used as a RF transformer with
two galvanic decoupled windings, transforming your loss resistance to 50
Ohm, and then a normal coax to the output of the PA. This prevents
unwanted common mode RF currents on the coax screen flowing somewhere,
through the ground of your power supply and then through the mains grid
back to earth.
73, GL, Stefan
Am 18.05.2016 18:04, schrieb Chris Wilson:
18 May 2016
Long story, but since stopping LF 136kHz TX for a while in lieu of
some HF working all the digital TV's in the house freeze or totally
blank on all bands above top band, up to 50MHz when I transmit at more
than about 5 or 10W. I have a loft mounted UHF TV aerial feeding an
amplifier cum distribution box. It has an internal mains PSU. Without
it we receive hardly any channels. It's been fine for years, and I
have never had TVI. My HF set up is unchanged from when I had no
issues. I put the TV antenna co-ax direct into my SA and on circa 531
MHZ see a broad digiTV signal that's strong. If I TX on say 20 meters
at high power into my antenna I see zero change in the TV signal. I
have also fed the output of my HF TX (TS-590 Kenwood) into the SA via
an attenuator and the output looks spotless on all bands. However, if
I connect the TV aerial amp / distribution box up and look at the
output from it the TV signal immediately drops into the noise when I
TX. My neighbours have no issues at all.
This seems to have occurred since I have been active on LF, and I had a
stage where full power would trip the main RCD in the consumer unit,
so RF was getting into the mains. Is it conceivable something has
occurred to perhaps a mains filter in the TV amp? I looked inside the
plastic case and it has a small mains transformer that feeds feed
throughs into a screening can with all the RF stuff within it. If
there is any mains input filtering it must be on the low voltage side
of the transformer. The unit has a moulded on three pin mains plug, but
with a plastic earth pin, so the unit is not grounded.
Is it possible to make a mains input filter for a piece of equipment
that would filter 136kHz but not affect normal 50Hz mains operation?
Has anyone experienced anything like this?
I have ordered a new amp / distribution box of known make, and will
just try replacing it, but would also like to know if my LF activity
could have done this?
Thanks.
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