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Old April 20th 18, 09:10 PM posted to rec.radio.amateur.antenna
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Default What actually happens?

What I seek is a graphical / pictorial representation of
the growth and collapse of fields around and antenna as it
radiates.

A corollary being the question that if the local inductive
and capacitive fields collapse and regrow on every half cycle,
why does not the radiative field also collapse back? What
causes them to radiate outwards?

Despite having (at some time, 45+ years ago) studied up
to the differential versions of Maxwell's Equations, this
(hopefully) elementary explanation has eluded me.

A picture paints a thousand words, and like Richard
Feynman before me, I can soar with the eagles with the
highest science but I need a seat-of-the-pants
understanding upon which to anchor my knowledge.

eg, I once had a problem coming to terms with
the Vector Magnetic Potential A, but understood
it when standing a bit close to the edge of the
platform as a railway train went past creating
little eddies of wind!



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Old April 23rd 18, 04:07 PM posted to rec.radio.amateur.antenna
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Posts: 18
Default What actually happens?

On 20/04/2018 21:10, Gareth's Downstairs Computer wrote:
What I seek is a graphical / pictorial representation of
the growth and collapse of fields around and antenna as it
radiates.

A corollary being the question that if the local inductive
and capacitive fields collapse and regrow on every half cycle,
why does not the radiative field also collapse back? What
causes them to radiate outwards?

Despite having (at some time, 45+ years ago) studied up
to the differential versions of Maxwell's Equations, this
(hopefully) elementary explanation has eluded me.

A picture paints a thousand words, and like Richard
Feynman before me, I can soar with the eagles with the
highest science but I need a seat-of-the-pants
understanding upon which to anchor my knowledge.

eg, I once had a problem coming to terms with
the Vector Magnetic Potential A, but understood
it when standing a bit close to the edge of the
platform as a railway train went past creating
little eddies of wind!


It may be interesting to apply a semi-graphical method; I was introduced
to one example way back in the mechanical content of my electrical
engineering studies -


http://rspa.royalsocietypublishing.o...0/329.full.pdf

Now I'm sure an erstwhile polymath could work something like this for
e-m waves.

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Old April 24th 18, 11:21 AM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Sep 2013
Posts: 53
Default What actually happens?

On 04/20/2018 04:10 PM, Gareth's Downstairs Computer wrote:
What I seek is a graphical / pictorial representation of
the growth and collapse of fields around and antenna as it
radiates.

A corollary being the question that if the local inductive
and capacitive fields collapse and regrow on every half cycle,
why does not the radiative field also collapse back? What
causes them to radiate outwards?

Despite having (at some time, 45+ years ago) studied up
to the differential versions of Maxwell's Equations, this
(hopefully) elementary explanation has eluded me.

A picture paints a thousand words, and like Richard
Feynman before me, I can soar with the eagles with the
highest science but I need a seat-of-the-pants
understanding upon which to anchor my knowledge.

eg, I once had a problem coming to terms with
the Vector Magnetic Potential A, but understood
it when standing a bit close to the edge of the
platform as a railway train went past creating
little eddies of wind!


Hello, and those are good questions. I remember years ago viewing an
animated video illustrating how propagating electromagnetic waves are
created around a transmitting antenna. Perhaps like your eddies
analogy. As to why (photons if you will) propagate at the speed of
light in vacuo, that property is contained in the Maxwell equations and
underscored by special relativity. In the immediate vicinity of the
antenna the E and H fields contain both energy storage (think capacitor
and inductor) and radiated components. (There is really only one local E
and H field but the terms associated with the storage components drop
off quickly with increasing distance from the antenna.)

So if we measure the feedpoint impedance of an antenna at some frequency
we can model that antenna (at that frequency) as a capacitor or inductor
in series with a resistor (the some of the losses in the antenna
structure and local environment like the earth plus a "radiation"
resistance) Sincerely, and 73s from N4GGO,

--
J. B. Wood e-mail:


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