VHF antenna construction.

Discussion in 'OnBoard Electronics & Controls' started by valvebounce, Nov 21, 2013.

  1. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    I have a Midland 5600 vhf radio and would like to construct my own antenna.
    I can't find any explicit instructions or drawings on the net.
    I can make the stainless brackets etc.but I haven't a clue on how they are connected,or how long the antenna needs to be.I realise that the height the antenna is mounted is the distance for the "line of sight".This is my first boat and radio,so I am a real newbie.Getting the connections and insulation points are my main concern.
    Any help,and perhaps a simplfied drawing would be greatly appreciated.
     
  2. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    Dont know. Sailboats have been using insulated stay vhf antenna for a long time. There is a minimum length, but i dont believe there is a maximum length. Im sure a Boatd esign net contributor will come on and clarify, explain..."db " and antennas.

    Normally the big consideration is line loss......The signal radiating out of the coaxial cable connecting the vhf to the antenna....before the signal can escape from the antenna
    A fiberglass whip is a hollow tube with a copper antenna wire inside the tube. The glass tube only makes the wire stand up.


    Perhaps its best to state how long you would like to make your antenna then ask a BDN guy how to best do it.
     
  3. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks Michael,you have answered one of my queries by telling me that the whip is hollow,and supports a copper rod/wire.I have some old hollow fishing rods I can use for the whip.They are fibreglass and composite,I believe carbon is a conductor,so I will avoid using any carbon rods.(Can do without a lightening conductor on board Haha.)I have found conflicting info.on the length,I think 6ft 3ins is about right.Something to do with the wavelength/cycles.I will wait for the correct info before I make a move.
    Many thanks V.
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The whip can be hollow fiberglass with a wire inside, aluminum tube or solid metal, whatever suits you best. The bracket is always metal and connects to the shield of the coax cable, the whip is connected to the conductor in the coax and isolated from the bracket. Using a fiberglass rod is the easiest solution.

    For best performance the length of the whip should be close to 1, 1/2 or 1/4 of the wavelength. Because you radio has 40 channels between 156 and 162 Mhz, calculate using 159 Mhz. That wavelength is approx. 188 cm or 6.18 ft.
    A 1/2 wavelength (lambda) antenna performs just a tiny bit less good than a full length one.
     
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  5. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Many thanks CDK,that answers my question perfectly.I will go for the 6.18ft antenna.My boat is a converted speedboat for fishing and doesn't have a lot of height above the water,so I will construct a small mast to give it more height below the antenna.I doubt that I will go more than 2miles from shore,and I won't be far from Liverpool,so I think that should more than cover it.
    Once again,many thanks.V.
     
  6. sharknet
    Joined: Dec 2013
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    sharknet Junior Member

    What about a stainless whip? Normally base loaded to provide maximum forward power.
    I ran one for years on a small fishing boat, had no trouble getting ship to ship or shore comms.

    Most of my boating was at lease 8Nm from the nearest shore station.
     
  7. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    Thanks for the advice S/Net.I don't have much problem aquiring suitable metals,my mate is a Tinsmith on Rolls Royce Marine engines,he's been there that long he is almost like a silent partner.Some of the new metals in use are unbelievable.The insulation points were my main problem,but with the advice from CDK,I think I can sort that out.A stainless whip is a great idea,so,back to the drawing board.
    Once again,many thanks.V.
     
  8. sharknet
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    Location: Auckland New Zealand

    sharknet Junior Member


  9. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    That's great S/net,thanks.I think I will probably make the whip 6.18ft like CDK suggested,because the signals are "Line of sight".I may raise the seating platform below the antenna to gain a little more height.
    Great info,thanks.V.
     
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