On Jan 24, 12:32*pm, Michael Coslo wrote
* * * * For those not familiar, the antenna consists of a tapped coil at the
bottom, a spiral wound lower section, a tapped loading coil, and an
upper stinger.

* * * * My question involves that spiral wound lower section. What would this
be thought of? As antenna below the loading coil, as an inductor in
addition to the other two, or some sort of hybrid of antenna/inductor?

* * * * How would this be modeled in EZNEC?

* * * * Apologies in advance if this was an incredibly stupid thing to ask! 8^)

* * * * - 73 de Mike KB3EIA -

Hi Mike, I built one similar to this myself. I always assumed the
spiral wound lower section added some distributed inductance that would
reduce the amount needed for the upper loading coil. The inductance
added by the spiral section could be modeled by adding a couple of
inductive loads on the lower section. The effect of the loads will be
to change the current distribution slightly on the lower section of the
antenna. The spiral bottom does radiate.

The goal on this type of antenna is to adjust the center loading coil
such that the antenna looks capacitive, and adjust the bottom loading
coil to cancel Xc. Hopefully your impedance now looks like 50 ohms
resistive. The spiral and center loading attempt to move the current
distribution up the antenna so that more radiation occurs on the upper
parts.

When I modeled mine, I only had a center inductive load, and the
results were satisfactory. My goal in modeling was to design matching
networks, I wasn't too concerned about current distribution. Also had
a top hat, modeled as crossed wires. Later discarded top hat, not
aerodynamic.

Homebrew mobile antennas IMO need center loading, no top hat, can
accept power (loads up), and will stay attached to your vehicle at
80mph in a driving rainstorm.

Gary N4AST