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Vhf radio tx issue

Discussion in 'General Electronics Discussion' started by portishead, Apr 7, 2016.

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  1. portishead

    portishead

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    Apr 5, 2016
    I don't know if any one can help with this..

    I have a vhf radio on a boat.
    I'm trying a new main antenna.
    When I press tx using the old antenna the ammeter moves forward a little.
    If I try the new antenna the ammeter goes backwards a little.
    Is there a reason for this??
    Thanks all.​
     
  2. 73's de Edd

    73's de Edd

    2,944
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    Aug 21, 2015
    Sir portishead . . . . . . .

    I don't know if any one can help with this..
    Not without more exacting specific information . . . . .even utilizing level 12 ESP capabilities . . . .

    I have a vhf radio on a boat.
    I'm trying a new main antenna.

    Brand of antenna and model . . . . . and do you stay on one fixed xmit frequency ?

    When I press tx using the old antenna the ammeter moves forward a little.
    Would this be an inline DC ammeter monitoring the units DC power consumption . . . .OR . . .an inline RF ammeter ?

    If I try the new antenna the ammeter goes backwards a little.
    Considering being on a fixed frequency . . . . . your transmitter must like the tuning / match being presented to it:

    Better with the old antennas . . . length/tuning/adjustment . . . . than with the new antennas length/tuning/adjustment.



    73's de Edd
     
  3. davenn

    davenn Moderator

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    1,892
    Sep 5, 2009
    ammeter ? I assume you mean the signal strength meter on the radio ?

    sooooooo
    what is the radio ?
    what are the antennas, old and new ?
    do you have a SWR meter ?
     
  4. portishead

    portishead

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    Apr 5, 2016
    Hi. So the antennas are just white 2.5mtr boat whips. No idea on make. Yes I am staying on the same channel trying it.
    The radio is a cobra F77.
    The ammeter I refer to is the boats next to the volt meter on the boats main switch control panel.
    No. I haven't got an swr meter.
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    OK

    you didn't answer my last Q
     
  6. portishead

    portishead

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    Apr 5, 2016
    Sorry, I did edit that last bit in. Re swr
     
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    get one, it's the only way you can check the antennas performance/tuning
     
  8. portishead

    portishead

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    Apr 5, 2016
    Should a new radio with a new antenna not just work or is there more to it?
     
  9. dorke

    dorke

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    Jun 20, 2015
    It should work.
    Take into account that there are actually 3 items:
    Transceiver,Antenna and the coax connecting them.
    The marine VHF is a very narrow band 156-162MHz ,
    so the antenna doesn't need any special tuning.

    The ammeter movement doesn't mean much.

    Even without an SWR meter.
    Use the cobra77 internal " Radio self test" option.
    It will check the battery, the transmit level and the antenna. (page 36 in the manual.)

    Then try to communicate in harbor on the low(1watt) level ?
    Same out at sea on the high level (25Watts-good for ~9 km )?
     
    davenn likes this.
  10. portishead

    portishead

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    Apr 5, 2016
    Thanks for your reply dorke. Thats is very useful help. The only thing I forgot to say was that the radio it's self is brand new but on self test it randomly says fail on the transmit level. It's odd.
    Any thoughts?
     
  11. duke37

    duke37

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    Jan 9, 2011
    Get or make a 50Ω dummy load and see if the radio objects when transmitting into this.
     
    davenn likes this.
  12. dorke

    dorke

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    Jun 20, 2015
    1. Make sure that the battery connection is good and firm.
    2. Make sure the antenna connection is good ,clean(no rust etc.) and firm.
    3. If it doesn't complain about the antenna,it could be a faulty unit ,I would contact Cobra technical support.
     
  13. davenn

    davenn Moderator

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    1,892
    Sep 5, 2009
    AGAIN

    A SWR meter in line will tell you if there is an antenna or coax problem

    just because it is new doesn't mean that it cant be faulty

    problem areas ....
    coax connector on the end of the cable may not have been done correctly
    there may be a fault in the coax
    there may be a fault in the antenna

    all these can cause a mismatch which will show up using the test meter


    Dave
     
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