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Old December 16th 05, 11:46 PM posted to rec.radio.amateur.antenna
Richard Clark
 
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Default Antenna reception theory

On Fri, 16 Dec 2005 13:56:12 -0800, Jim Kelley
wrote:

one might be inclined to argue that the voltage induced on a wire should be the same at every
point along a finite length


One might, if it were an exceptionally short piece of wire. Otherwise
it must exhibit some greater reactance than nearly 0 and support a
difference of potential in proportion to the current through it. There
is also the radiation (non-0) resistance to consider (same observation
of a potential difference there too).

Presumably, this all hinges on what is meant, in practical terms, for
"finite length."

The "rise" in voltage as the field sweeps
past would be with respect to time,

With respect to what time? The time for the wave to sweep past? What
frequency? The full wave, or its peak? or its average (0)?

rather than with respect to position.


Position of what?

If we could generate a step-wave, and that wave impinges upon the wire
broadside, and we were to arbitrarily assign the frequency of 3 MHz
(so that the wire length of 1 Meter is one percent long - a short
antenna by any definition); then the first degree would immediately
demand a response in 1nS. 1nS is a very appreciable dT for 1 Meter's
worth of inductance.

Still and all, 1nS is hardly the slice of the wave an antenna wire
sees, unless it is 1nS thick - or 33cm. This comes close to a tower's
size, but receivers work with far thinner stuff and the dV/dT
certainly pushes the voltage drop across 1 meter of that wire up
higher.

73's
Richard Clark, KB7QHC
 
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