Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Old January 21st 04, 12:10 AM
Macman
 
Posts: n/a
Default 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.


  #2   Report Post  
Old January 21st 04, 12:33 AM
Cecil Moore
 
Posts: n/a
Default

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



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----
  #3   Report Post  
Old January 21st 04, 12:35 AM
JGBOYLES
 
Posts: n/a
Default

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
  #4   Report Post  
Old January 21st 04, 12:39 AM
Macman
 
Posts: n/a
Default

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.


  #5   Report Post  
Old January 21st 04, 12:44 AM
Macman
 
Posts: n/a
Default

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.




  #6   Report Post  
Old January 21st 04, 01:25 AM
Cecil Moore
 
Posts: n/a
Default

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



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----
  #7   Report Post  
Old January 21st 04, 01:38 AM
Ralph Mowery
 
Posts: n/a
Default

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.



  #8   Report Post  
Old January 21st 04, 02:10 AM
'Doc
 
Posts: n/a
Default



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
  #9   Report Post  
Old January 21st 04, 02:46 AM
Ralph Mowery
 
Posts: n/a
Default

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.


  #10   Report Post  
Old January 21st 04, 11:54 PM
JDer8745
 
Posts: n/a
Default

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
Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Inverted ground plane antenna: compared with normal GP and low dipole. Serge Stroobandt, ON4BAA Antenna 8 February 24th 11 11:22 PM
Mobile Ant L match ? Henry Kolesnik Antenna 14 January 20th 04 05:08 AM
Poor quality low + High TV channels? How much dB in Preamp? lbbs Antenna 16 December 13th 03 04:01 PM
QST Article: An Easy to Build, Dual-Band Collinear Antenna Serge Stroobandt, ON4BAA Antenna 12 October 16th 03 07:44 PM


All times are GMT +1. The time now is 08:45 PM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017