November 11th 14, 02:02 PM
posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: May 2011
Posts: 550
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A short 160M antenna
On 11/11/2014 7:00 AM, Wimpie wrote:
El 09-11-14 19:40, Lostgallifreyan escribió:
wrote in :
What Maxwell's Equations say is a moving magnet produces a moving
magnetic
field.
A moving magnetic field is not an electromagnetic field.
Never mind Maxwell, why is no-one asking "why is an electromagnet not
used as
an antenna when driven by AC?" That should be a basic reality check
because
I've neve heard of such a thing working, and if it did it would be widely
known.
You may know that an AC current through a loop does generate near
fields, and far (EM radiation) fields. This follows from Maxwell's
equations. The solution for radiated power from a loop carrying AC
current is:
Prad = 3.85*10^-30*(A*I)^2*f^4 [W]
The product of A (Loop surface area) and I (rms current through loop) is
the magnetic moment (m). m is used in formulas frequently.
The formula contains f^4, when you halve the frequency, the radiation
drops to 6.25%. This is the reason that at low frequency the EM
radiation is negligible in most cases. This behavior is also the reason
that you need large antennas to produce low frequency EM radiation
fields with useful efficiency.
If you take two vertically oriented loops that are perpendicular to
eachoter, and feed them 90 degrees out of phase, you create a
rotating/spinning magnetic field around the Z-axis. Because of the
orientation of the two coils, they don't interact with eachother. So by
using two oscillating magnetic dipoles, you can generate a rotating
magnetic dipole.
The summation of the two fields (our rotating field), generate a
vertically polarized EM radiation field in the XY-plane. This is like in
a normal loop antenna, but now the pattern is omnidirectional (because
of two loops instead of one). In positive and negative Z direction there
is a pure circular field.
How you generate the rotating magnetic field doesn't matter. Instead of
using two out-of-phase fed loops, I can generate exactly the same
rotating field by using a rotating permanent magnet with its N-S
direction in the horizontal plane. I can also use a rotating coil fed
with DC current. Due to mechanical limitations, you can't generate
high rev/s (that is the frequency). Because of the f^4 behavior EM
radiation is negligible in practical mechanical systems involving
permanent magnets.
In the "AC fed two coils case" energy is delivered by the sources
providing the current, in the "rotating permanent magnets case" the
energy is provided by the drive mechanism.
Excellent explanation, Wim. Many thanks for that. I learn a lot from you.
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