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Tuning a 1/2 wave GMRS antennna



I thought I understood this, but I guess not.  I want to put a 1/2 wave whip on my Jeep window frame NMO mount (no ground plane).  A ham friend gave me a 1/4 wave VHF antenna that I thought I could just cut down to 1/2 wave UHF.  As I cut I get no meaningful changes on the SWR meter.  I DO get a 1.05 SWR using a Midland "ghost" antenna but I dont  know how much it will need for a ground plane.  I was hoping to cut down the new antenna and do a range test with the ghost.

Does SWR only work on 1/4 wave or does the reading translate to 1/2 wave? 

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Swr works on any antenna.

It really doesn't tell you if the antenna is resonant or not.

Low swr doesn't always indicate the antenna is efficient or resonating where you want it to. However, it is easiest way to get to resonance.

The unity gain (most phantom/low profile style antennas) antennas are usually designed well with low swr to begin with.

Nmo antennas may or not be tunable. The base usually has a coil. This is used to electrically make the antenna longer. Without knowing the coil type and other variables you can't/may not be able to realistically ever get it tuned.

You should talk to your ham friend to guide you in tuning it.

Many hams have a crap ton off old whips because when chasing low swr, we tend to over trim. Sometimes rendering the whip useless for the band we where chasing.

Learned the hard way tuning a nmo cb whip. Wasted alot of whips before realizing the nmo(coil) was coupling with a near by structure. Completely, throwing off the swr readings. I was 2:1 on my truck bit when a buddy tested it on his truck it was 6:1 vswr.

Sent from my SM-S901U using Tapatalk

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3 hours ago, kirk5056 said:

Does SWR only work on 1/4 wave or does the reading translate to 1/2 wave? 

A Halfwave antenna has a “matching network” on the end, usually in the base area, that a 1/4 wave doesn’t have. You can’t simply cut a 1/4 wave antenna for a lower frequency and expect it to work as a 1/2 wave with no ground plane at a much higher frequency.

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A half wave antenna is normally fed in the middle with a balun or some type of matching device (gamma match for a yagi driven element for example).  Simply cutting a 1/4 wave for 2-meters down to a half wave on the GMRS frequency would present a horrible match at the base of the NMO mount.  You would be much better off cutting the antenna down to a 1/4 wave on the GMRS transmit frequency center (about 6-1/4").  The next best thing would be a 5/8 wave antenna, but this would take a matching coil at the base.  Both designs require a ground plane, for instance, a permanent NMO mount through a metal roof.

You did mention that you wanted to put the antenna on your Jeep window frame, with no ground plane.  They do make no ground plane antennas which a lot of people in my area are using on tractors with fiberglass cabs.  They are a compromise, however, because there's nothing to stop the RF from traveling back down the coax (the reason mounting through a metal roof is best).  When RF travels back down the coax, it distorts the radiation pattern and you've got you performance basically resembling a hand-held radio with a little more power.  You can put ferrite chokes on the coax near the base of the antenna to help a little.

Here's a link to a Maxrad no ground plane antenna that's pretty popular around here:


Another option is to use a through-glass antenna which mounts on the windshield.  Here's an example:


Either one of these would be suitable for GMRS operation.

Best of luck,

Derek, WRMD298 - N4DBM




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