[RFI] RFI Question

David Eckhardt davearea51a at gmail.com
Fri Feb 12 16:02:11 EST 2021

I'll cease with this last example.  Having once played the pipe organ, the
organ pipe is also a good example of resonant excitation.  The wind blown
into the bottom of each pipe is pretty much white, almost literally from an
audio standpoint.  Once it hits the lip of the pipe - the indentation near
the pipe base and just above the conical pipe expansion - turbulence is set
up which ultimately excites the resonant structures within the pipe.

Nature has all sorts of examples of resonant and shock excitation.  Even
your cranium filled with the neural matter of the brain resonates with a
half wavelength of nominally 70-cm in the amateur band.  Then consider the
full-wave eigenfunction of that half wave that hits the cell frequencies
around 900 MHz not to mention our HTs with their rubber ducks held very
close to our heads.  Every wonder why there is concern of cell phone energy
having a biological effect on the brain matter and inner ear?  It's all
about resonance.

Dave - WØLEV

On Fri, Feb 12, 2021 at 8:43 PM K9MA <k9ma at sdellington.us> wrote:

> 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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*Dave - WØLEV*
*Just Let Darwin Work*

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