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Antenna length advice

parkerd619

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Sep 13, 2013
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Camden NC
Ok folks. I have a few stainless steel whips from old CB mag mount antennas.

I checked a few antenna calculators to find out the length for a 5/8 wave antenna for the center of the GMRS band.

It gave me a measurement of 38.4 cm.

My question is this. Do you cut the antenna at exactly 38.4 cm, or do you allow for the depth of the antenna mount. The mount is 2cm deep.
 

rivardj

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Michigan
First of all, check to see if you need a loading coil.
I don't think(I could be wrong) it's not only the elements but a coil is needed too.
BTW I am never wrong. HI HI.

DW
So. Cal
A 5/8 wave-length antenna does require a coil. A 3/4 wave-length antenna does not, however, the radiation pattern of a 3/4 wave-length antenna is up off the horizon which is not always desirable.

Also, you likely can't use the base of the CB antenna unless you modify it.
 
Last edited:

mmckenna

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Jul 27, 2005
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I am a lineman for the county.
Ok folks. I have a few stainless steel whips from old CB mag mount antennas.

I checked a few antenna calculators to find out the length for a 5/8 wave antenna for the center of the GMRS band.

It gave me a measurement of 38.4 cm.

My question is this. Do you cut the antenna at exactly 38.4 cm, or do you allow for the depth of the antenna mount. The mount is 2cm deep.

38.6cm would be a 5/8th's wave antenna on the UHF band. You would need an appropriate UHF matching coil to bring the impedance down to 50Ω. The CB loading coil would not work for what you want.

The length would be the total length of the radiating element, and that would need to include the cone that the whip sits in. Usually it's a good idea to cut it long and check SWR, then adjust as needed. The exact length needed will vary depending on the installation.
 

parkerd619

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Camden NC
38.6cm would be a 5/8th's wave antenna on the UHF band. You would need an appropriate UHF matching coil to bring the impedance down to 50Ω. The CB loading coil would not work for what you want.

The length would be the total length of the radiating element, and that would need to include the cone that the whip sits in. Usually it's a good idea to cut it long and check SWR, then adjust as needed. The exact length needed will vary depending on the installation.
I'm not using anything from the CB antenna, except for the whip itself. The whip was going to be installed in an NMO mount that currently has a dual band antenna attached to it.
 

mmckenna

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I am a lineman for the county.
I'm not using anything from the CB antenna, except for the whip itself. The whip was going to be installed in an NMO mount that currently has a dual band antenna attached to it.

OK, good deal.


You'd cut your whip a bit longer than the 38.4cm you calculated and start there with the whip all the way seated in the cone. Hook up to your antenna analyzer or SWR meter and check. If all your calculations are correct, the total length of the cone and the whip should come pretty close to 38.4cm, but don't be surprised if it's off a bit. Different variations in mounting will throw it off a bit.
 

prcguy

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So Cal - Richardson, TX - Tewksbury, MA
If you calculate the length of a 5/8 antenna then cut and trim for a good match and you don't have a matching coil, you will end up with a 3/4 wave which will have a good match but will point up in the sky. You really need the matching circuit to end up with an actual 5/8 wave which has a little gain at the horizon. Don't waste your time unless you make a proper matching coil for your 5/8 whip.
 
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