Tuning a 1/2 wave GMRS antennna

[kirk5056]

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? 

[kidphc]

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.





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[Lscott]

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.