Hello Sailors,
I rained so hard.... (..how hard did it rain?) that I
was only able to play with a silly six meter antenna
project over the weekend. I can't imagine how some of
you deal with all the snow and rain you get in other
parts of the country/world.
****
I've got a snot full of low band ~42MHz antennas removed
from service. The CHP in California is on low band (and
it works well for them). Their old pulled antennas end up
at surplus sales and flea markets for very low prices.
My goal is to find an easy mod to relocate the 42 MHz
antennas up to about 52.525 or slightly higher, which
I hope will cover the upper end of the Six-Meter FM
Band.
I've got a small amount of Antennex, Comtelco, Larsen
and Maxrad low band antennas to use as test jigs. To
be honest the mostly plastic body Larsen stuff doesn't
look like very hardy or easy to convert. Most of the
commercial antennas use the same base/jig materials for
construction through a mostly mechanical assembly.
I've found the Maxrad antenna coils/loads unscrew if
you know how to fix a lathe or wrench onto them. Using
a wrench will often mark the metal so access to a lathe
makes a much cleaner dissassembly.
The Antennex unit coil covers are a very firm press fit
over a dual direction seration, which can be removed by
a strong - firm lathe pull.
I'm working with the Antennex brand loads now... this
weekend I started by shorting the upper loading coil
turns. When the smoke cleared I had 4 upper load coil
turns shorted and had moved the impedance tap up 1/4 inch
on the coil for 50 ohms. The rod length required about
1.5 inches to be removed.
The results are great! The antenna gives a near 1.1
match at 52.600 (my actual target frequency) and retains
a < 1.5 swr/match for well over 1MHz each side of the
band.
The final modification was to remove the excess turns
vs shorting, which didtn't make a big change in the
antenna tuning. I'm going to move the tap point back
to the original position to see how far/where the impedance
changes. The goal here would be to write up an easy to
reproduce antenna modification for surplus commercial
antennas.
****
One last part... I can easily see why and where these base
loaded coil antennas go bad. The Antennex brand of antennas
use a very weak thin brass foil strape press-fit contact
at the ground end of the loading coil. The coil high current
causes this location to fail over relatively short time
lengths from serious arcing.
Every one of the low-band Antennex base loaded coil antennas
I've pulled appart have this problem (serious black arc
and pitting marks). A wire brass or stainless steel
brush cleans it back up, but I'm not really one to trust
this type of dorky connection for the long term. Since
90% of the surplus antennas I've found at the flea market
(from the ex chp application) are the Antennex brand, I''m
going to assume the ground foil arcing issue is more than
a small problem as yet unresolved by the mfgr.
This week will produce the final resultant antenna modified,
cleaned-up and ready to use on 6 meters.
More information to follow as this project works out.
73's
skipp
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