Converting 102" CB Antenna

[onexco]

Hello. I'm relatively new around here so I hope I'm posting this in the correct area.

I did a search for this and found an older thread that partially addressed this issue but that thread was 7 years old so I thought I'd start a new one.

I have an older jeep that I use for ranch work. It already has a 102" Whip CB antenna with spring mounted on it that is not being used anymore. I was wondering if I could cut the whip down for use on 2 meters? If I could cut it to work on both 2 meters and 70 cm that would be even better. I know antennas for these bands are very cheap and it would probably best to just buy something new but since this mount is already there I thought I would ask. If it would work, what length should I cut it?

Thanks for your help.

 

[popnokick]

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You could make it a 1/4 wave by cutting it to 19 inches. But a quarter wave needs a ground plane, and I'm betting that ball spring on your Jeep is not in the center of the roof or on a horizontal metal surface. So no ground plane... You won't be happy with performance, and will be cutting off 4/5?of the 102 whip.

 

[LtDoc]

Yes, you could cut that antenna down to size on 2 meters. I would consider it a lot impractical though. As you said, a 2 meter/70cm antenna is fairly cheap. Or, just make one.
The required 'groundplane'/counterpoise/'other half' of a VHF/UHF antenna isn't usually a problem on most metal bodied vehicles. It would certainly help to get it above surrounding metal things. It isn't the additional height so much as getting it to where it isn't 'shielded' because of the vehicle's body.
If cutting that whip to size is your only alternative then have at it!
- 'Doc

 

[onexco]

Thanks for your reply. The ball mount is mounted vertically on the rear quarter panel. I would assume it is grounded since it is connected to the tub on the jeep. It couldn't be cut to 2 meters to be a full wave antenna? Sorry for the stupid questions but I'm very new to amateur radio and antenna systems. I have used business frequency VHF radios for work but they were already installed in the vehicles so I never really understood the details of how the antenna systems worked. Thanks.

 

[onexco]

Thanks Doc. I was writing my earlier response before yours was posted.

Since this ball mount has a spring what length would I need to cut it for 2 meters. I know it is not the best option but it is better than just having a hole back there.

Thanks.

 

[rfradioconsult]

Measure from the base insulator to the whip and cut at 19 inches. I would remove the spring, it could possibly act as a loading coil making your results unpredictable. FYI some commercial 2Meter antennas can be found with a 3/8 x 32 stud that will screw directly to the ball mount.

 

[davedaver1]

Most springs have a wire braid that serves to prevent it being a coil - bend the spring over and you should see it.

I agree with rfradioconsult, I think you would be much better served by putting something like the Hustler CG-144 (84" tall) on the mount. The 19" whip down along the body would not do very well.

 

[rfradioconsult]

Back when radios had little bottles in them that glowed in the dark and low band was popular I've seen several springs that the braid was rusted or broken, used to be common antenna failure problem.

 

[jhooten]

Just a suggestion, even Techs have 10 meter privileges. Use the whip for 10 meters and put a proper dual band antenna on the jeep.

 

[onexco]

Thanks everybody for your help. I appreciate it.

 

[zz0468]

A ballmount would make a poor 2 meter antenna, even if you could cut it right. The antenna starts in the coax where the shield is separated from the center conductor, and in the case of a CB ball mount, that could be a couple of inches inside the vehicle. So, start measuring there. It could turn out that you end up with an 10" whip mounted on the side of the vehicle, with a high current node on the antenna hidden behind the rear quarter panel. Performance would be horrendous, and even worse on UHF.

 

[onexco]

Thanks.

 

[LtDoc]

Antenna starts where the coax stops, that's true. But there are 'ways around that' if the effort is worth the time and effort. If that ball mount and whip is the only option then I would say to do it. But it isn't the only option with just a little ingenuity/imagination.
The other liability in in where that ball mount is mounted. Typically, that sort of mount/antenna won't be on top of a vehicle, but on the side of it. With HF that wouldn't be a 'deal breaker'. With VHF/UHF it certainly could be.
Lots of qualifications to it, but in general, cutting down a CB antenna probably isn't too feasible.
- 'Doc

 

[mark40]

Doc, jumping in on the topic- how much is compromised by cheating to the side of the truck vs. the center of the trunk?