Full wave dipole?

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Jon Rudolph:
I want to put up a dipole for 6 meters and have plenty of room for a full wave dipole.  Would this be a good idea?  Almost all the information I see concerning dipoles refer to halfwave, why is this?

Pete Allen:
HI: Successful dipoles are almost invariably half wave because the feedpoint impedance gets out of hand very quickly, leading to an almost total loss of power in feedline losses.

A RESONANT full wave dipole has a feedpoint impedance of 3,000 ohm up.  Usually up. Far up. Under some circumstances the feedpoint impedance can be as high as 50,000 ohms. Since there is no way to match this much mismatch at the antenna feedpoint, under any circumstances most of your expensive RF goes to heat your feedline - your "antenna tuner - or the finals in your transmitter.

A RESONANT half wave dipole has a feedpoint impedance of somewhere between 20 and 120 ohms, depending on it's height above ground. Fed with quality coax the SWR is less than 3:1, usually less than 2:1, and the feedline and tuner losses are trivial.

73  PEte Allen  AC5E

Sean Doran:
There was a book written (printed) a few years ago by 73.  In it, there is a multiple wave length "dipole".  If one of the legs uses a 1/4 wave, the other length of the dipole can be multiples of the wavelength.   You'll have either a 72 ohm or 52 ohm match (if I am remembering correctly), depending on the distance above ground and/or the closeness to buildings or other objects.  It recommends a 4:1 balun or a 1:1 balun depending.  If you are using a tuner, you'll have less concern as long as you are preventing energy from moving down into the coax.   For DX, you may want to consider a 1/4 wave vertical for the low angle of radiation. I believe you may be pleased with the performance of a vertical on 6 meters.

Pete Allen:
Hi again: Most 6 Meter DX is horizontally polarized and there is very little Faraday rotation at 50 Mhz.

  Nominally, cross polarization loss is 20 dB - 100 microvolts from a properly polarized antenna will get you 1 microvolt when you turn the antenna 90 degrees, from vertical to horizontal or vice versa - but my experience has been that it's more like 25 dB.
  So 6 Meter verticals make great antennas for fairly local work, where most of the stations also use horizontal polarization but they handicap you for DX.

  As far as an antenna with one leg a quarter wave and the other some number of half waves - I tried that back in the 1940's and was not pleased with the results. It took more space but did not perform as well as a conventional dipole.

  Since you only need 9 feet 2 inches or so overall for a conventional dipole - I would make a conventional half wave dipole, feed it with 9913 or RG213, and enjoy the magic band.

  73  Pete Allen  AC5E

Norman Sullivan:
A full wave dipole can be very effective. It can be made directional if sloped. To feed it with 50ohm coax, the feedline should be placed 1/4 wvlngth from one end.  Refer to article "Antennas for QRP" on this site.  


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