A different kind of mobile 2m antenna.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KJ4ZQK, Dec 28, 2010.

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  1. KJ4ZQK

    KJ4ZQK Ham Member QRZ Page

    Does anyone know if anybody makes a 2m mobile antenna with a counterpoise?

    I drive a gas tanker. I am in and out of this truck daily. I can't us a magmount as the roof is aluminum, so I will be using a clip on mirror mount. The problem, no metal under the antenna. The built in coax is appalling so I won't screw into the built in system. Given the poor operation of CB in these trucks I am pretty sure the problem lies in the lack of an RF mirror for the antennas.

    The antenna can be no more than 1/2 wave long or my max legal height will be exceeded. At this point I am waiting to get my Antenna Book and am thinking of trying to build something on my own, though this seems like a waste of time considering the low cost of most mobile 2m antennas out there.

    Thanks,
    Norman
     
  2. W5RB

    W5RB Guest

    A collinear antenna would work better in the circumstances , but is likely to exceed your height limit . If your mirror mount is metal , it probably will offer a suitable counterpoise , since ~ 19 inches is plenty , compared to the much greater length needed to be effective for CB .

    Dunno if it's a possibility in your installation , but Hustler makes a ground plane skirt , designed to convert its mobile antennas for base use , have a look and see if it's something that could work for you .

    http://www.hamradio.com/detail.cfm?pid=H0-003397
     
  3. W8JI

    W8JI Ham Member QRZ Page

    The mirror, assuming it is reasonable size truck mirror with metal frame, should be an adequate counterpoise for two meters. It would be like having a ten foot mirror frame for CB.

    73 Tom
     
  4. AC0FP

    AC0FP Ham Member QRZ Page

    Several manufacturers make a "no ground plane" 1/2 wave length antenna for 2 meters, I think MaxRad makes some. These antennas are generally intended for use on fiberglass boats and cars where there is no ground plane available.

    Don't make the mistake of listening to those who will tell you how much better this type of antenna will work if you mount it over a ground plane! :(

    73,

    fp :)
     
  5. K9ASE

    K9ASE Subscriber QRZ Page

    what about mounting to the frame of the tractor behind the cab instead of the mirrior the highth may put the antenna above the cab but not beyond your restriction?
     
  6. KA9MOT

    KA9MOT Ham Member QRZ Page

    Your max legal height is 13' 6". You should be able to run an antenna at least 5 feet long. I don't think antenna height counts for your max legal height anyway. You just need to be able to clear bridges. Do you need to clear a Boom for loading?

    I used to use my right CB antenna as a "feeler" for low bridges. If the antenna hit (very low speeds) move left or back out.

    On my last truck, I modified the mount of an Arrow Antennas Dual Band J-Pole to work on the right mirror of my 379 Pete. That son of a gun worked good and I still use that antenna today here at the shack.

    http://www.arrowantennas.com/osj/j-pole.html

    Look close, you'll see it on the right mirror (The truck is dirty). You will also see a Larson Dual Band NMO that did not work as well as the Arrow:

    [​IMG]

    BTW, This truck had 1.6 million miles on it. The paint is faded but she was in excellent Mechanical condition and LOOK! NO DENTS!!!! A big truck with no dents is hard to find now-a-days.
     
    Last edited: Dec 28, 2010
  7. KA9MOT

    KA9MOT Ham Member QRZ Page

    Yeah because all VHF/UHF antennas work so well when surrounded by steel! NOT!!!!
     
  8. AD5MB

    AD5MB Ham Member QRZ Page

    decoupling sleeve. solder a 1/4 wavelength conductor - copper pipe, or a shield from a larger cable - to the mounting end of the coax shield. if you disassemble a standard 2.1 dBi wifi antenna you will see that it consists of 1.25" of coax with the shield stripped off, and a 1.25" decoupling sleeve soldered on. that's why they have that characteristic taper halfway up.

    I would verify the SWR and resonant frequency with an MFJ-269 before I trusted my finals to it.
     
  9. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    How about putting a magmount out on the hood! (if you aren't driving a cab-over).
     
  10. KF5GCL

    KF5GCL Ham Member QRZ Page


    I have wondered about that. Are you suggesting that it won't work better than a conventional antenna, or that it won't work at all? What's the story on that?
     
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