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Larsen NMO 2/70 SWR & Reactance Chart, Plus a Tuning Question

Discussion in 'General Technical Questions and Answers' started by K4GMO, Apr 12, 2017.

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  1. K4GMO

    K4GMO Ham Member QRZ Page

    Good evening, all. I just analyzed my Larsen NMO 2/70 with a new MFJ-259C. I then graphed the SWR and reactance in .1 MHz increments from 140.00 - 148.30 MHz, as shown in the attached picture (if the image is too small, just know that the sweep shown is 140.00-148.3 MHz, the shaded blue area is 144.00-148.00, the green line is reactance with a scale of 0-16, the red line is SWR with a scale from 1-3).

    Unless I am mistaken, it seems that this antenna's SWR and resonant frequency are below the 2m band. If that is the case, I would like to get advice re: "tuning" this antenna if at all possible. I've read dozens of posts re: the antenna not being tunable, and others re: people trimming the antenna to tune it ... and I really don't know what to make of it all. What I do know is that my antenna seems to be optimized for below the 2m band in my specific installation, and most of my area repeaters are unfortunately above 147.00 Mhz, which is in the area of my highest SWR (2.0-2.5).

    Some preemptive answers to questions I might get:

    1. Installation is the driver side front fender on a Jeep wrangler.
    2. NMO Mount is a Laird MBOX, plus an Accessories Unlimited AUJ1.
    3. The mount is grounded/bonded to the chassis with 3" of Electric Motion Company EM2081, and continuity verified with a voltmeter. I have the chassis, frame, hood, tailgate, etc., bonded in several places with the EM2081, all verified with the voltmeter.
    4. Cable is 9' of genuine Times Microwave LMR-240 Ultra Flex. Coax tested good and shows .3db loss as expected.

    I appreciate any help that any of you might provide! Thanks!


    SWR and Reactance graph:
    Antenna Analysis.png

  2. N7WR

    N7WR Subscriber QRZ Page

    I have used that antenna for many, many years and have never had a need to "tune" it. The problem I see with your install is that there is no ground plane under the antenna. To work properly and, IMO, to give you the best SWR in the ham bands the antenna needs a ground plane under it, not to one side of it. With the Jeep top being fiberglass (I assume) I don't know where you would mount it to improve the situation. An SWR of 2.0:1 or 2.5:1 is not impossible to live with unless you are a perfectionist.
    N8EKT and K4GMO like this.
  3. K7JEM

    K7JEM Ham Member QRZ Page

    Since this is a dual band antenna, what does the SWR look like on UHF? I wouldn't cut it unless you know that, unless you are not going to use the antenna on UHF.

    It could be that your mounting situation is throwing the length off somewhat. My thought is that the hood is in very close proximity with the coil, normally the coil will be mounted above the "plane" area.
    K4GMO likes this.
  4. KK5JY

    KK5JY Ham Member QRZ Page

    I suspect that's the cause of a lot of the detuning you are seeing. The antenna is capacitively coupling to the roof frame and pillars, since they are in the near field on 2m. You can trim the antenna, but IIRC, Larsen recommends against it, since the changes also affect the 440 tuning.

    I have had great luck with all of the Larsen dual-band antennas, but unfortunately, the design does assume a roof-mount for good SWR. Every time I try a trunk or L-bracket install on a trunk or hood, the detuning is rather pronounced. I used one on a Camry trunk, and I could never get it better than 2:1 on either band. When I mount them on a truck roof, they work FB.

    If you are only interested in 2m, you could trim the antenna to raise the resonant point... it should be pretty straightforward, since you already have good SWR, just a few MHz too low.
    K4GMO likes this.
  5. K4GMO

    K4GMO Ham Member QRZ Page

    OK, thanks everyone! Yeah it is a tricky situation, and unfortunately my analyzer does not cover UHF (which I don't use much of anyway). The Jeep is a fiberglass roof, so my options are in the middle of the hood, off to the side, or on the tailgate. This is unfortunately the best place I can think of without obstructing my view.

    I've got some thinking to do...


  6. N7WR

    N7WR Subscriber QRZ Page

    For what it's worth I'd leave it as is and see how it works for you. It may work just fine and the SWR is not so high as to degrade output or hurt anything. I've done hundreds of public safety radio installs and some of them wound up with antennas just like your install---and they worked fairly well which is a good thing since options were non existent.
    W6KCS and K4GMO like this.
  7. K7JEM

    K7JEM Ham Member QRZ Page

    Here's a thought. If you don't use 440, you might remove the whip part and replace it with a straight SS whip, about HW long. Then you can run your graphs and cut that antenna if you want to, reserving the original whip. You should be able to trim the antenna much closer than 2 to 1 over the 2M band.

    Or purchase another 2M only complete antenna and just swap them out as needed.
    W6KCS and K4GMO like this.
  8. K4GMO

    K4GMO Ham Member QRZ Page

    Excellent idea. I just might do that; it's always good to have a spare element anyway!
  9. W6KCS

    W6KCS Ham Member QRZ Page

    Welding or brazing rods make pretty good experimental antennas that are easy to trim, to see what the rod should be cut to.
    AD5KO and K4GMO like this.
  10. K8JD

    K8JD Ham Member QRZ Page

    Most V and UHF mobile antennas have optimal performance when mounted on a flat roof with at least a QW radius of metal around the base (at the lowest freq used) . anything else, and you may have to trim the length down slightly, when the resonance seems below the band.
    If the resonance seems above the band you are out of luck since you can't stretch the metal whip. .
    K4GMO likes this.

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