Full wave dipole?

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Pete Allen:
Actually, that Off Center Fed dipole is more commonly known as a Windom. And it works fairly well as a multi-band dipole with a wide range tuner. It works even better when the feedpoint is around the 30 percent point. That is, 30 percent of OA length, feedpoint, 70 percent of OA length.

But the question was about an antenna for six meters - and feedline losses become substantiallly more of a problem on VHF. A resonant center fed half wave dipole will maximize radiation and minimize losses.

  73  Pete Allen  AC5E

Dale Hunt:
There are lots of different ideas floating around here, and sometimes
getting confused with each other.

A full-wave dipole, or "2 half waves in phase", gives about 2dB
gain over a half-wave dipole.  This basic antenna is conbined into
arrays at many SW broadcast stations.  The feedpoint impedance
is high, and depends on the diameter of the wire:  the larger the
diameter as a fraction of a wavelength, the lower the impedance.
(Multiple wires can be used to simulate a fatter element and lower
the feed impedance.)  I'd have to dig out my copy of LaPorte's
Radio Antenna Engineering to see how fat the antenna would have
to be on 6m to have a 600 ohm feed point impedance, but it
probably is possible using aluminum tubing.

Even with a 1000 - 2000 ohm impedance (typical for HF wire
antennas), the antenna is easily matched with a quarter wave
shorted stub of open wire line.  The position of the shorting bar and
the coax tap point on the stub are adjusted for minimum SWR.
A similar vertical antenna for 2m has appeared in many ARRL books
over the years, using copper tubing and elbows for the antenna and
the matching stub.  (You can get an extra 1dB of gain by extending
the wires to 5/8 wave on each side, and shortening the matching
stub accordingly.)

The 2dB gain is a result of a sharper pattern, so the radiation will
be less in some directions.  This may be a problem if the antenna
is not rotatable, unless you are just interested in working one
particular direction.  If a fixed direction (at right angles to the wire)
works in your situation, you can extend the antenna with additional
half wave sections, as long as you put a quarter wave phasing
stub (shorted at the far end) between the ends of adjacent
elements.  This array can be fed in the middle of one half-wave
element (probably around 100 ohms) or with the open wire stub
between two adjacent elements.  The more elements, the more
gain, and the sharper the pattern.

Now, some comments on some of the antennas mentioned in the
other posts.

A full-wave wire fed 1/4 wave from the end will have a relatively
low feedpoint impedance and can be used with direct coax feed.
The pattern will be a cloverleaf with 4 lobes, just as if the wire were
fed at one end.  This is because the two half wave sections are out
of phase.  (Adding a quarter wave phasing stub in the middle will
change the pattern to the bi-directional broadside radiation that
you get from center-feeding it.)  This may be useful if your supports
are not broadside to your prefered radiation direction.  This antenna
in either form is primarily a single-band antenna.

The "Windom" with the same length radiator would give the same
pattern, but the feed point is shifted to about the 1/3 point, and the
feed impedance is roughly 300 ohms.  This is used as a multi-band
antenna, as the impedance is roughly the same on all even
multiples of the design frequency.  (So you may be able to design
a 24 MHz windom and use it on 6m... maybe...)  But, for single band
use, I think the other options are preferable.

If you are designing a rotatable antenna using aluminum tubing,
you will probably get better performance with the same length of
tubing by making a simple 2-element yagi.  The gain is higher, the
turning radius is less, and you don't have to worry about trying to
insulate the high-impedance at the feedpoint.  And you get some
front-to-back ratio as well.

Have fun experimenting! - Dale WB6BYU

Griff:
Jon,

When hams talk about a dipole, they are traditionally talking about a 1/2 wave wire, center fed with coax. This is the standard referrence antenna that most designs are compared against (usually in free space). It is THE "basic" antenna and they get more complicated from there... Use good low loss coax and trim it for a low SWR. Since you have "extra" room, you should be able to mount the 1/2 wave wire in the clear and away from objects. When it comes to 6 meters, propagation can be the biggest challenge ;-)

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