Knarf wrote:
Sorry, this may sound dumb, I think I must have missed the point.

Doesn't sound dumb - you just missed the original argument. It
occurred over on eHam.net, was titled "In Search of 'The Perfect
Mobile Antenna'" and overflowed to this newsgroup. W8JI said there
is essentially no current change from end to end in a mobile loading
coil and used the lumped ideal dimensionless inductor from EZNEC to
"prove" his statement. K3BU took issue and the argument still rages.

The article on eHam.net is at:

http://www.eham.net/articles/5998

W8JI's take on the subject is on his web page at:

http://www.w8ji.com/mobile_and_loaded_antenna.htm

K3BU's take on the subject is on his web page at:

http://www.k3bu.us/loadingcoils.htm

My take on the subject is on my web page at:

http://www.qsl.net/w5dxp/current.htm

Why are
people arguing about current in a loading coil? NEC, and experiment, seem
to provide the answer.

Lumped point inductances used by some models are dimensionless
and therefore have no current drop. Of course, lumped point
inductances don't exist in reality. The Helix feature of EZNEC+
illustrates the change in current through the coil. Based on
the quote from Balanis below, the total current through the
coil is If+Ib where 'If' is the forward current and 'Ib' is
the backward (reflected) current. That is, of course, phasor
addition. The net total current, If+Ib, for a 1/2WL dipole is
close to a cosine function with the center feedpoint at zero
degrees. So Itot = If+Ib = ~Ifeed*cos(degrees), where (degrees)
is the number of degrees away from the feedpoint of the dipole.
Halfway out one leg of a dipole the current is approximately
Ifeed*cos(45) = 0.707*Ifeed (Ifeed is feedpoint current)

The current at the tip of a dipole is Ifeed*cos(zero) = 0
Itot = If+Ib = 0 at the tip of the dipole because the two
currents, If and Ib, are equal in magnitude and opposite in
phase. (Note: the convention is that the current reflected at
an open-circuit changes phase by 180 degrees. Even though
the current is a phasor, only the Real part exists in reality.
Current in a wire has a magnitude and direction. There are
only two possible directions.)

The total current on the dipole is a standing wave with the
total current decreasing to zero at the tip of the dipole.
If and Ib are flowing in opposite directions and therefore
their phases are rotating in opposite directions. It is that
phase rotation difference that causes If+Ib to be different
at each end of a typical loaded mobile antenna. It's a pretty
simple concept and is explained on my web page above.

Incidentally, the feedpoint impedance of a 50 ohm dipole depends
upon the magnitude of the reflected voltage and reflected current
on the antenna. If a 1/2WL dipole were turned into a traveling-
wave antenna by terminating both ends, the feedpoint impedance
would be hundreds of ohms.
--
73, Cecil http://www.qsl.net/w5dxp
"The current and voltage distributions on open-ended wire antennas are
similar to the standing wave patterns on open-ended transmission lines ...
Standing wave antennas, such as the dipole, can be analyzed as traveling
wave antennas with waves propagating in opposite directions (forward and
backward) and represented by traveling wave currents If and Ib ..."
_Antenna_Theory_, Balanis, Second Edition, Chapter 10, page 488 & 489


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