Help Ant Problem

Discussion in 'General Technical Questions and Answers' started by W8GFT, Jun 14, 2002.

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  1. W8GFT

    W8GFT Ham Member QRZ Page

    I had a Comet SBB2nmo mount on a magnet mount
    and when I did my swr was about 1.4 -1.6 depending
    on power out and freq.

    Well I installed a perm mount nmo on the roof of my
    truck. Since then when I am running 50watts my swr
    jumps to 2.5-4.0 depending on freq.

    Now I put my sbb-2 back on the mag mount good swr
    across the band. I put a simple 1/4 wave on the mag
    and 1.1 - 1.4 across with 50 watt output

    But put either antennas on the perm mount the swr
    goes up.

    Now I have checked no shorts in the feed line. I cleaned
    the underside of the cab roof to get a good ground.

    I can touch the antenna ground and the chassis using a
    ohm meter and it is good. 1.2 is the reading.

    But use either antenna on the perm mount my swr goes up on 50 watt at 5 and 10 it is ok. One of my friends usedhis swr meter and I used mine. we tried different lengths of patch cable to hook them up etc.

    We used 2 different volt ohm meters. the mount is made
    by maxrad it is a ma nmo mount.

    the mag mount is by maxrad. I am perplexed. I thought
    making the ant perm would help and improve the swr
    not the other way around.

    Any help is greatly appreciated,


  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Are you seriously measuring "1.2" Ohms using an Ohmmeter to measure the cable's shield at the connector end, to chassis ground? If so, that indicates something seriously wrong. It should read "0.000" Ohms, and nothing higher. Even 1/2 an Ohm is waaay high, indicating a bad connection, somewhere.

    If possible, have an experienced professional check your installation.

    SWR should not be higher, or lower, or really any different, using the permanent mount. The advantage of the permanent mount has nothing to do with SWR; however, there certainly are advantages, not the least of which is less damage to your automobile finish, less risk of coaxial cable damage from slamming the door on it, an installation far less likely to draw attention and be stolen, etc.

  3. K9STH

    K9STH Ham Member QRZ Page

    Take the NMO mount loose, remove it from the hole (of course the coax will still be attached and go through the hole). Using a knife blade, scrape the underside of the metal around the mounting hole. Make sure that you get rid of the paint / rust protection that is on the underside of the metal. Reinstall the mount making sure that the outer ring is very tight.

    99 percent of the time this will fix you problem. Most vehicles have some sort of paint on the underside and the "fingers" of the mount do not make an acceptable contact through this paint. By the way, while you have the mount loose, check the coax connections (both the center and shield) to make sure that they are good. Sometimes the ring that attaches the shield comes loose. If so, you can solder it in place (only one connection is necessary).

    Also check the rig end of the coax for proper connector installation.

    In many years of commercial communications work the vast majority of time that things didn't seem right with an ohm meter check it was the paint on the underside of the hole.

    Glen, K9STH
  4. WJ5O

    WJ5O Ham Member QRZ Page

    Hi Tim,
    I run into the same situation a few years back........ seems that the capacitence of the mag mount causes the length of the antenna to be electrically longer to get proper SWR.
    When the antenna is removed from the mag mount and placed on a "thru the hole" mount the capacitence is gone and the antenna is too short. ..... . Hold a "cupped" hand close to the antenna as the SWR is checked. If the Hand capacitence causes the SWR to go up.... Shorten the antenna.... If the SWR goes down .... lengthen the antenna.....Works for Me
    73 Bill WJ5O
  5. W5ATX

    W5ATX Guest

    I'd never had that problem before, but perhaps Bill is correct. I still vote for the paint issue considering your ohmmeter reading. But to do the test Bill describes, you could just as easily test the SWR at different frequencies. If the SWR is lower on lower frequencies, the antenna is too long. Conversly, if it goes up as you go lower, it's too short. Most antennas should cover the entire 4 MHz 2m band with a reasonable SWR - no more than 2:1 for certain.

    Whatever you do make SURE you don't touch that antenna accidently while transmitting. The burn you will get cannot be described in words. No, you won't need the emergency room, but trust me, you'll suffer for WEEKS! RF burns are nasty to begin with, but the higher in frequency the RF, the "deeper" it feels. A burn on your skin is rough, but when it feels like it is under your flesh, on the bone . . . Just be careful.

    (Now you know why microwave ovens cook from the inside out!)

    Good luck with the antenna though, let us know what you find. Scrape that paint!!!!


  6. W8GFT

    W8GFT Ham Member QRZ Page

    Thanks all for the replies so far. When I installed the
    antenna I used the dremel tool with a sanding drum on
    it and sanded down to bare metal. but I am wondering
    if the mount isn't defective. Funny thing happened
    when I got home I hooked up my swr meter to it and
    my highest swr at 50 wts was 1.6 at 145.000 and lowest was 1.4 at 148.900 50 wts but on the 2 lower power
    settings my swr was 1.1 to 1.3. But from what several
    have said my swr should stay the same no matter what
    the power. and maybe very just a very little over the
    spread. That is the weird thing about the when we
    used the mag mount both a 5.00 1/4 wave rod and
    the 30+ Comet had very low swr. The 1/4 wave was
    no more than 1.3 full power across the spectrum.
    The Comet which is a 1/4 wave on 2 meters and 5/8
    wave on 440 2.15/3.8 dbi but the antenna is only 18
    inches long. The nmo kit from maxrad I got was not
    the top of the line. It uses a 3/8 hold and you have to
    push the threaded base up thru the roof and then put the
    nut/brass base for the antenna on up on the roof. the
    cable is just rg-174. my total cable length from roof
    to radio is about 11ft. the one strange thing that happened when I had the mag mount on was I had no
    noise when I went to the roof mount I picked up engine
    noise I put a dc filter on the radio and it solved the
    problem. Ford Late Model 97-2002 F150's Ext cab esp
    have problem with rfi from the fuel pump in the tank.
    Ford has issued several tsb's on this.

    So my deductions from this is the antenna has been
    lenghtened all it can be so I need a longer antenna.
    The mount probably isn't the best. the coax could be
    part of the problem.

    when using the 1/4 rod on the perm mount the swr
    goes thru the roof also.

    So here are my thoughts. I can change mounts and go
    with a uhf style and drill the hold out to 5/8 inch and
    clean the underside more. get a uhf mount ant that is
    a little longer.

    Get a new nmo mount with better coax and connector
    and try it with the same antenna.

    Or buy a new truck and don't drill holes in it and use
    a mag mount. LOL I don't think my wife will buy that

    Thanks for your help and guidance.



  7. W5ATX

    W5ATX Guest

    NMO hole type mounts are readily available.  I see them on Ebay all the time, or from any of the ham radio dealers.  Or look up a 2way radio dealer in your yellow pages.  Yes, it sounds like the mount or coax is defective.  I'd bet on the coax, but . . .   do as you see fit, but something is wrong there, and it isn't the antennas. Oh and there's nothing wrong with Maxrad quality. I'd say you have a coax problem, but if you're not comfy replacing it, do the whole mount.

    Good luck,

  8. W9GB

    W9GB Ham Member QRZ Page

    Tim -

    The NMO has been the standard mount for commercial radio installations (e.g. fire, police, ambulence, government) for years.

    The installation you describe must be properly installed (no shorts to ground).  A VOM can determine this within seconds.

    I would not used RG-174 coax for a 2/440 MHz antenna.
    Although it is very easy to route, it can be easily shorted (heat or meachnical).  At 11 feet and 450 MHz, you have over 2 dB loss - just in feedline. (RG58 is about 1 dB and 8X is less than 1 dB).  

    I have only seen RG 174 cable on the mount, you described, used for lower frequency (< 150 MHz) scanning and GPS antennas - where the +20 db preamp within the GPS antenna makes up for feedline loss.

    73 de w9gb
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