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Test a Marine VHF Antenna

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  • 02-12-2020 1:34pm
    #1
    Storm 10
    Registered Users Posts: 5,889 ✭✭✭


    I have a Marine VHF Antenna on the chimney and it seems to be deaf on the marine channels all I am getting is a cracking noise and broken voice, I have a whip which I put up and its perfect, can someone tell me how to test the antenna without taking it down with an OHM meter, I know with a dipole antenna the outer braid goes to the bottom and the centre to the top, with this antenna you only get a reading when both wires are joined, I don't have a clue what setting to set the OHM meter to and how to check the antenna, I have attached the meter not sure how to test an antenna any help appreciated.


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    #2
    Tow
    Registered Users Posts: 6,679 ✭✭✭Tow


    There are a number of ways to test it, but multi meter is not the best. Depending on the design the antenna may (electrically) be a direct short on the meter or open circuit.

    With a closed dipole, put the meter on Ohms 200 and you should get close to 0 when measuring between the inner core and outer braid of the coax.

    You already know it does not work, it is 90% the coax (water) or it's connection into the base of the antenna.

    If can transmit into it then a SWR meter is the normal way of testing. There are also antenna analyzers, which sweep then for a range of frequency's. Or a grid dip meter is the old school version.

    When is the money (including lost growth) Michael Noonan took in the Pension Levy going to be paid back?



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    #3
    Storm 10
    Registered Users Posts: 5,889 ✭✭✭Storm 10


    Tow wrote: »
    There are a number of ways to test it, but multi meter is not the best. Depending on the design the antenna may (electrically) be a direct short on the meter or open circuit.

    With a closed dipole, put the meter on Ohms 200 and you should get close to 0 when measuring between the inner core and outer braid of the coax.

    You already know it does not work, it is 90% the coax (water) or it's connection into the base of the antenna.

    If can transmit into it then a SWR meter is the normal way of testing. There are also antenna analyzers, which sweep then for a range of frequency's. Or a grid dip meter is the old school version.

    Thanks for that I will try what you say I don't transmit on it receive only like listening to Coastguard Helicopters and shipping on the West Coast, as a matter of interest what reading would you get with a dipole antenna and would you use the same setting on the meter.


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    #4
    Storm 10
    Registered Users Posts: 5,889 ✭✭✭Storm 10


    When I selected 200k on the OHM meter I got a reading of 000, when I switched to 200 on the meter I got a reading of 0.14, how does that look


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    #5
    humaxf1
    Registered Users Posts: 865 ✭✭✭humaxf1


    Test the coax first...
    • disconnect from the aerial end AND the radio
    • set your multi-meter to continuity
    • one probe to the centre of the radio end coax plug - the other probe to the shell of the connector
    • if the meter beeps, there is a short which will explain the lousy receive

    If using Ohm setting on the MM, set to lowest scale, e.g 200. If there's a short, you will see a low reading anywhere between 2.0 or 5.0 (depending on how long the coax run is.

    As another user has said, if it's a short in the coax, it's most likely due to water ingress. In this case, replace the coax and aerial end connector. Ensure to seal up the aerial end to avoid water ingress.

    When you got the .14 reading, was the aerial end of the coax disconnected?


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    #6
    Storm 10
    Registered Users Posts: 5,889 ✭✭✭Storm 10


    humaxf1 wrote: »
    Test the coax first...
    • disconnect from the aerial end AND the radio
    • set your multi-meter to continuity
    • one probe to the centre of the radio end coax plug - the other probe to the shell of the connector
    • if the meter beeps, there is a short which will explain the lousy receive

    If using Ohm setting on the MM, set to lowest scale, e.g 200. If there's a short, you will see a low reading anywhere between 2.0 or 5.0 (depending on how long the coax run is.

    As another user has said, if it's a short in the coax, it's most likely due to water ingress. In this case, replace the coax and aerial end connector. Ensure to seal up the aerial end to avoid water ingress.

    When you got the .14 reading, was the aerial end of the coax disconnected?

    No the coax was connected to the antenna because its on the chimney and with the lousy weather I did not get the chance to take it down, however the coax to the cable that comes from the antenna which I had to join because it would not reach the radio is perfect, I always had a good reception on any antenna on the roof but this one beats me because its a Marine antenna.


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    #7
    pixbyjohn
    Registered Users Posts: 6,250 ✭✭✭pixbyjohn


    Have you ever received anything on your Marine antenna? When did you install it?


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    #8
    Storm 10
    Registered Users Posts: 5,889 ✭✭✭Storm 10


    pixbyjohn wrote: »
    Have you ever received anything on your Marine antenna? When did you install it?

    I always received Valentia Coastguard on the small steel whip, but all I am getting now is cracking voice noise on the new antenna, I took down the steel whip as its on my small boat but nothing on the new antenna, I get Galway and Lough Ree Coastguard loud and clear but their transmitters are close by, I put the antenna up 3 weeks ago, I hope weather permitting to take it down and check it out for damage.


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