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January 25th 07, 02:05 AM posted to rec.radio.amateur.antenna

Mike Coslo Mike Coslo is offline

Mike Coslo is offline

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First recorded activity by RadioBanter: Nov 2006

Posts: 116

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Mobile antenna question

wrote in
ups.com:

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.

Sounds sensible. Is there any future in trying to get this thing to work
on 40 meters?

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.

Thanks much, Gary. When the weather breaks, It should be ready to
roll.

- 73 de Mike KB3EIA -

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