Testing VHF antenna with ohm meter

Discussion in 'Boat Electronics' started by Vinny, Jun 8, 2006.

  1. Vinny

    Vinny Guest

    Hello,



    I suspect that my VHF antenna is shot, I think it's open circuit.

    I'm looking for a quick and easy way to verify it with an ohm meter.

    I know that the 50 ohm impedance is an AC impedance

    at RF frequencies. But I'm guessing a DC ohm meter should

    measure something between a few 10s of ohms to a few hundreds

    of ohms if the antenna was good.

    I'm measuring ~ 10 meg ohms.



    Time for a new antenna?



    Thanks in advance,

    Vin
     
    Vinny, Jun 8, 2006
    #1
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  2. Vinny

    Lynn Coffelt Guest

    "Vinny" <> wrote in message
    news:...
    > Hello,
    >
    >
    >
    > I suspect that my VHF antenna is shot, I think it's open circuit.
    >
    > I'm looking for a quick and easy way to verify it with an ohm meter.
    >
    > I know that the 50 ohm impedance is an AC impedance
    >
    > at RF frequencies. But I'm guessing a DC ohm meter should
    >
    > measure something between a few 10s of ohms to a few hundreds
    >
    > of ohms if the antenna was good.
    >
    > I'm measuring ~ 10 meg ohms.
    >
    >
    >
    > Time for a new antenna?
    >

    Depending on the design, the VHF antennas that I'm familiar with will
    read either "open" or "shorted" with an Ohmmeter. 10 megohms is a strange
    reading for an antenna. Almost sounds like one that is supposed to read
    "open" but has a little (or a lot) of water/corrosion someplace where it
    doesn't do any good at all.
    Old Chief Lynn, W7LTQ
     
    Lynn Coffelt, Jun 8, 2006
    #2
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  3. Vinny

    Larry Guest

    "Vinny" <> wrote in
    news::

    > Time for a new antenna?
    >


    Not necessarily. Some antennas are a DC (your ohmmeter is DC) open
    circuit, naturally from their design. Others have a matching transformer
    in the base of them, so act like a DC short.

    Which antenna is it? The ohmmeter proves nothing.
     
    Larry, Jun 8, 2006
    #3
  4. Vinny wrote:
    > Hello,
    >
    >
    >
    > I suspect that my VHF antenna is shot, I think it's open circuit.
    >
    > I'm looking for a quick and easy way to verify it with an ohm meter.
    >
    > I know that the 50 ohm impedance is an AC impedance
    >
    > at RF frequencies. But I'm guessing a DC ohm meter should
    >
    > measure something between a few 10s of ohms to a few hundreds
    >
    > of ohms if the antenna was good.
    >
    > I'm measuring ~ 10 meg ohms.
    >
    >
    >
    > Time for a new antenna?
    >
    >
    >
    > Thanks in advance,
    >
    > Vin


    Don't mess around with meters. If you are near a chandlers, buy an emergency
    antenna. Try it out, if that does the trick, take it back and exchange it
    for a permanent antenna. Any chandlers will oblige. Problem solved!

    Dennis.
     
    Dennis Pogson, Jun 8, 2006
    #4
  5. Vinny

    Geert Maene Guest

    chuck wrote:
    > Vinny wrote:
    >
    >> Hello,
    >>
    >>
    >>
    >> I suspect that my VHF antenna is shot, I think it's open circuit.
    >>
    >> I'm looking for a quick and easy way to verify it with an ohm meter.
    >>
    >> I know that the 50 ohm impedance is an AC impedance
    >>
    >> at RF frequencies. But I'm guessing a DC ohm meter should
    >>
    >> measure something between a few 10s of ohms to a few hundreds
    >>
    >> of ohms if the antenna was good.
    >>
    >> I'm measuring ~ 10 meg ohms.
    >>
    >>
    >>
    >> Time for a new antenna?
    >>
    >>
    >>
    >> Thanks in advance,
    >>
    >> Vin
    >>
    >>

    >
    > Why do you suspect the antenna is shot?
    >
    > Are you measuring at the end of 70 feet of coax attached to the antenna
    > or directly at the antenna terminals?
    >
    > Chuck
    >
    > ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet
    > News==----
    > http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
    > Newsgroups
    > ----= East and West-Coast Server Farms - Total Privacy via Encryption =----


    Try to get a SWR meter. This will mesure the power that go's and gets
    back to your transmitter.

    --
    Geert Maene.
     
    Geert Maene, Jun 10, 2006
    #5
  6. Vinny

    Larry Guest

    Geert Maene <> wrote in news:gvvig.474444
    $-ops.be:

    > Try to get a SWR meter. This will mesure the power that go's and gets
    > back to your transmitter.
    >
    >


    http://www.walcottcb.com/valor-v6050-vhf-marine-swr-power-watt-meter-p-
    920.html

    Don't forget to get a 3' coax jumper to hook the TX port to your radio.

    Mount it permanently next to the radio by using two right-angle coax
    adapters male to female. Screw them onto the meter so they go back into
    two matching holes in the panel. If you make the holes a tight fit with
    the PL-259 coax connectors, no mounting of the little meter is necessary.
    Just leave it on the surface between the holes. That way you can check for
    RF output power and antenna performance every time you use the radio...IF
    the radio isn't mounted out in the weather the meter won't tolerate....

    Works great.
     
    Larry, Jun 10, 2006
    #6
  7. Vinny

    Vinny Guest

    I don't have a real good reason for suspecting the antenna except
    that theDC resistance I measure is strange. I would expect it to be open
    circuit or low resstance, not a few Meg ohms.
    The VHF seems to receive OK and when I transmit, I know there
    is power being generated beause it interferes with my portable
    FM radio. Bu when I call for a radio check, I don't get a response.
    It's an old antenna, it was on the boat when I bought it. There's no
    markings left on it but it has a blue and green pin stripe near the base
    which I think is shakespeare (sp?)

    "chuck" <> wrote in message
    news:...
    > Vinny wrote:
    > > Hello,
    > >
    > >
    > >
    > > I suspect that my VHF antenna is shot, I think it's open circuit.
    > >
    > > I'm looking for a quick and easy way to verify it with an ohm meter.
    > >
    > > I know that the 50 ohm impedance is an AC impedance
    > >
    > > at RF frequencies. But I'm guessing a DC ohm meter should
    > >
    > > measure something between a few 10s of ohms to a few hundreds
    > >
    > > of ohms if the antenna was good.
    > >
    > > I'm measuring ~ 10 meg ohms.
    > >
    > >
    > >
    > > Time for a new antenna?
    > >
    > >
    > >
    > > Thanks in advance,
    > >
    > > Vin
    > >
    > >

    >
    > Why do you suspect the antenna is shot?
    >
    > Are you measuring at the end of 70 feet of coax
    > attached to the antenna or directly at the antenna
    > terminals?
    >
    > Chuck
    >
    > ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet

    News==----
    > http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+

    Newsgroups
    > ----= East and West-Coast Server Farms - Total Privacy via Encryption

    =----
     
    Vinny, Jun 11, 2006
    #7
  8. Vinny

    Larry Guest

    "Vinny" <> wrote in
    news::

    > It's an old antenna, it was on the boat when I bought it. There's no
    > markings left on it but it has a blue and green pin stripe near the base
    > which I think is shakespeare (sp?)
    >
    >


    If in doubt - Dump it and feel better....

    What kind of boat is this on?.....

    Sailboats should use Metz Manta 6...
    http://www.metzcommunication.com/manta6.htm

    The one I gave away with the Sea Rayder jetboat under it was on its third
    fast boat. CG uses them on their boats, too.

    NO FIBERGLASS for the UV to eat and fall apart.....

    Guaranteed for LIFE.
     
    Larry, Jun 11, 2006
    #8
  9. Vinny

    Vinny Guest

    "Larry" <> wrote in message
    news:Xns97DF1EC1C6EEnoonehomecom@208.49.80.253...
    > "Vinny" <> wrote in
    > news::
    >
    > > It's an old antenna, it was on the boat when I bought it. There's no
    > > markings left on it but it has a blue and green pin stripe near the base
    > > which I think is shakespeare (sp?)
    > >
    > >

    >
    > If in doubt - Dump it and feel better....
    >
    > What kind of boat is this on?.....
    >
    > Sailboats should use Metz Manta 6...
    > http://www.metzcommunication.com/manta6.htm
    >
    > The one I gave away with the Sea Rayder jetboat under it was on its third
    > fast boat. CG uses them on their boats, too.
    >
    > NO FIBERGLASS for the UV to eat and fall apart.....
    >
    > Guaranteed for LIFE.
    >


    It's on a '95 seaswirl striper walk around. This is my 3rd year with the
    boat and I'm almost certain the antenna is the orginal. I don't know how
    long they typically last but 10 years of sun and salt seems like a long time
    for
    a low end antenna to last.
    I'll do a few tests with the guys next to me in the marina. Even with no
    antenna, they should pick up a signal from 10 feet away. That will
    at least prove the radio probably works OK.
     
    Vinny, Jun 14, 2006
    #9
  10. Vinny

    Larry Guest

    "Vinny" <> wrote in
    news::

    > 10 years of sun and salt


    Yes, dump it.....

    Now, let's figure out which antenna you should get. The biggest antenna
    isn't the one you need, or want....

    Is this antenna mounted on a lay-over base or fixed mounted? If it has a
    lay-over base, does it also have an extra mount to hold it up on the side
    of the cabin, for instance? How much do you have to lay it down? As you
    probably trailer it, laying over another monsterous 8' long antenna is
    just another chore you probably don't need.

    I'm looking at the 21' walkaround's picture on Striper's webpage. Nice
    boat, by the way....(c;

    Ok, what would you think of putting an antenna on top of the hardtop that
    only sticks up 40", is stainless steel, not fragile plastic, mounted
    through a 3/4" hole so there's no coax exposed to the weather? Is there
    an overhead radio enclosure under the top? If so, we can use just a
    jumper cable between the radio and the antenna, which has a standard SO-
    239/U female coax connector sticking down into the cabin. NO coax
    losses, nothing exposed to passengers, weather, corrosion....great, easy
    install.

    http://www.metzcommunication.com/manta6.htm

    It won't split, crack, weather, produce fiberglass shards to cut your
    hands after the gelcoat weathers off and you can move it from boat to
    boat for years. It requires no ground or ground plane. You can hold it
    in your hand and it will produce a great signal. Because its cable is
    SEPARATE from the antenna, when the cable breaks, we replace the cable,
    not the whole antenna, like a Shakespeare. How stupid to have coax
    potted into a base....idiots.

    Why not a monstrous 8db antenna? Pattern....radiation pattern.

    The 1/2 wave little Manta's radiation pattern looks like a donut laying
    on the table...the table being the sea surface. If you tilt the donut as
    the boat rolls and pitches, a good part of the donut still points to the
    spoon laying on the other end of the table, the target radio system
    you're trying to talk to. Now, the bigshot, high gain antenna gets its
    signal gain from flattening out the donut as if you drove the car over
    it. All this gain is out the ends of a very flat radiation donut,
    perpendicular to the antenna. If the antenna rolls to port, for
    instance, the signal to port is pointing into the sea a short ways from
    the antenna. The signal to starboard is pointing towards the sky.
    Neither of those signals are going to the targets off the beams. If the
    boat/antenna pitches, the signal isn't pointed at the targets off bow and
    stern. The fat donut of the 1/2 wave antennas lets lots of signal hit
    the target's antenna, even if it's pitched or rolled over 30 degrees.

    My last little boat with this antenna in it was a Sea Rayder 16' jetboat.
    The Manta was mounted to a plastic handle in the bow just inside the
    gunwale and the top of it was about 4' off the water. Range on 25 watts
    was about 20 miles in normal conditions....except, of course, if we were
    jumping wakes...(c;

    If you don't like it, simply return it to Metz. I bet you'll keep it.
    You'll hardly remember it's there...going under a bridge, for instance.
    If it hits anything, it will bend back horizontal and spring right back
    to vertical.
     
    Larry, Jun 14, 2006
    #10
  11. Vinny

    Vinny Guest

    Thanks for the comment about the boat.
    I have the 20' model which is a 19' hull with a bow pulpit.
    (Can't figure out how they get away with counting the pulit though.)
    I wish I had a hardtop, that would certainly be where the antenna would go.
    I have a canvas bridge enclosure so there's nothing to mount to.
    The antenna I have now is 8' and I'm guessing 6dB. It's on a laydown mount
    but I keep the boat in the water so it only gets lowered when I trailer
    it home and winterize it. It does not have a support bracket since
    I have no place to mount it. That probably doesn't help the lifespan of
    the antenna either. Right now it's mounted on the starboard side of the
    cabin
    near the windshield. I was going to just replace what I had but I'll look
    into the Manta. According to the link you sent, they show examples of low
    mount positions so it should work.
    I like to flyfish off the bow and the 8 footer really screws up my backcast.
    A shorter antenna would help.

    Thanks for the tips,
    Vinny

    "Larry" <> wrote in message
    news:Xns97E2132059BDAnoonehomecom@208.49.80.253...
    > "Vinny" <> wrote in
    > news::
    >
    > > 10 years of sun and salt

    >
    > Yes, dump it.....
    >
    > Now, let's figure out which antenna you should get. The biggest antenna
    > isn't the one you need, or want....
    >
    > Is this antenna mounted on a lay-over base or fixed mounted? If it has a
    > lay-over base, does it also have an extra mount to hold it up on the side
    > of the cabin, for instance? How much do you have to lay it down? As you
    > probably trailer it, laying over another monsterous 8' long antenna is
    > just another chore you probably don't need.
    >
    > I'm looking at the 21' walkaround's picture on Striper's webpage. Nice
    > boat, by the way....(c;
    >
    > Ok, what would you think of putting an antenna on top of the hardtop that
    > only sticks up 40", is stainless steel, not fragile plastic, mounted
    > through a 3/4" hole so there's no coax exposed to the weather? Is there
    > an overhead radio enclosure under the top? If so, we can use just a
    > jumper cable between the radio and the antenna, which has a standard SO-
    > 239/U female coax connector sticking down into the cabin. NO coax
    > losses, nothing exposed to passengers, weather, corrosion....great, easy
    > install.
    >
    > http://www.metzcommunication.com/manta6.htm
    >
    > It won't split, crack, weather, produce fiberglass shards to cut your
    > hands after the gelcoat weathers off and you can move it from boat to
    > boat for years. It requires no ground or ground plane. You can hold it
    > in your hand and it will produce a great signal. Because its cable is
    > SEPARATE from the antenna, when the cable breaks, we replace the cable,
    > not the whole antenna, like a Shakespeare. How stupid to have coax
    > potted into a base....idiots.
    >
    > Why not a monstrous 8db antenna? Pattern....radiation pattern.
    >
    > The 1/2 wave little Manta's radiation pattern looks like a donut laying
    > on the table...the table being the sea surface. If you tilt the donut as
    > the boat rolls and pitches, a good part of the donut still points to the
    > spoon laying on the other end of the table, the target radio system
    > you're trying to talk to. Now, the bigshot, high gain antenna gets its
    > signal gain from flattening out the donut as if you drove the car over
    > it. All this gain is out the ends of a very flat radiation donut,
    > perpendicular to the antenna. If the antenna rolls to port, for
    > instance, the signal to port is pointing into the sea a short ways from
    > the antenna. The signal to starboard is pointing towards the sky.
    > Neither of those signals are going to the targets off the beams. If the
    > boat/antenna pitches, the signal isn't pointed at the targets off bow and
    > stern. The fat donut of the 1/2 wave antennas lets lots of signal hit
    > the target's antenna, even if it's pitched or rolled over 30 degrees.
    >
    > My last little boat with this antenna in it was a Sea Rayder 16' jetboat.
    > The Manta was mounted to a plastic handle in the bow just inside the
    > gunwale and the top of it was about 4' off the water. Range on 25 watts
    > was about 20 miles in normal conditions....except, of course, if we were
    > jumping wakes...(c;
    >
    > If you don't like it, simply return it to Metz. I bet you'll keep it.
    > You'll hardly remember it's there...going under a bridge, for instance.
    > If it hits anything, it will bend back horizontal and spring right back
    > to vertical.
    >
     
    Vinny, Jun 23, 2006
    #11
  12. Vinny

    Larry Guest

    "Vinny" <> wrote in
    news::

    > According to the link you sent, they show examples of low
    > mount positions so it should work.
    >


    Worked great on a Sea Rayder F16XR2 jetboat....even at 50...(c;

    Even jumping the boat out of the water couldn't destroy it...
     
    Larry, Jun 23, 2006
    #12
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