# [RFI] RFI Question

K9MA k9ma at sdellington.us
Fri Feb 12 15:43:28 EST 2021

```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 at 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 at sdellington.us

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