Wimrs NVIS mobile antenna from Motorola

Discussion in 'Mobile Radio Systems' started by K4KWH, Oct 9, 2018.

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  1. K4KWH

    K4KWH Ham Member QRZ Page

    I was looking at this antenna @ https://www.ameradio.com/doc/Micom_FAD1410_brochure.pdf-. It is a commercial tuner-based all-band HF antenna. It is in use by, for example, Civil Air Patrol and other commercial and military stations. I've seen them up close, but I don't know how they really perform. Prolly pretty expensive, too! At first glance, I would think they can't work all that well. First of all, they are folded across the roof of the vehicle due their length (20'feet or more). Obviously, there is no choice due to the length of the thing.

    I've never been a fan of tuner-based mobile HF antennas. While a tuner/coupler can yield convenience and *some* fair performance on a base station, they fall far short in mobile operations. A mobile, first of all, is at a disadvantage from the get-go compared to the same, comparable, base station on the same frequency. It loses height, it is moving thru a changing (fluid) environment, it encounters different objects such as other vehicles and buildings. It's relation to ground changes as it encounters different soil compositions, moisture, and density. So a mobile antenna needs to be the most efficient one possible in order to overcome these obstacles. This is why the old (ugly?) Bugcatchers and screwdrivers with large center loading coils usually get the nod when it comes to performance. The screwdriver, IMHO, is the one that *I* endorse, especially since it is able to rapidly move from one frequency to another and becomes a resonant, low SWR radiator (if tuned correctly by the operator) at the frequency of choice. It also overcomes many of the obstacles mentioned well enough to make a mobile punch thru the hash with a fairly decent signal. Since a tuner/couple does NOT "tune" the whip, but merely provides a 50 ohm match and absorbs heat, the antenna system is severely handicapped when compared with other types of mobile HF antennas.

    So I saw this commercial "Mobat" antenna, and it's made me curious. How well DOES it work? And one that I looked at in person appeared to be installed incorrectly (?) For one thing, it was mounted on the back bumper of a van, then the whip went across the roof of the van to the right front side. It was then pulled down within 4" of the top of the van, and I thought, "This can't work". There was a metal bracket, insulated with plastic about 4" mounted on the roof, thru which the whip resided. IMHO, most of your signal is going into the ROOF of the van!:confused::eek: Changes I would make are. 1) get rid of the metal bracket, 2) use PVC with a "T" and a base plate, and raise the whip at least 2' above the top of the truck. This would help raise the whip out of the near field and also insulate the whole business.

    So all you antenna gurus take a look and tell us what you think of the Mobat antenna.;)
     
  2. WB7OXP

    WB7OXP Ham Member Volunteer Moderator Volunteer DX Helper QRZ Page

  3. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Just spend a bit of time on my web site. Read the Antenna Myths article. Oh! And make sure you read the NVIS Myth sub-section.
     
    K8AI, WA8FOZ and KE5PPH like this.
  4. K4KWH

    K4KWH Ham Member QRZ Page

    And I posted this as a curiosity item, Alan, and as a point of mere discussion. It is NOT an antenna *I* would purposely choose! However there are some commercial and military units that DO use this thing. IMHO, there are not very good performers. Based on my experience and theory, the Mobat antenna can't be a very good performer; I saw that during Hurricane Matthew a couple years ago. The very fact that it is bent across the top of the vehicle (signal absorption, etc) is enough to make it suspect. Now I have also taken a screwdriver and fashioned a half-dipole of about 12-15 feet, and slung it up at an angle into a tree in the opposite direction of the vehicle and used the screwdriver unit to tune the whip. I got quite satisfactory results with that!
     
  5. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Jerry, I won't dispute the results you got. However, it WAS NOT a result of NVIS propagation. In order to accomplish NVIS, you have to have a significant amount of very-high angle radiation (≈80°, which is almost straight up!). All of the issues surrounding NVIS can be easily proved by modeling any so-called NVIS antenna using EZNEC or other numerical engine.

    If you really want to try to play with NVIS, you'll need a very low dipole antenna, no more than 6 to 8 feet off the ground. A shorter reflector almost on the ground is also required. If you do the placement and spacing correctly, you'll end up with a lobe which is rather broad is width, covering an arc of about 75° at its 3 dB points. And, the most important ingredient of all, the right MUF. Oh! And forget trying to get an NVIS pattern over about 5 MHz under any MUF condition. At this point in the sun spot cycle, you'd be lucky to get any NVIS on any frequency over about 2 MHz.
     
  6. KL7KN

    KL7KN Ham Member QRZ Page

  7. K4KWH

    K4KWH Ham Member QRZ Page

    These days, we're lucky to get much of anything!;) And most antennas work to some degree. The Mobat just wouldn't be my choice for my own operations.
     
  8. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Myths die hard, and this is yet another example why they do ( http://radionerds.com/index.php/AS-2259). Anyone with the ability to use EZNEC can prove to themselves, that the depicted antenna will not produce significant radiation about about 60°. Think about this... Near VERTICAL Incidence Skywave.
     
  9. K4KWH

    K4KWH Ham Member QRZ Page

    But yet people and agencies prefer convenience rather than performance. This is the objection I got when I argued with some people that ALE in a mobile doesn't work very well because of the limitations and restrictions of current HF mobile antennas. Most screwdriver antennas cannot react fast enough to deal with the possibly rapid change in frequency, or to give accurate results for what is known in certain circles as a "handshake" signal where the radios in a network send out signals to each other in order to lock onto each other. SO these agencies, and in particular, one I am involved with, use these Mobat antennas for 1) convenience, 2) to make up for lack of training in HF ops, and 3) to be able to cope with these rapid changes in frequencies that *may* occur. The results that I am seeing are a distinct reduction of communications effectiveness. So I suggested that we abandon ALE in favor of analog, single-frequency operations for mobile until such time some unknown new innovation in antennas comes along economical enough for our use. I got all kinds of objections, including that we would be going back to the "dark ages":) HUH?:confused: My objective was/is to provide effective, reliable message handling out in the field. Example: Hurricane Andrew a couple years ago. People came back to me saying as how they were unable to communicate from the beach area of eastern NC to the EOC in Raleigh. I knew why. It was because of the tuner-based, all-band HF antennas (Mobat) on those mobiles. I.E. COMPROMISE! By contrast, I demonstrated a few years back how, using a conventional, screwdriver style antenna, we at our local unit were able to reliably call the state-wide HF net on 60 Meters, and do it day in and day out.
    The NCS took check-ins, passed/received traffic sitting in a van using an analog, single frequency Motorola Micom II. IN disaster relief, we traveled to all points in the state and passed traffic on frequencies from 4 to 30 MHZ at will in varying conditions. When I presented the results to the powers that be, they (rather a "he"), it was called "garage tech", and "ham radio garbage". I said nothing, but I took high offense at this!:( And I thought, I'll blow your a** off any day of the week in a radio contest of signal effectiveness:D using my "ham garbage":D:D

    Its pretty obvious what I think of the Mobat, eh?;)
     
  10. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    So would any random dipole. But it would be random, not optimized. We as hams can get vastly better performance with purpose built antennas, installed in optimum installations.

    There is no reason for a ham to not use a resonant half wave dipole for the target band, and install it in a level configuration somewhere around 0.125 - 0.15 Wavelengths high.

    Buying one of these military antennas would be a waste of money by every standard.
     

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