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Rubber Duck vs Mag Mount?

Discussion in 'Mobile Radio' started by K6BAT, May 8, 2015.

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  1. K6BAT

    K6BAT Ham Member QRZ Page

    I recently got a mag mount antenna, which I stuck on my Jeep wrangler, inside on the wheel well. The antenna element is above the metal part of the body, and I believe the signal should have no problem escaping the canvas soft top or plastic windows.

    http://www.amazon.com/gp/product/B0045EQUBK/ref=oh_aui_detailpage_o09_s01?ie=UTF8&psc=1

    I am transmitting about 5 watts on 2m and 70cm, with an SWR of 1.2 and 1.4 respectively. Using an Anytone TERMN-8R radio.

    When I try to hit distant repeaters, I seem to do better with the rubber duck antenna than when plugged into the mag mount. Yes it's an un-scientific test, but it's repeatable every time - the rubber duck gets there and opens it up, and the mag mount does not. Nearby repeaters are no problem for either antenna.

    Would it help me to scrape paint off that wheel well for a better contact? Should I actually see continuity between the coax shield and the car body, or wouldn't you expect that from a mag mount?
     
  2. K4VLF

    K4VLF XML Subscriber QRZ Page

    Try putting it on your hood. I assume the roof is non-metallic?

    Generally, the higher, the better. And you need metal under it to act as a "ground" or counterpoise (capacitive coupled).
     
  3. K6BAT

    K6BAT Ham Member QRZ Page

    Thanks. Yeah next I'll try moving it around the vehicle to see if that helps. The top is a soft one, a canvas material.
     
  4. K6BAT

    K6BAT Ham Member QRZ Page

    I think you were right. I tried scraping paint away from the wheel well (it's under carpet), and it made no difference. I tried various places on the vehicle, and the best place I can find is the cowl or hood. Maybe I'll have to start looking for a hood lip mount instead of the mag mount, because I think the mag mount will eventually scratch my paint, taking it on and off.
     
  5. KK4PLD

    KK4PLD Ham Member QRZ Page

    Would changing to a bumper mount make a difference? Being metal I would assume that you would have better grounding. I am sure there are off road and jeep forums that installed radio equipment as well, they may provide good installation practices that would help you as well.
     
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Your antenna needs to ideally have at least 19 inches around the base in all directions to work properly . . . and there is no real advantage on VHF to actually connect the braid to the metal, as there is sufficient coupling by capacitance. (unlike on HF, where magmounts don't work very well, especially on the lower bands).

    I'm confused about your vehicle . . . although it may be a pick-up style, with an open back covered by plastic or canvas, surely the cab has a metal roof?

    In which case the best place for your magmount would be right in the middle of the cab roof.

    Most mag mounts I've ever seen have rubber underneath or even felt (like on a pool table), so they don't scratch the paint. If yours hasn't, why not glue some on?
     
  7. K6BAT

    K6BAT Ham Member QRZ Page

    Mine is a Jeep Wrangler, and those either have a fiberglass "hard" top, or a soft top - no metal roof. This means there aren't a lot of horizontal metal surfaces to attach an antenna! Some people put theirs on mounts between the tailgate and spare tire, which I've heard isn't great, because there is very little ground plane. There's also a tail light mount, that sandwiches between the body and tail light, and I might try that. Then there is the cowl mount, which requires drilling some holes and I might try that as well. The bumper is a solid piece of steel, but it may position the antenna a bit low for the signal to clear the body of the Jeep. Thanks so much for your input!

    2006-jeep-wrangler-tj-front.jpg
     
  8. G3YRO

    G3YRO Ham Member QRZ Page

    Oh I see the problem . . .

    The mounting positions you've suggested are really way too low for a VHF antenna . . .

    So how about a simple vertical dipole, mounted halfway along one of those horizontal bars at the back? That would work well, as of course you no longer need a groundplane! (I've often used cheap aluminium FM Broadcast dipoles on 2m, with each leg cut down to 19 inches). You could just make something up so you just clip it on when operating mobile.

    OK for 2 metres . . . but no good as a dual-band antenna, of course.
     
  9. K4VLF

    K4VLF XML Subscriber QRZ Page

    Dead center in the hood is the best option I see - any other place will prolly give you very poor RF performance.

    Paint protection: Mag mount may have a plastic film on it already, and you can add some more, like visqueen, if it is not too slippery. And it is an off-road jeep - what's a couple of scratches amongst friends anyway???
     
  10. KE5MC

    KE5MC Premium Subscriber QRZ Page

    At first I assumed mag mount was placed directly on the horizontally mounted spare's rim in the back cargo area. Is your mag mount only placed on top of the carpet/fiber panel covering the spare in the cutout below the level of the cargo area? If so that explains the poor performance and you can see why it is working better on the cowl and hood where the magnets are in direct contact with the sheet metal. Mag mounts will over time scratch the finish.

    I use a lip mount at the corner of a Toyota Prius. I can only tell that the antenna pattern is distorted by the location when making U-turns or clover-left turns on the interstate and the change in strength is minimal and only for distant repeater with mid-scale signal strength. Its a compromised location that works well enough for me.

     

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