Cutting CB Antenna For Scanner Use

[supernoma]

What I have is a Cobra mag mount CB antenna, and I'd like to tune it to about 140-150 MHz.

It's got a coil on the bottom, but the physical antenna is 32-1/4".

So about how long do I have to cut this?

 

[WouffHong]

Antenna length...

Well, the 1/4-wave length is 19.36", but the coil has to be removed (or bypassed) as it is designed to turn that 32.25" whip into an 8-9 foot antenna.

Removing the coil totally will probably negate the ability to mount the whip, so it may be more trouble than it's worth, unfortunately.

However, if so motivated:

IF you can open the Coil housing, remove the multi-turn coil, and directly connect the bottom (where the coil was connected to the coax to the top (where the whip sits in), that will eliminate the Coil problem, but remember to reduce the 19.36" quarter-wave whip's length BY the length of wire you used inside that Coil carcass.

(That gives you a 1/4-wave overall length from the Coax connection to the end of the whip)

It's not an easy project, but might be an interesting challenge to explore.

FYI: To determine the 1/4-wave length of an antenna in feet, just divide the frequency in MHZ into 234..
73
Tom
W4NOV

 

[supernoma]

There's no way to use it with the coil intact?

 

[kb2vxa]

No, it's a >>>CB<<< antenna, it's a a long way from 11M to 1.5M, a LONG way.

"...but remember to reduce the 19.36" quarter-wave whip's length BY the length of wire you used inside that Coil carcass."

There's not a heck of a lot of room in there so as long as you keep the exposed center conductor reasonably short you can't go wrong. Just cut the whip down to 19" even and be done with it, total length an inch either way makes little difference with a ground plane type antenna.

 

[supernoma]

Okay, I got into the coil and bypassed it, thanks guys.