Yuri Blanarovich wrote:

I would urge any young person who reads this and wants to understand
electromagnetics to get a good book on the subject, read what the
authors say, and forget what Richard just posted. He's all wrong.
73, Tom Donaly, KA6RUH

See what I mean?
I urge any young person to read the book about jerks.

The key to understanding the total current in standing-wave antennas
is in understanding the forward current and reflected current components
and their superposition at different points along the antenna. The cosine
distribution of standing-wave current in a 1/2WL dipole is the result
of the superposition of forward-traveling current and rearward-traveling
reflected current.

An unterminated rhombic is a standing-wave antenna because the forward
current gets reflected at the open end of the wire. The forward current
causes radiation in the forward direction and the reflected current
causes radiation in the rearward direction. The radiation "loss" causes
the reflected current at the feedpoint to be a lower magnitude than
the forward current at the feedpoint. There are standing waves all
up and down an unterminated rhombic and the Vtot/Itot feedpoint impedance
depends partially upon the phase between the forward current and
reflected current and, of course, upon their magnitudes.

Properly terminating a rhombic virtually eliminates reflections and
turns the antenna into a traveling-wave antenna which radiates mostly
in the forward direction. There are virtually no standing waves on
such an antenna.

These same ideas can be applied to other standing-wave antennas, including
a 1/2WL inverted-V. In EZNEC, we can terminate the ends of such an antenna
to ground through resistors that eliminate standing waves on the antenna.
The feedpoint impedance of such an antenna is in the ballpark of 600 ohms.
Where does the low feedpoint impedance of an unterminated 1/2WL inverted-V
come from? It comes from the superposition of the forward current and
the reflected current at the feedpoint. These two components are in phase
and phasor-add to a large current. The two voltage components are 180 deg
out of phase and add to a small voltage. small-voltage/large-current is
a low feedpoint impedance. Using a minus sign for 180 degrees, the feedpoint
impedance of an inverted-V is approximately (Vf-Vr)/(If+Ir).

We can understand a standing-wave antenna by doing an analysis of a lossy
piece of transmission line. If the losses in the transmission line approximately
equal the radiation "loss" of the antenna, the feedpoint impedances will have
approximately the same value.

Once standing-wave antenna currents are understood, it is easy to see why
the total superposed currents at each end of a 75m Bugcatcher coil are nowhere
near equal even though the forward current and reflected current at each end
of the coil are close to the same value.

Circuit analysis works well when there is only one current flowing in a coil.
Circuit analysis falls apart when forward and reflected currents are flowing
in a coil and distributed network analysis is required when such coils are
installed in standing-wave antennas.
--
73, Cecil, http://www.qsl.net/w5dxp


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