Find a ham in your area who has an antenna analyzer.

Use the ham/analyzer to find the resonance of your antenna. Once it is known,
then adjustments can easily be 'gesstimated intelligently'.

I'm in New Hampshire. If close I'll do the measurement for you.

Where do you live? Nearest large city and state will do for info.

/s/ DD

A. Pismo Clam wrote:
Hello All,

I have just found my old Hi-Gain mobile-mount whip antenna, circa
1980's, I'm guessing.

I have just looked at the antenna and it is made in 4 parts; the whip,
the chrome spring that the whip gets screwed into, a metal tube [with a
plastic sleeve covering it. could this be the loading coil?] and lastly,
the mag base. A small quantity of RG 58 is soldered to the underside of
the "loading coil", making a very compact antenna.

If I loosen the set screw in the base and pull out the whip, it is 28"
long; I'd like to cut it so that it will receive/resonate center of the
151-174 MHz band. where I do most of my summer monitoring, using my old
[but still working] BC 210.

How long should I cut the whip to accomplish this? Or is the whip too
short already!?

I wouldn't bother the group with such an easy question, but I don't know
where to look for the formula!

Many thanks for taking the time to respond!

Alain

I have just found my old Hi-Gain mobile-mount whip antenna, circa
1980's, I'm guessing.

I have just looked at the antenna and it is made in 4 parts; the whip,
the chrome spring that the whip gets screwed into, a metal tube [with a
plastic sleeve covering it. could this be the loading coil?] and lastly,
the mag base. A small quantity of RG 58 is soldered to the underside of
the "loading coil", making a very compact antenna.

If I loosen the set screw in the base and pull out the whip, it is 28"
long; I'd like to cut it so that it will receive/resonate center of the
151-174 MHz band. where I do most of my summer monitoring, using my old
[but still working] BC 210.

If it's an old 2-meter antenna, then I'd guess from the whip length
that it's probably a 5/8-wave, with some form of inductive loading to
make it resonant.

If that's the case, it'll probably receive reasonably well for
monitoring purposes in the 151-174 range without any modification.

Or, you could shorten it by an inch or two, gradually trimming the
whip until the received signal strength around 160 MHz is at a peak.

Another option would be to bypass the loading coil (or whatever it is)
with a wire jumper, and shorten the whip to around 16.5". This will
convert the antenna into a quarter-wave vertical, resonant at around
160 MHz.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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