Mod'ing a fiberglass CB antenna for use on VHF

[suleske]

I've a vehicle with a standard Radio Shack fiberglass CB vertical antenna mounted to the roof rack with a decent bracket. I've no desire to run 27 MHz but was thinking I could tune it to use on 144 or maybe even 6m. Anyone have experience with doing this easily? Would think I could cut it down to become a 5/8 wave on either 6 or 2m. Haven't looked much further but assume its a coiled wire inside the fiberglass. Just take the end cap off, cut it down and replace the cap. Is there a calf to do this without air of trial and error? Obviously measure twice and cut once. It does not appear to have a loading coil. It's about 4 feet long.

I don't currently have a VHF SWR meter. I have an old RS Micronta one I think is geared to CB but seems to work in the lower part of VHF.

 

[jhooten]

Straight small diameter solid copper wire inside. When they smacked a tree limb and splintered above the halfway point we cut them to 54" for 6 meters. When you cut mix up some epoxy to seal the cut end or it will splinter and fray. Somewhere I have on cut even shorter that I tried on 2M for giggles once. It worked as well as any other quarter wave.

 

[n9mxq]

Firestick makes a 2meter fiberglass antenna.. And you can find them on ebay and elsewhere cheap enough..

 

[jhooten]

3/8-24 antennas are out there.
A short one: Hustler SF-2 Mobile Antenna

A little bigger: Hustler CG-144 Antenna CG144
This one is far and away one of the best made and highest performing 2M mono band antennas available.

 

[AgentCOPP1]

Without an SWR meter, it will be next to impossible to get to the 2:1 or 1.5:1 that you typically want. I would strongly recommend you get one before even trying because trying to tune an antenna blind is very tricky. Hell, when I was first setting up my vertical antenna, the specifications in the manual for how long each segment was supposed to be were completely wrong. They were giving me about 5:1 on every band. Do you have any ham radio friends who could loan you an SWR meter?

 

[LtDoc]

Can it be done? Yes. Is it worth doing? Beats me, you have to answer that question. One question that comes to mind is if that antenna is base loaded? If so, then that 'yes' may turn into 'no'. At least without a lot of effort.
I know it's how 'everybody does it', but an SWR meter isn't very good to use in tuning any antenna. It can only tell you a little bit about impedance matching, doesn't know resonance from a fried pie. Both of those are important, impedance matching and resonance. A 50 ohm resistor makes for very nice impedance matching and can radiate all the power getting to it. It just makes a terrible antenna.
- 'Doc

 

[suleske]

This is the fiberglass whip on the car

Pretty sure this is the installed fiberglass whip (CB) with a mirror mount, attached to the luggage rack in the center of the roof:

Adjustable 48 Fiberglass Whip Element for CB Antenna : CB Radio Antennas | RadioShack.com

So there is no matching network / loading coil feeding it. Sounds like replacing it with a 2m 1/4 wave whip of some sort is the way to go, though I'd like to keep it on the cut-it-and-thus-free side of things.

About a VHF SWR meter... The old Micronta (Radio Shack) SWR meter I have is no doubt targeted at CB, though it says nothing of the sort. I'll post a separate thread about that.

Thanks!

 

[cgervasi]

2m antennas

Many years ago I bought an antenna that looked just like this one at Radio Shack: Amazon.com: Ambient Weather 20-176 Indoor / Outdoor Weather Radio Antenna: Electronics I'm not sure if it's the same. It's marketed as a scanner antenna but resonates at 2m, dead-nuts in the center of the band. I don't know if that's a coincidence. It almost seemed like it was made for 2m.

I just wrote a post about an experience I had trying to use a highly loaded CB antenna as a VHF receive antenna. The results were god-awful. Many times if an antenna works at one frequency it will work at least somewhat at higher frequencies -- but not always.

I think you should get/borrow an SWR meter and cut your 11m antenna until it resonates decently on the band you want. You can use the formulas for half wave or full wave, but I find they're not very predictive of how much I have to trim an antenna.

 

[SCPD]

It probably would not be too difficult to trim it to 10 meters, or maybe 6 meters.
Carefully get the rubber tip off best you can without damaging it.
Then, with a hacksaw, start cutting off about a 1/3" or so off the top, and check the match each time.
Be great if you had an MFJ analyzer, you would know what resonant freq you are at with each cut.
Like someone said, you definitely need an SWR meter.
After you get where you want, superglue the cap back on.

You could get it to 2 meters, but it's probably not really gonna be much more than a 19" quarterwave. The bottom half of antennas like the one you showed are often just a straight wire.