Re: [SI-LIST] : Antenna Problem on the Board

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Tue May 29 2001 - 13:58:21 PDT


Chris,

This E(f) relation being used looks like the magnetic
dipole equation. My question is in terms of radiation,
why is the magnetic dipole equation being used and
any more valid than the electric dipole equation?

Now, I understand the loop circuitry of a trace over a
ground plane, but traces can also act very much like
dipoles. Especially folded dipoles in some cases.

Where you have

H(f) = k(H) * [ I * f^2 * A / d ] (Amps/meter)

where

k(H) = 1.316*10^-14
I = amps
f = frequency in Hertz
A = area in meters squared
d = distance from antenna in meters.

You also have for the electric field

E(f) = k(E) * [ I*f*l/d ] (Volts/meter)

k(H) = 6.3*10^-6
I = amps
f = frequency in Hertz
A = area in sq. meters
d = distance from antenna in meters.

Two points:

One - I've seen this loop equation used quite a lot and
I have to disagree with it. There has to be some sine
or cosine factor in there as well as to compensate for
position from the loop.

Second, even if the magnetic relation is accepted, the
constant factors alone show that there are orders of
magnitude differences in amplitude between the two.

My bottom line here is that where people seem to
assume a loop, I see quite clearly a folded dipole.

- Doug McKean

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