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  • UHF Antena Resistance

    Guys,

    I"ve installed a UHF antena but I'm not sure if I've terminated the cable correctly to my FME connector.

    When I check the resistance from the centre core to the cable screen I'm getting 0.6ohms. Is this about right or have I stuffed up the fme termination?

    Thanks
    2019 GXL, Bullbar, UHF, Redarc Brake Controller, Tow Bar, Secondary Fuel Filter

  • #2
    Re: UHF Antena Resistance

    I think you have
    stuffed up the fme termination?
    . If you measure the resistance between the centre core and the outer shielding you should read a very high resistace ( Infinity). That is it should read the same as when the leads arent connected to anything.
    You must have caught a bit of the shield when you pushed the connector together. Try again leaving less of the shield exposed.

    Wayne

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    • #3
      Re: UHF Antena Resistance

      Sparksy,

      Does it matter if the antena is ground independant design or not? After I posted I found a link stating that ground independant antenas would be close to zero ohms and ground dependant antenas would be infinity ohms.
      2019 GXL, Bullbar, UHF, Redarc Brake Controller, Tow Bar, Secondary Fuel Filter

      Comment


      • #4
        Re: UHF Antena Resistance

        Taken the antenna off and the cable should be an open circuit between the conductors, if not you may have a single shield wire touching the centre conductor. This will reduce your transmission power.
        [url=http://www.myswag.org/index.php?topic=12264]My Prado[/url]

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        • #5
          Re: UHF Antena Resistance

          I have a RFI CD5000. The cable is fixed at the antena end.
          2019 GXL, Bullbar, UHF, Redarc Brake Controller, Tow Bar, Secondary Fuel Filter

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          • #6
            Re: UHF Antena Resistance

            You should be able to unscrew the whip
            [url=http://www.myswag.org/index.php?topic=12264]My Prado[/url]

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            • #7
              Re: UHF Antena Resistance

              Its always good practice when you finnish connecting both ends to test with a mulitimeter the continuity (test for a open or closed circuit ie short circuit) in the ohms or when you touch the 2 probes together it makes a buzzing noise. This means you have a closed circuit, you may get a slight reading even 0.01 ohms will effect your signal strength when transmitting. Your ground is not so much a problem as most new UHF radios will transmit and receive without one, although if the centre core is touching the outer sheild it wll greatly affect the transmitting output. Some antennas (with the wire on a spring) sold are fixed and the plug is molded, theses are known as tuned antennas. If you happen to damage the cable you cannot fix it. You cant join the cable without some difficulty and if the plug has to be cut off to fit through the fire wall your just thrown away your money.
              97 Vitara 1.6 JX Day Tripper(550klms range), 98 Prado V6 LPG GXL for Touring (1400klms range), Offroad Cub Camper for Overnight.

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              • #8
                Re: UHF Antena Resistance

                The only proper way to test the antenna is with a VSWR meter. If you know someone in the two way radio industry, or a HAM radio operator you may be able to get a loan of one. It's always a good idea for anyone to get their antenna checked with this device after installation and then every year or so, even if you think it operating OK. It'll show up issues that a multimeter can never do.
                This will give you a reading of forward power ( going up the stick ) vs reflected power ( coming back ). Anomolies including shorts, breaks, water infiltration, corrosion, incorrectly tuned or wrong band antenna whip etc will show as a high reflected power.
                For a simple test an ohmmeter is OK but you need to know the how the antenna is designed, it may look like a short at DC ( which is what you are measuring with a multimeter ) but not at RF frequencies. The VSWR meter above tests at the actual UHF frequencies you are using, using the power output of your radio.
                The simple 1/4 wave ground plane type antenna is a DC open circuit between centre pin and outside ferrule of the plug but it might not always be the case with ground independent ones.
                Cheers, Wayne

                Comment

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