combining

[WQFM513]

I have an idea about combining a gmrs repeater and a cb base into one antenna. I havent checked either aginst this antenna with a swr meter yet.
My question, besides bandpass/reject filtering the feed lines to the radios, could this be done safely? will my 50 watts uhf mess up the cb receiver? will my cb mess up my gmrs receiver?

 

[wyomingmedic]

Being that CB is 27MHz and GMRS is 460MHz, it is like comparing apples and Fords.

I can see no way to combine and use a CB antenna on GMRS or GMRS antenna on CB.

WM

 

[WQFM513]

a full wave cb antenna is 434 inches right? and a full wave gmrs antenna is 25.5 inches and some. 17 or 18 full wave gmrs antennas would be 433.80 inches. so a "stacked" or "multiple" wave antenna should come out pretty nice on an swr meter. iv seen this before, i just dont know if its right or if its a "fluke" in the meter or if it will damage the radio.

 

[rfradioconsult]

SWR may be acceptable at the GMRS frequencies; however you would have a radiation pattern off each end of the antenna, practically zero radiation to the side of the antenna. The becomes a problem when you try to use a VHF antenna as a 3/4 wave UHF antenna, a noticable loss of range as your pattern tends to have major side lobes at a high angle..

If you want to experiment there are diplexers on the market with bandpass for HF/VHF/UHF to a common antenna, cost under $100.

 

[wyomingmedic]

I don't really know anybody using a full wave antenna on CB. Most folks use either a 1/4 wave or a 1/2 wave dipole.

You can certainly check the SWR of a CB antenna on GMRS, but I'm pretty sure it wont work the way you want. And as was said, the radiation pattern will NOT be anything you want.

Antennas that are multiple wavelengths in length can be a conundrum. As for the actual act of combining, that is no problem. MFJ sells ready made units for just that. $29.

WM

 

[WQFM513]

the antenna is 1/4 wave. i was just using full wave to simplify my math.

 

[ramal121]

Do you have an antenna up for GMRS now? I'm from the school of "what the hell". You'll never learn anything unless you try it. I'd hook the GMRS repeater up to CB antenna and check the SWR. It needs to be a fairly good match (not only to save the transmitter, but also keep the repeater duplexer happy and in tune). If for some magical reason SWR is low, run the repeater and do a walk test. Does it preform as well as you expect? If you have a UHF antenna to compare it to all the better. Stranger things have happened!

All of the above comments are valid though. In addition, if it is a gain type CB antenna, there will be a matching network for it and that could just plain suck the life out of a UHF signal no matter if the length pencils out or not. Have fun and good luck!