Inside the mag mount is a tuning circuit for 146 MHZ!! So, it's not
simply cutting the length from 49 inches to another value. It will
require changing the tuning inside the mag mount.

So, scaling the 49 inches from 146 MHZ to 223 MHZ yields a length of 32
inches for 5/8 wavelength at 223 MHz. But, you still have to solve the
retuning of the internal circuit.

W1MCE


Darrin wrote:

I found an old mag mount 2 meter antenna, and want to convert it for
use on 224MHz. The whip is 49". Is there a simple formula to figure
out how short to cut it?

Darrin wrote:
I found an old mag mount 2 meter antenna, and want to convert it for
use on 224MHz. The whip is 49". Is there a simple formula to figure
out how short to cut it?

What you have is a 5/8WL on 2m. It, no doubt, has a coil in the base
which is too big for 224MHz. You will have to modify the coil along
with the whip. I hope it is not a potted coil.
--
73, Cecil http://www.qsl.net/w5dxp



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I beleive there is a section in the HandBook on how to convert a 2 meter 5/8
wave to a 220 5/8 wave.

Dan



--
If I am tired now, why do I have to RETIRE later?

It's not as hard as people are making it sound.

This sounds like you have a 5/8's 2M antenna, with 49" of whip. The loading
coil is not DC grounded, meaning that the coil is simply in series with the
center conductor of the antenna.

Electrically your antenna is approximately 54" in length (You'll notice your
current antenna works well on the 6M band too!), meaning that the coil is
providing 5" or so of the length (It doesn't really work this way, but it's
close enough). 5/8 on the 220 ham band is about 33", so if you subtract the
5" of the coil, the whip should be about 28".

Go find some solid #12 copper wire, and replace the 49" whip with about 30"
of copper wire. Then with an SWR meter, determine what the correct whip
length needs to be.

With the SWR meter in-line, do a plot of the SWR, say every 1 MHz. You
should see something like this:

MHz SWR
222. 2.5:1
223 2.7:1
224 3.0:1
225 4.0:1

This is telling you the resonant frequency is somewhere below 222 MHz. Cut
1/4" off the copper, and try again and repeat till you have a good SWR. You
are looking for at least a 2:1 SWR maximum across the entire band.

Once you find the length in copper, cut your 49" whip down to the same
length, plus 1/4", and re-test the SWR. You may find you have to adjust the
length of the stainless whip just slightly, as the copper has different
properties than the stainless whip, but it shouldn't be more than 1/4"
(Which is why you added 1/4" to the length before cutting).

If the SWR appears lower at 225 MHz, then the antenna is too short, try a
longer piece of copper.

Stay away from the 3/4 wave resonant point, which would be about 34" of
whip. It will be resonant, but you will not be happy with the performance.

When you get all this done you may also find your not real happy with the
5/8's 222 antenna either. I have one, and like the 1/4 wave 222 antenna
much better. If you decide to go the 1/4 wave route, remove the coil
entirely, and cut a 13" whip. This should be perfect for 222 MHz. You can
also make a nice 1/4 wave 222 antenna from all the extra Motorola whips you
find at the hamfest for $1. They are too short for 2M, but are perfect for
222. Simply heat the whip with a propane torch to remake the little curl in
it's base.




"Darrin" wrote in message
...
I found an old mag mount 2 meter antenna, and want to convert it for
use on 224MHz. The whip is 49". Is there a simple formula to figure
out how short to cut it?

Well, don't measure your SWR in the garage.

It's best to be at least 1 wavelength away from metal objects when you are
adjusting the antenna, which for 220 is 4.5'.

"Darrin" wrote in message
...
On Thu, 02 Oct 2003 13:05:05 GMT, "KB9WMJ"
wrote:

It's not as hard as people are making it sound.

This sounds like you have a 5/8's 2M antenna, with 49" of whip. The
loading
coil is not DC grounded, meaning that the coil is simply in series with
the
center conductor of the antenna.

Electrically your antenna is approximately 54" in length (You'll notice
your
current antenna works well on the 6M band too!), meaning that the coil is
providing 5" or so of the length (It doesn't really work this way, but
it's
close enough). 5/8 on the 220 ham band is about 33", so if you subtract
the
5" of the coil, the whip should be about 28".

Go find some solid #12 copper wire, and replace the 49" whip with about
30"
of copper wire. Then with an SWR meter, determine what the correct whip
length needs to be.

With the SWR meter in-line, do a plot of the SWR, say every 1 MHz. You
should see something like this:

MHz SWR
222. 2.5:1
223 2.7:1
224 3.0:1
225 4.0:1

This is telling you the resonant frequency is somewhere below 222 MHz.
Cut
1/4" off the copper, and try again and repeat till you have a good SWR.
You
are looking for at least a 2:1 SWR maximum across the entire band.

Once you find the length in copper, cut your 49" whip down to the same
length, plus 1/4", and re-test the SWR. You may find you have to adjust
the
length of the stainless whip just slightly, as the copper has different
properties than the stainless whip, but it shouldn't be more than 1/4"
(Which is why you added 1/4" to the length before cutting).

If the SWR appears lower at 225 MHz, then the antenna is too short, try a
longer piece of copper.

Stay away from the 3/4 wave resonant point, which would be about 34" of
whip. It will be resonant, but you will not be happy with the
performance.

When you get all this done you may also find your not real happy with the
5/8's 222 antenna either. I have one, and like the 1/4 wave 222 antenna
much better. If you decide to go the 1/4 wave route, remove the coil
entirely, and cut a 13" whip. This should be perfect for 222 MHz. You
can
also make a nice 1/4 wave 222 antenna from all the extra Motorola whips
you
find at the hamfest for $1. They are too short for 2M, but are perfect
for
222. Simply heat the whip with a propane torch to remake the little curl
in
it's base.


Thanks for the advice. I'm going to print it out, and maybe save this
project for a rainy day in the garage.