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eHam Forums => HomeBrew => Topic started by: XY3LON4 on March 26, 2007, 09:15:19 AM



Title: converting a cb whip antenna to a 2 meter.
Post by: XY3LON4 on March 26, 2007, 09:15:19 AM
I am doing some research on antennas and it seems to me that a full wave 2 meter antenna needs to be 2 meters long. Couldn't a person take a tunable tip cobra antenna, -strip off the plastic coating,
-cut the wire at the tip,
-unwind the wire and rewind to match the length of the wire to a full wave 2 meter,
-evenly rewind it and resolder it to the tip and cover it with a length of heat shrink tubing.
This way you could adjust the tip for the best SWR.
What do you think?
1. How long would I need to make the wire?
2. How many turns?
3. Would the turns need to be even the whole length of the antenna or would I load it at a certain point?


Title: converting a cb whip antenna to a 2 meter.
Post by: WB6BYU on March 26, 2007, 03:25:51 PM
There have been a number of discussions about converting CB antennas
to 2m here in the forums - might be worthwhile doing a search, here
in Homebrew as well as Elmers and TowerTalk.  I've converted some of
the base-loaded CB antennas to half-waves on 2m and they work
fine.

But to address your specific questions:

> I am doing some research on antennas and it seems to me that a full > wave 2 meter antenna needs to be 2 meters long.

No, a 1-wavelength antenna doesn't work well at all for a vertical whip.
1/4 wave is the standard length - which is about 19" on 2m.


> Couldn't a person take a tunable tip cobra antenna, -strip off the plastic coating,
> -cut the wire at the tip,
> -unwind the wire and rewind to match the length of the wire to a full wave 2 meter,
> -evenly rewind it and resolder it to the tip and cover it with a length of heat shrink tubing.
> This way you could adjust the tip for the best SWR.
> What do you think?
> 1. How long would I need to make the wire?
> 2. How many turns?
> 3. Would the turns need to be even the whole length of the antenna or would I load it at a certain point?

The CB antenna is wound with wire because the physical length is less
than a quarter wavelength (9 feet for CB.)  But since a quarter wavelength
on 2m is only 19", you don't need any shortening at all - just make it
full sized.  If you want to convert the CB antenna, disassemble it as
you suggested and cut the wire to 19".  Run it straight up the side of
the fiberglass support (or with a very slight  wrap - perhaps 4 turns for
the whole wire.)  Cut off the fibreglass at the top of the wire and cover
the remainder with heatshrink.  That is all you need.

There are ways that you may be able to make a better 2m mobile antenna
using the same materials, but they will require knowing the existing whip
length and rod diameter.  But the theory behind it is different from that
for the same whip used for CB.  The key difference is that the antenna
is physically shorter than 1/4 wavelength on CB, while it is longer than
1/4 wavelength on 2m, so the same techniques don't apply.



 


Title: converting a cb whip antenna to a 2 meter.
Post by: KE7RIP on March 24, 2008, 10:35:57 PM
Sounds like the "Sound FX Magnetic Mount CB antenna with signal Amplifier" i bought from walmart should be easy to convert.  It comes in 4 pieces.  Mag base. the  coil with bottom half of the whip. The screw on amplifier. and the top half of the whip.

part no. 3450
Specs
26.8-27.405 Mhz
Over all lenght 26 inches. Not including cable.

I bought it for a by the windo inside antenna. It was convert a CB whip or build my own Jpole. Honestly the Jpole sounds easier and funner since i dont have a SWR meter yet.

Thanks for any replys.


Title: converting a cb whip antenna to a 2 meter.
Post by: KB9WIS on June 06, 2008, 01:45:36 PM
I recently came across a few websites of a guy the converted IMAX 2000 antennas as well..  He makes them better, but can also retun them more bands.


Links to his websites are below.  

I think the page you can check out his CB antennas are: http://www.bighairantennas.com/CB_Antennas.html

And the page, that shows how he converts and upgrades an IMAX 2000 Antenna: http://www.bighairantennas.com/BigHairsAntennasversuscompetitors.html