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CB to GMRS mobile antenna Mod ?
- Thread starter FlyBoy50
- Start date
A quarter wave GMRS antenna is about 6.5" tall. 5/8 would work better, but there will be some matching involved.
The main concern I would have with GMRS is coax loss.
Thankfully, the loss is easy to measure with an SWR meter or nanoVNA. Simply disconnect the coax at the antenna so the far end is open (you can use a short too) and measure the SWR at the frequency of operation. The matched line loss in dB (loss assuming you had a perfect 50ohm match into the antenna) will be 10*log((SWR+1)/(SWR-1)).
If you measure SWR with nothing connected to the other end and you still get a low SWR, the cable is eating up the power. My rule is no SWR under 4. Here are some SWR-loss values:
SWR 4 = 2.2dB loss
SWR 3 = 3dB loss
SWR 2 = 4.8dB loss
SWR 1.5 = 7dB loss
SWR 1 = your cable is a dummy load.
The main concern I would have with GMRS is coax loss.
Thankfully, the loss is easy to measure with an SWR meter or nanoVNA. Simply disconnect the coax at the antenna so the far end is open (you can use a short too) and measure the SWR at the frequency of operation. The matched line loss in dB (loss assuming you had a perfect 50ohm match into the antenna) will be 10*log((SWR+1)/(SWR-1)).
If you measure SWR with nothing connected to the other end and you still get a low SWR, the cable is eating up the power. My rule is no SWR under 4. Here are some SWR-loss values:
SWR 4 = 2.2dB loss
SWR 3 = 3dB loss
SWR 2 = 4.8dB loss
SWR 1.5 = 7dB loss
SWR 1 = your cable is a dummy load.
Thanks for the quick comeback. I almost missed it because the little bell didn't give me a alert to your answer. So what your saying sounds like it is possible if you know your stuff, but a guy like me might just be better to buy the proper antenna. LOL I might bite off more then I want to chew ! LOL
Well, antennas are the easiest thing to homebrew, and you could certainly cut an antenna down and make it work for UHF, but it takes an SWR meter or analyzer capable of operating properly those frequencies. An HF SWR meter may not get the job done.
If you do have a means to measure the antenna and have extra CB antennas, by all means, try it. Feel free to post pictures of your options, some may be better than others. You will want to avoid CB antennas with sealed coils at the base though.
If you do have a means to measure the antenna and have extra CB antennas, by all means, try it. Feel free to post pictures of your options, some may be better than others. You will want to avoid CB antennas with sealed coils at the base though.
I don't know if my SWR meters are up to the job so if I have to buy a analyzer I might as well buy a antenna ready made for the job. It's all really interesting and wish I would have got back into radio years ago when I was younger and didn't have so many finish up jobs on my plate. I have 2 little cheapy Walmart mag mounts antennas in the garage that are center loaded that would be good to experiment with so it would be No Great Loss if you messed them up. They were good for a quick, temporary antenna to throw on the roof of your vehicle for using your hand held. Thanks for the help with the question and at least I know it can be done, but it's going to be one of those Back Burner Projects in case I live longer than expected. When you get almost 75 YO there is a lot of ????? marks. LOL
I like testing antennas with a NanoVNA. Very versatile with the klunky free software to boot.
Get the measurement right and a straight forward 1/4 wave is the easier for the do-it-yourselfer. Matching network should not be needed when done right.
From what I'm reading in the gmrs sites the Midland - Mxta26 antena one of the better gmrs antennas it's around 50 bucks then just need mouth of you choice.. I tried to mod an old fransis whip not to successful.. So keeping the cb and 6 foot whip and adding the gmrs radio with the midland antenna just need to figure out a mounting point..
[Edit: Apologies, I forgot you didn't have an analyzer and preferred buying an antenna over an analyzer. I will leave my reply anyhow, feel free to ignore.]
Have you tried a coupled radiator? I learned it from the ARRL Antenna Book. Although their focus was on HF, which is where I used this trick, I assume the same works at UHF. Let me talk about my wire dipole just to illustrate the concept. I am fairly sure it can be adapted to a vertical whip.
This trick works with wire antennas and coaxial antennas. With 14awg dipoles, a higher frequency is added by hanging a wire half wave radiator 4 inches below the main dipole. No physical connection, just close coupling. The same works if the shorter radiator is a sleeve too.
What I wonder is if you can do the same thing to your CB whip. Lets say you connect a piece of wire to ground near the feed point of the CB antenna and then use some insulator to support it in close proximity to the main vertical. You could then use a simple LC diplexer to run both radios on the same antenna.
This would add wind loading, but GMRS would be under a foot, so I think you would be ok. The wire approach would be better than the sleeve when it comes to maintaining the match at the lower band, but both seem like they would be easy to do. I have never tried this, but if it works with wire dipoles, I think it would be worth a try.
Have you tried a coupled radiator? I learned it from the ARRL Antenna Book. Although their focus was on HF, which is where I used this trick, I assume the same works at UHF. Let me talk about my wire dipole just to illustrate the concept. I am fairly sure it can be adapted to a vertical whip.
This trick works with wire antennas and coaxial antennas. With 14awg dipoles, a higher frequency is added by hanging a wire half wave radiator 4 inches below the main dipole. No physical connection, just close coupling. The same works if the shorter radiator is a sleeve too.
What I wonder is if you can do the same thing to your CB whip. Lets say you connect a piece of wire to ground near the feed point of the CB antenna and then use some insulator to support it in close proximity to the main vertical. You could then use a simple LC diplexer to run both radios on the same antenna.
This would add wind loading, but GMRS would be under a foot, so I think you would be ok. The wire approach would be better than the sleeve when it comes to maintaining the match at the lower band, but both seem like they would be easy to do. I have never tried this, but if it works with wire dipoles, I think it would be worth a try.
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You guys are a lot smarter then myself when it comes to radio stuff seeing I've had no training in that area. If my plate wasn't so full of projects already I might take on learning more but as it stands now a $50.00 antenna is making more sense for me. LOL
I'll look into that thank you I have a dozen or so older whips of different size... Sound like a fun experiment
I've got a Midland MXTA26 and the Luiton version. Excellent antennas.