Full vs Half vs Quarter Wave Mobile Antenna

Discussion in 'Mobile Radio' started by AF7JA, Jul 4, 2021.

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  1. AF7JA

    AF7JA Ham Member QRZ Page

    I decided to go with a separate GMRS unit install instead of just accessing GMRS using my VHF/UHF (why. . . well, there is this "it's the law" thing that is nagging at the back of my mind). This means installing another antenna on my FJ Cruiser.

    This is enough antennas to push me over the line, drop the headliner and install NMO mounts instead of the "clamp-On" that I am currently using. This got me to looking at the Larson NMO tunable, a favourite when I was playing with radios more often. Realizing how short the wavelength is, has anyone played with different lengths?

    The "no ground plane" feature of half-wave is pointless as it is going to go on a big sheet of steel (the roof). The bit of gain is more valuable to me. I then got to thinking of full-wave, I can't think of anyone I know ever using full-wave, there has to be a good reason not to, but I am not thinking of any. There is the point that going full-wave gives very little gain over half-wave. . .

    The antenna comes pretty long, with the intention of being cut, but it can't be un-cut. Any thoughts?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Full-wave needs the same kind of matching network as half-wave, but the antenna is longer and will likely hit more stuff. Also, the full-wave will have more high angle radiation (and reception), which may only be of benefit if you're driving in a deep canyon; under more normal circumstances, it actually detracts from performance.

    I strongly prefer 5/8-wave, which requires a very simple matching network (since it has -jX, only a small series inductor is required to cancel that) and is the most common length for mobile VHF whips.
    K0UO and W0GSQ like this.
  3. K6EEN

    K6EEN Ham Member QRZ Page

    At GMRS frequencies (462-467 MHz) most "gain" antennas are going to be co-linear designs anyway, 5/8 wave over 1/2 wave or whatever, longer than a quarter wave. Look at Larsen's commercial UHF offerings. Larsen NMO450 you can tune to cover GMRS, either the "C" model with the enclosed coil, or the "B" model with a black whip and the exposed coil. You can add a Larsen spring (SKU SPRING or SPRINGB) to the whip if you want to flex more than the whip alone gives you. The "wide band" version with a special base coil is also an option, NMOWB450C, might be able to make it cover the upper portion of the 440-450 MHz ham repeater band as well as 467 MHz.

    A plain old cut-to-tune quarter wave could work, too. Just use the Larsen cut chart supplied with the antenna and an analyzer for fine tuning.
    K7JEM likes this.
  4. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Go 5/8
    AB2YC and K3RW like this.
  5. K8JD

    K8JD Ham Member QRZ Page

    Practical V/UHF Antennas
    A fullwave vertical for U/VHF mobile is unheard of ! It's too long and it radiates too much energy up into the sky that's wasted.
    The BEST simple (single metal whip length) is certainly a 5/8 (or 230 electrical degrees exact) This has the strongest low angle radiation, closest to the horizon for a vertical antenna.
    Next would be a Phased segmented antenna used on UHF mostly due to the size.uually a 5/8 over 5/8 wave for the highest gain toward the horizon. (about 30 inches tall) The small bump in the middle is a wound up Stub to revrse phase so all the RF is going in the same direction (for gain).
    Beyond that, they get too complicated for mass productions . too tall for a mobile.
    I once had a 5/8 wave over a QW vertical for 2M on top of my vehicle. That was 7 ft tall on top of the car. Nearly 12 Ft overhead clearence !
    It made for GREAT simplex on FM but trimmed low hanging tree branches. :eek:
    Last edited: Jul 8, 2021
  6. K8JD

    K8JD Ham Member QRZ Page

    Answering last first; First you calculate the length for your frequency (band) with a formula or look it up on the cutting chart that comes with factory made antennas.
    THEN cut it a bit LONGER than that value. Mount it in place and Measure the SWR (maybe wirite it down) .
    Then snip a small part of the end off and RE-Mesure SWR. reading should be a little lower. .
    Repeat in small increments until you see a slightly higher SWR. STOP HERE.
    Often the longer antenna rods we trim had a "static ball" or eye poke protector on the thin end. We had to unscrew the set screw from the base coil and notch the thicker end of the rod with a file where the cut was made and snap it with pliers.

    Backing up one Question to "NGP" antennas, usually a halfwave with a transformer coil on the base, THEY are DESIGNED for a nonmetallic roof like the back box of an ambulance , that is made with a fiberglas roof.
    If you have a metal roof vehicle , you shoud not even think of buying a NGP antenna.

    Back to first part. TOO MANY ANTENNAS :eek: You should have seen some of the service vans I drove around for work. I mounted a whole string of antennas on the roof from front to back 6 in whip for UHF, 18 inch for VHF, colinear for 800 MHz, 4 ft base loaded for 30 - 40 MHz, another one for 40 - 50 mHz Finally another one for the company radio . Most of the coax lines went to a workbench in the back to test customer's radios on air for a final working test after servicing them. Sometimes I had my own 2M FM rig and another roof mounted antena! :rolleyes:
    Last edited: Jul 8, 2021

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