This works in both directions, of course. I once had a neighbor with a
wall-mounted telephone with a long coiled handset cord. The usual
filters just couldn't keep the RF out of that phone, unless I replaced
the cord with a shorter one. I'm pretty sure the long cord was resonant
on 20 meters.
73,
Scott K9MA
On 2/12/2021 12:46 PM, David Eckhardt wrote:
The phenomenon is well known as resonant excitation. Hit a bell with the
clapper. The impulse of the clapper hitting the inside wall of the bell
theoretically contains all frequencies. The structure of the bell can not
support ringing at all frequencies. However, the bell can ring at
frequencies at which its structure exhibits resonance. In the specific
case of the bell, this is shock excitation. The bell is 'hit' with all
frequencies, but acts as a filter to 'ring' at only frequencies that
exhibit resonance in the bell structure. Oh,......did I already write
that.
An interesting experiment: take a lightning discharge. While there may be
resonances in the strike due to the electrical length of the plasma, it can
be considered to contain all frequencies as a first approximation. When
that energy strikes an antenna, it acts like the clapper of the bell. If
you have a method of looking at the frequency domain of the response of
your antenna to the lightning energy 'strike', you will easily see the
resonance structure of the antenna.
However, I'd much prefer the use of a NANOVNA!!!
Just a short lesson in physics.
Dave - WØLEV
On Fri, Feb 12, 2021 at 6:17 PM Gary Peterson <kzerocx@rap.midco.net> wrote:
This reminds me of a broadband noise that appeared on 75 meters. It would
come and go suddenly, happened at about the same time every day and sounded
like the rhythm of an old, Maytag washing machine. I DFed it to the
nearest neighbor’s house, almost a thousand feet to the north. It was his
new treadmill. He’s a ham and, in spite of being inactive, was
sympathetic. We ran some tests. When he used an extension cord to power
the treadmill from a different, nearby outlet, the noise was undetectable
at my QTH. Go figure. I volunteered to supply some ferrites to snuff the
RFI. He was comfortable with the extension cord and different outlet
solution...so problem solved.
Gary
KzeroCX
“I can't be the only one who read this and made a mental note to check for
extension cords around the house....kc2ieb chris”
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Scott K9MA
k9ma@sdellington.us
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