My antenna snapped and I could fix it if...

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Jun 20, 2015.

  1. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    ok, will give that a go.
    My idea is glue in a short section of copper pipe.
    Then overcoat the copper, maybe a light wrap of glass. Or maybe some black PL polyurethane smoothed over the copper.
    Sand a little , then white spray paint on top.

    I will have to take the antenna off the boat to do a decent job of this. To make it look good, It needs to be lined up straight, otherwise it will be goofy looking. ;)
     
  2. peterjoki
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    Location: Turku, Finland

    peterjoki Junior Member

    I like your approach. If your going to do it, then do it right!
    Otherwise it's not worth doing :)

    Let us know how it worked out.

    I sent my uncle an email (Electrical engineer and lifetime HAM radio enthusiast) asking about this fix and the possible ill effects of the collar. Will let you know if he responds.
     
  3. peterjoki
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    peterjoki Junior Member

    New info:

    In terms of quality the collar should be as short as possible and non-magnetic. So Copper, SS, Aluminum.

    The collar should preferably be in electrical contact with the original conductor in the antenna.

    In practice there shouldn't be any noticeable difference either way.
     
  4. BertKu
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    BertKu Senior Member

    agreed

    I am not completely in agreement, in view that we do not know whether sdowney after having mounted the areal back, whether that part could touch metals which are grounded. I would just leave it floating. There will be no chance for static if left floating. Also we don't know whether may touch it accidental, we don't know the output power nor whether he will electrical insulated again from touching.

    agreed
    Bert
     
    Last edited: Jun 22, 2015
  5. peterjoki
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    peterjoki Junior Member

    That is a valid point. Perhaps a soldered connection from counductor + heat-shrink rubber insulator over the collar, for the best fix?
     
  6. BertKu
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    BertKu Senior Member

    Yes, I think so. I would not leave the copper exposed to being touched. Should he connect the copper to the core of the areal (which I do not think is a good idea) there is then no chance in shortening the areal and have a different labda input impedance.
    Bert
     
  7. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    I decided to fix it better than what I intended.
    This break had multiple issues with some broken pieces.
    I started by removing the wire. Upper wire broke loose from lower wire and was stuck in the upper tip. A heat gun softened whatever glue they used, and the wire came out from the broken tip. I figured they glued or used that sticky foam to secure the wire and they did. I suppose the idea with the foam is to keep the wire from moving the pole making noise and eventually breaking from work hardening the soldered wire? I will reuse the existing foam, but this upper wire I will just slide back in and that will be it.

    I soldered it back together and heat shrinked it.
    You can see the antenna pole is missing some pieces.

    [​IMG]

    So then I mixed Loctite epoxy with milled fibers and carefully lined up the skeleton of broke bits. the milled fibers make it thicker and add strength.
    [​IMG]
    That left some holes to fill, so I used a sheer thin FG cloth to fill in. Let that set, then wrapped several spiral layers of glass around using epoxy with 1/32 milled fibers. I overlayed with painters tape to squeeze it together.
    [​IMG]

    Sanded it smooth and also added a coat of epoxy with milled fibers on top.
    It was difficult to get the break to line up perfectly, but I managed it. Sighting down the pole will reveal if it is straight or crooked.
    It will have a small bump out where It is repaired. Two spiraled layers of glass should be enough glass to work.
    [​IMG]
     
  8. BertKu
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    BertKu Senior Member

    Hi sdowney717, not bad, you must be proud at yourself. I have a question. At what frequency do you receive and transmit and at what power? It looks like shortwave with vertical polarization. Bert
     
  9. sdowney717
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    sdowney717 Senior Member

    This antenna is standard VHF marine band.
    It is 8 feet long but the transmitting element is much shorter, quarter wave?

    The white coax from the VHF radio comes in and is bent 180 degrees with a short section of black coax and then the unshielded element is soldered to the black coax.

    I never really use the radio except to listen to weather band. One time about 10 years ago when I called for some assistance I used my cell phone, and that sort of surprised whoever we called, they were not used to a boat using a cell phone to get assistance, but it worked fine, The Virginia Marine police came out and gave us 5 gallons of gas to get back to port. There was a fierce storm going on at the time and we burnt up all our fuel withing a mile of the marina. Then we drug anchor and slammed into a fishing pier for a while. Got clear of that and finally thank God, the anchor held or I would have sunk on sold old pilings. The antenna broke as the boat was hitting the fishing pier. Then I replaced with this antenna. The first antenna was a very nice long antenna, but replacement cost was close to $250 and this one I have now was cheaper.

    I had to fix a few items like a piece of plywood on the flybridge, replace the antenna, replace a 6 foot section of 1 inch SS tubing railing. The boat would go down on wave trough, then the wave would rise and push the hull up and the flybridge was caught under the fishing pier. So the weakest link was the side plywood of the flybridge. It was so rough we were all seasick that day.
     
  10. BertKu
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    BertKu Senior Member

    The rule is as follow. Labda = 300 / Mhz . By 8 feet it means Frequency = 300 divided by 2.438 = 123 Mhz

    Often by transmitting it is 1/4 Labda = about 60 cm. The reason why you have it longer for receiving, your input signal may be slightly stronger. For any aerial it is important that the ground plate is well secured to the ground of the transmitter, to allow a high impedance at the tip of the areal. I made my own aerials, by cutting stainless steel 2 mm welding rods in 8,6 cm pieces. But I am working on 868 Mhz. Pity that stainless steel welding rods does not come in 8 feet lengths, you could have easy made your own for a few bucks. (provided the ground plate is truly a grounded plate.) Bert
    P.S. sdowney, you could make your own aerial, but use 1/4 labda and use a piece of SS welding rod. They come in lengths of 1 meter (i.e. more than 2 feet)
     
    Last edited: Jul 2, 2015
  11. sdowney717
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    sdowney717 Senior Member

    Hi Bertku,
    this is a US marine VHF antenna.

    Here are the frequencies it is supposed to work with.

    http://www.navcen.uscg.gov/?pageName=mtvhf

    I spray painted the pole today and will hook it back up on boat friday.
    My repair is holding even with light flexing of the pole.
    Feels real solid, like new.
     
  12. BertKu
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    BertKu Senior Member

    Well, if you want to receive those frequencies, i.e. from 152.05 to 162.55, your aerial length should be 152.05 + 162.55 divided by 2 = 157.3 Labda is thus 1.907 meter. Probably why you have on the receiving side Labda and on the transmitting side 1/4 labda , is probably in case the boat aerial is mounted low and then it may not receive always a good signal. 1.907 = 6.256 feet. Just measure it and also the transmitter side. 1/4 labda at 1.564 feet long I do not think that would be able to notice the difference with a few centimeters longer or shorter. But you WILL notice a difference if the ground plate is weak.
    You mentioned about 8 feet, therefore I calculate the frequency in the previous thread as 123 Mhz. You are happy with your success in repairing your aerial, we all are also happy . Bert
     
  13. sdowney717
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    sdowney717 Senior Member

    The coax runs up inside the antenna pole, it is only unshielded on the upper 2.5 feet or so of the pole. (the point where I had to resolder the wire to the coax.) It is what the manufacturer designed it to be, a vhf marine boat antenna. It happened to break at the position where coax becomes a single unshielded wire. that was likely the weakest point on the pole. The heavy shielded coax line likely gave the pole a little more bend resistance, the little unshielded wire is pretty floppy. AFAIK, that unshielded wire is where the signal goes out, not the shielded coax lower down.
    I have it mounted up high on the flybridge, so likely 10 feet or so above the water is the mount point. So antennas tip maybe 18 feet above the water level.
     
  14. BertKu
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    BertKu Senior Member

    Your ground plate is the end of the coaxial cable. I am puzzled, why is the areal is so tall for 1/4 labda to receive the 162.55 Mhz optimum. i.e. 1.564 feet and not 2.5 feet. Look my experience with aerials is, that it probably only matters receiving a bad signal, if you are far away from the source of the signal. I would just have taken a half meter of stainless steel welding rod, a piece of copper, drill a hole in it and mount it so that the ground base (your coaxial cable) is nicely insulated from the ss i.e place a shrink sleeve over the 4 to 5 cm (+/- 2 inch) copper rod, which has an hole in the middle and then the SS is firmly pressed into the copper. The core of the coaxial you solder onto the copper and then push the shrink sleeve over it. Thereafter mount the whole lot into a piece of pvc pipe and fill it up with hot glue. just measure for any possible short circuit. If your 4 -5 cm is not firm enough, make it 10 cm long. Bert
     
    Last edited: Jul 3, 2015

  15. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Sdowney, should you break the aerial again , I recommend, buy a few meters of electrical 1/2 inch PVC pipe, for UV light processed, pull a coaxial cable through it, make from a round piece of yellow copper a pipe, of which the diameter is the same as a stainless steel welding rod. Make in that piece of yellow copper 4 holes (vertical) and put thread in it, place 2 screws in it, put your SS areal in it, tighten the screws, place the core of your coaxial cable in the other end of the yellow copper rod, screw the center of the coaxial cable in the copper rod with those 2 other screws. Place a shrink sleeve around it, move the outer ground wire up and tape it with some insulation tape. Push it into the PVC tube back and put hot glue to ensure that it does not pull out. and you have a first class aerial for strong winds. Just calculate the length: 300/frequency and then divide by 4 = the length of your top. A few centimeter up or down is not that critical.
    Just in case your aerial breaks again.
    Bert.
     
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