Trimming Antenna To Size -Confirm my measurement-
 

I recently came across a radio shack CB/HAM mag mount antenna (#21-972).
I want to trim it down for 2m use. What would be length I would look for?
The steel whip is 39inches and it's total height with base is 46 inches.
I think it may be between 19.5 and 19.75 inches. Could someone confirm my
measurement?
Thanks.

Macman wrote:

I recently came across a radio shack CB/HAM mag mount antenna (#21-972).
I want to trim it down for 2m use. What would be length I would look for?
The steel whip is 39inches and it's total height with base is 46 inches.
I think it may be between 19.5 and 19.75 inches. Could someone confirm my
measurement?

If it has a loading coil in the base, forget it for 2m.
--
73, Cecil http://www.qsl.net/w5dxp



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I recently came across a radio shack CB/HAM mag mount antenna (#21-972).
I want to trim it down for 2m use. What would be length I would look for?

Hi Mac, Just make it a little long, and trim for best match (swr). You should
be OK with around 19 inches, but if you want the best match at your frequency
of interest, make long and trim.
73 Gary N4AST

No loaded coil, as far as I can tell. There is a spring adapter at the base
but I believe it's just to allow for flex.

Thanks for the feedback.

Take a look.

http://www.radioshack.com/product.as...%5Fid=21%2D972

No loading coil that I can see. Only a spring adapter at the base to which
I believe is for tension release.

Macman wrote:

Take a look.

http://www.radioshack.com/product.as...%5Fid=21%2D972

No loading coil that I can see. Only a spring adapter at the base to which
I believe is for tension release.

There's a loading coil beneath the spring. To use it on 2m, you would
need to short out that loading coil in the base under the spring.
--
73, Cecil http://www.qsl.net/w5dxp



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I recently came across a radio shack CB/HAM mag mount antenna (#21-972).
I want to trim it down for 2m use. What would be length I would look for?
The steel whip is 39inches and it's total height with base is 46 inches.
I think it may be between 19.5 and 19.75 inches. Could someone confirm my
measurement?
Thanks.

I have not look at that antenna, but when it is advertised as a cb/ham
antenna it is usually for the ham 10 meter band or the CB. At the bottom of
the antenna is a loading coil. To use it on 2 meters you will have to
remover the coil in the bottom of the antenna. If it is a 2 meter ham
antenna that is about 40 inches long it will be a 5/8 wavelenght antenna .
You need to look at another antenna for 2 meters if you want a 1/4 wave whip
without any loading or matching coils. Then it would be close to 19 to 20
inches long.

Macman,
There is a loading and impedance matching coil in the base
of the antenna just below the spring. It may be possible to
find a length of 'whip' that in combination with the loading/
matching coil will allow operation on 2 meters. Then again,
it may not be possible (don't bet on it).
Another option would be to remove the covering of the base
coil and re-tap it. One problem with that is that the covering
will probably be destroyed in the process. Another problem is
that the supplied coil probably doesn't have the correct
dimensions
required (you can always re-wind and tap it).
It won't be as simple as just 'trimming' the whip...
'Doc

Take a look.


http://www.radioshack.com/product.as...%5Fid=21%2D972

No loading coil that I can see. Only a spring adapter at the base to
which
I believe is for tension release.


It does have a loading coil somewhere in it. That is the way any mobile
antenna around 3 feet tall works for 10 and 11 meters. It will not work on
2 meters unless you can find the coil, take it out , and then cut it to
about 20 inches.

If there is a coil, maybe you cud just short it out with a piece of flexible
wire.

It's been done.

73 de Jack, K9CUN