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CB Radio Forum - Discussions regarding Citizens Band Radio (CB)

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Old 06-03-2017, 3:02 PM
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Default Larsen NMO cutting chart

They have 27 to 30 MHz in .5 increments...

I've got a roof mount NMO antenna connected to a Uniden Pro510xl radio. Which is the proper MHz for that radio? I thought these base loaded whips were only supposed to be for Cb?
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Old 06-03-2017, 3:26 PM
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Quote:
Originally Posted by Sonicadventure View Post
They have 27 to 30 MHz in .5 increments...

I've got a roof mount NMO antenna connected to a Uniden Pro510xl radio. Which is the proper MHz for that radio? I thought these base loaded whips were only supposed to be for Cb?
Those whips could also be used for amateur 10 meter band or certain commercial services around 30 MHz.

For CB use, you would want to tune the antenna to the middle of the band (ch20 - 27.205 MHz). You would then check the SWR on ch1 and ch40 to see if they are roughly the same.

John
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Old 06-03-2017, 3:36 PM
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Default Cutting chart

If your reception is satisfactory, I would leave the antenna alone. A small mistake can turn an antenna into just another piece of wire. Many of the antennas I have used have a set screw on the coil to adjust the whip length, and you can make adjustments without worrying about cutting too much off.
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Old 06-04-2017, 12:24 AM
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Quote:
Originally Posted by chief21 View Post
For CB use, you would want to tune the antenna to the middle of the band (ch20 - 27.205 MHz).
The actual center of the band is channel 19, 27.185MHz, due to gaps in the CB channel lineup.
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Old 06-04-2017, 10:07 AM
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Since the mounting location will affect tuning I would leave the first cut an inch or so long for the low end of the CB band to allow for tuning.
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Old 06-07-2017, 7:58 AM
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I have several mag-mount antennas of various lengths that I use on several vehicles. Since I only use them to monitor and report problems on the road, channel 19 is my frequency of interest. I stop cutting my antennas when I get to 1.5 or below on 19. This sometimes leaves me at an SWR of around 2.0 or so on channel 40, but I don't care since I don't talk on the higher channels anyway. Leaving the antenna a little long allows me more flexibility on vehicles and antenna placement locations. I can always cut more, but adding more no-can-do.
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Old 06-07-2017, 9:35 PM
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The Larsen NMO-27 is designed to cover a span of frequencies depending on the length of the whip. They are not just for CB, although they do make good CB antennas.

I have one that I use. I used my antenna analyzer from work to tune it. I achieved 1.03:1 SWR on Channel 19, center of the band, and 1.3 or so on channels 1 and 40.
46 7/16" is the length of mine from the base of the cone (where it screws onto the coil) to the top of the whip.

That is for my specific setup and location on permanent NMO mount. Your exact length will vary slightly depending on your exact installation.

I agree with prcguy, leave it a bit long and trim the whip using a proper SWR meter to get it correct for your installation.
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