Mobile dualband antenna gain comparison - shorty mag mount vs. Diamond NR770HBNMO

Discussion in 'Mobile Radio Systems' started by KX4O, Apr 27, 2018.

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  1. KX4O

    KX4O Ham Member QRZ Page

    I thought you folks might enjoy a sneak preview of data from my upcoming review of the Diamond Antenna NR770HBNMO and how it compares with a Comet dualband shorty mag mount it replaced. To be fair, Diamond makes a model identical to the Comet so what we are comparing here is antenna styles more than brands. Key points...
    • Each of these plots is a cut along a Subaru Forester's center-line. The front of the vehicle faces left. We are looking at the left side.
    • The Diamond is mounted to the right roof rail towards the rear.
    • The Comet mag mount antenna is on center-line in two positions: one directly behind the Forester's enormous "non-conductive" moon roof and then moved rearward to the center of the actual metal roof area... pretty far back.
    • The relative gain difference between the Diamond and Comet include whatever attenuation difference exists between the Diamond's 13.6 foot RG-316/B or Comet's 12 foot RG-58. Put another way, the antenna "systems" under test include their stock feed lines.
    • The plots are offset together such that the outer ring of 0 dB is set to the highest gain of the three plots for easy relative comparison.
    • These center-line cuts don't tell a much more interesting story of how the vehicle body messes with things in other directions, but I reserve that 3D analysis for my web article.
    • These measurements confirm, more or less, my observations after using the new antenna for about a year.
    Fair warning - Nothing is perfect in the antenna simulation or measurement biz, but the plots below show solid trends to help understand things. Enjoy!

    For 2m...
    [​IMG]

    For 70cm...
    [​IMG]
     
    N0TZU likes this.
  2. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    The patterns look realistic to me. The shorty is just a 1/4 wave 2m antenna that will load at 70cm but with an upward pattern. Apparently the little coil in it doesn't do much.
     
  3. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Would you be so kind as to include the input parameters you used?
     
  4. KX4O

    KX4O Ham Member QRZ Page

    You mean measurement chamber details? It's a near field probe system in an anechoic room. Uniquely it maintains a non anechoic floor emulating ground to a degree. Hence the chamber specifically tests vehicles and their antenna systems as a whole.

    I also have a decent wire NEC model with lots and lots of wires... per your suggestion Mr. Applegate, by the way. Soon I will compare the measurements with the simulations in the eventual article. For now, measurements will have to do.
     
    N0TZU likes this.
  5. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    I agree. I answered your latest conversation, and as I alluded to, we really need one (anechoic chamber) large enough to do HF measurements. It would sure point out a lot of fallacies, braggadocio, and misconceptions, that's for sure!
     
    N0TZU likes this.
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    How about an informal open air test "site" on someone's acreage? It wouldn't need absolute calibration as the desired result is relative numbers between antennas. For antenna testing, incidental signals in the environment shouldn't be an issue (as opposed to EMC compliance testing).
     
  7. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    We could talk on the telephone or air about this, and not be done in 2 hours. One of the major issues with relative measurements outside, are the ambient conditions. For example, the ground humidity effects the conductivity, and over say a 2 or 3 hours period, with nothing else changed, the SWR will vary more than one would expect. This obviously points directly at a change in ground loss. Imagine several vehicles, placed at "about" the same spot, over the several hours it takes to do a shootout, and trying to sort the changing ambient to reflect the true ranking? It just cannot be done satisfactorily.

    And speaking of shootouts... I'm sure you've read what I say about them. All they accomplish are bragging rights (rites?), and most of the time, hostility. And 99% of the time, near-field signal strength is all that's measured. That alone is fraught with problems.

    If you really want to get my ire, talk about bending over a mobile antenna, and converting it to NVIS! On 20 meters yet!
     
    N0TZU likes this.
  8. KX4O

    KX4O Ham Member QRZ Page

    Just so everyone understands... the system that made the measurements resulting in the graphs above measure the near-field (a far-field range of vehicle/antenna combinations in VHF would be enormous) amplitude/phase in a full hemisphere and convert this to far-field via well known conversion techniques. The graphs are relative far-field measurements.
     
    N0TZU likes this.
  9. K7JEM

    K7JEM Ham Member QRZ Page

    I have been using one of these "shorty" type antennas for years (but NMO mount), different manufacturer, but similar in design. It always worked "OK" on 2M, but it has been a pathetic performer on UHF. This chart looks right as to what I experience. I would change to a longer antenna, but it tends to hit objects too much. I might do that anyway, probably would have years ago, but most places I travel have pretty good UHF repeater coverage, so it isn't normally much of a problem.
     
  10. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Yes a proper validated chamber is certainly better (and for automotive you need a "10 meter" chamber - $$$$$ :eek:) but couldn't an OATS could be used depending on what error band is acceptable? I would think it would work well enough to separate the good performing mobile antennas from the dogs which is what I had in mind, not little differences.

    But admittedly I haven't tried it, so I could be very wrong.
     

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