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Author Topic: Dipole question  (Read 1169 times)
W1JKA
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Posts: 1664




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« on: November 09, 2012, 04:37:42 AM »

     I would like to put up a 30m hw dipole on my small lot and have the following question.Because of space limitations one radiator would have to be about 5-8 ft. above a metal roof on a one story ranch house,the other radiator would be over clear ground.Would the metal roof have any negative effect and if not does it make a difference wheather the coax feedline shield side is over the roof or clear ground side?Max. antenna height about 23 ft.Any input appreciated,thanks.   Jim
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WX7G
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« Reply #1 on: November 09, 2012, 05:15:16 AM »

The metal roof will tend to lower the radiation resistance but you dipole will still work well.

The coax shield can be connected to either side of your dipole. Many hams use 1:1 baluns (current chokes) at the feedpoint but this is optional.
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N4UM
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« Reply #2 on: November 09, 2012, 06:28:35 AM »

If I had a nice metal roof, I'd give serious consideration to installing a vertical  if you're interested in chasing DX.
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W5LZ
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« Reply #3 on: November 09, 2012, 12:09:06 PM »

Will it work?  Yes, it will.  Won't be the 'ideal' place or height, but so what, it will certainly work.  Do it.
 - Paul

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K3VAT
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« Reply #4 on: November 09, 2012, 03:45:17 PM »

    I would like to put up a 30m hw dipole on my small lot and have the following question.Because of space limitations one radiator would have to be about 5-8 ft. above a metal roof on a one story ranch house,the other radiator would be over clear ground.Would the metal roof have any negative effect and if not does it make a difference wheather the coax feedline shield side is over the roof or clear ground side?Max. antenna height about 23 ft.Any input appreciated,thanks.   Jim

Jim, as the other commenters have said, yes, there will be an interaction: mutual coupling between the  the antenna and any close by metal object.  Often, the more massive and close in the metal object, then the more interaction.  You may see the interaction manifest as 'difficulty to obtain a good match on a particular band or portion of the band'.  Additionally, the radiation pattern may be skewed but this is hard to predict and hard to visualize exactly what it means.  Our antennas work best when located well out into the clear (>1/2 wavelength ~).

If your max H above ground is 23' feet, then you would want to concentrate on operation for the 5 bands of 10M thru 20M.  The antenna may even load 'nicely' on 30M and 40M but the majority of the radiation is  skyward (esp 40M), so don't expect P29 to come back to you on 40M, though you never know!

GL, 73, Rich, K3VAT
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W1JKA
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« Reply #5 on: November 10, 2012, 04:40:31 AM »

      Thanks to all you gentlemen for your thoughts and advice.I have decided to nix the roof dipole,xyl was not keen on it and although I would like a mono band vertical on the metal roof,that is a definite no go with her.N4UM Tim's very nice set up gave me some ideas,so yesterday I homebrewed a little 8 ft. PAC-10 30m mono to test out the ground conditions for radials at my qth.I was very happy with the results and if it was not so flimsy I  would use it all winter,so will now build a 23 ft.mono vert. for 30m.Thanks again.  Jim
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