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March 9th 06, 04:07 PM posted to rec.radio.amateur.antenna

On Wed, 8 Mar 2006 23:37:57 -0600, (Richard
Harrison) wrote:

Tom, W8JI wrote:
"Thinking the inductor or loading coil represents 60 degrees of
electrical length is EXACTLY where the big myth is at and it can easily
be proven to be a myth!"

A vertical antenna is often driven against a reflecting ground system.
It is desirable that it be self resonant at nearly 1/4-wavelength
(90-degrees), in many instances, to eliminate reactive impedance to
current into the antenna, avoid loading coil loss, and avoid bandwidth
limitation which comes with high-Q coils. Even with its drawbacks, a
base loading coil is often the practical way to resonate a too-short
antenna.

Suppose the vertical is only 2/3 the height needed for self resonance,
or 60-degrees high. The loading coil must replace about 30-degrees of
missing antenna to bring the vertical to resonance.

30-degrees is not an inductance value. An inductor is impure because it
has resistance and capacitance in addition to inductance. Also, the
inductance needed to replace the missing 30-degrees of antenna depends
on where it is sited, high, low, or in-between.

Siting affects performance as it determines current distribution along
the antenna.

Where is the myth?

Maybe part of the myth is that the antenna must be resonant to work.
Nothing could be further from the truth.

How about this thought experiment: Assume a too short for resonance
monopole, that has its feedpoint impedance made non-reactive by the
insertion of a "base loading coil." All kinds of arguments, including
this one, arise about what the role of the coil is, what its current
distribution is, how it affects efficiency and so forth.

To minimize these arguments, let's stop calling the inductor a "base
loading coil" and call it part of an "L-network feedpoint matching
network." Now the radiator isn't resonant and the resulting feedpoint
reactance (and resistance) is matched separately with the external
network.

Has the radiator current distribution changed? No. Does the inductor
in the L-network "make up" some number of electrical degrees in the
radiator? Not from my viewpoint.