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Author Topic: Mystery Dipole  (Read 1471 times)
KF5MEZ
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Posts: 2




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« on: October 03, 2012, 04:44:35 PM »

I have inherited a boxful of misc. ham odds and ends.  One of these is a brand-new dipole antenna, wires only, no balun.  The sides are not even.  One side is 12'6" long and the other end is 22'6" long.  This works out almost exactly to a 10m full-wave, or 20m half-wave.  Here's what I'm trying to figure out:

How should I connect my HF rig to this dipole?  if I go RG8 from the back of the transceiver should I use a 1:1 balun to make the connection?  Does it matter which output from the balun goes to the short side and long side of the dipole?

I'm thinking the other option is to connect to the antenna with ladder line?  I have plenty of RG8 and 450 ohm ladder line.  The total distance from the transceiver to the center point of the dipole will be about 60'.  Going this route, i would expect to use a 4:1 balun.  Is there a good rule of thumb on how much RG8 I should use to connect my transceiver to the balun?

Thanks for any advice you can give.  Also, any references to online resources would be appreciated.

73

Leon
KF5MEZ

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AI7RR
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Posts: 164




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« Reply #1 on: October 03, 2012, 04:56:23 PM »

I think for you a little "Off-Center Fed Dipole" research might be in order.

73, Roger
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KF5MEZ
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Posts: 2




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« Reply #2 on: October 03, 2012, 05:37:50 PM »

Thanks Roger.  I have done some research, and I'm familiar with the idea of an OCF dipole.  The dimensions on this one are what's stumping me, and I'd like to hear some ideas about where to start on this.  I'm guessing this antenna will have about 200 ohm impedance, which would suggest a 4:1 balun but I'm asking because I don't have access to an antenna analyzer (just an SWR meter), and it's difficult for me to work at heights, so I'm trying to minimize the guesswork.

All my previous HF antenna experience has been with vertical omnis that already had loading coils and SO239 connectors at the base, so this is new territory for me.

73
Leon
KF5MEZ
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KC9Q
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Posts: 49




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« Reply #3 on: October 03, 2012, 08:25:45 PM »

UR 12.5 ft/22.5 ft split calculates to a half Wave at about 13.485 MHz, with a 35.%/64.3% split.  This will provide operation on 13.486 MHz, 26.971 MHz, and 53.943 MHz, or there about.  The OCF would operate a bit better using the 33.333%/66.666% split.  That would be around 11' 2" and 22' 4", giving you a Half wave length for 14.0895 MHz or so.  The operation would fall on 14.0895 MHz, 28.179 MHz, and 56.358 MHz, (useful for 20 and 10 Meters).  Use a 4:1 CURRENT Balun, and any length of 50 ohm cable you want.  A longer OCF length will get you more frequencies if you have the room for it.

73,
Mike
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