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Old December 16th 05, 10:56 PM posted to rec.radio.amateur.antenna
Jim Kelley
 
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Default Antenna reception theory



Roy Lewallen wrote:

Richard Harrison wrote:

. . .
I agree the voltage induced in 1/2-meter of wire properly placed within
a 1V/m uniform field is 0.5 volt, not 1 volt. The induced voltage in a
wire within a uniform field sweeping the wire rises uniformly along the
wire. It can be assumed to be the summation of tiny increments of
voltage all along the wire. The voltages of the too-short dipole halves
add just as two cells in some flashlights add. Their vectors are head to
tail. But, current will be limited by radiation and loss resistances of
the wires. It will also be limited by reactance in the wires.
Open-circuit, 0.5 V + 0.5 V = 1V.



There are two incorrect statements here.

First, the voltage induced in the wire doesn't rise uniformly along the
wire. It's sinusoidal, even for a very short wire.


Interesting. Assuming a plane wave sweeping broadside, with the field
being the same at every point along the wire, one might be inclined to
argue that the voltage induced on a wire should be the same at every
point along a finite length. The "rise" in voltage as the field sweeps
past would be with respect to time, rather than with respect to
position. Sort of explains why a radio receiver works just fine with
only one wire attached when you think about it. ;-)

73, ac6xg

 
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