Actually, it IS a half-wave dipole, period. The ends are open circuits,
consequently they exhibit the impedance of open circuits [very high],
and the current, which creates most of the radiation, is in the center.
Their radio advantage is that it is somewhat easier to elevate the
high current portion of the wire. Logistically, they're light, small,
easily transported, and not hard to deploy.
They require some matching legerdemain to get your TX to cram power into
the several thousand ohm end of the wire, but a half-wave dipole is a
half-wave dipole, regardless of where you feed it. The familiar
Buddipole in one of the horizontal configurations is an off-center-fed
loaded half-wave dipole. Efficiency suffers of course because much of
the electrical length is in the two inductors which don't radiate much,
but I used one successfully for several years. Pretty heavy though.
Don't overlook small magnetic loops such as the Alexloop [a bit pricey]
or any of the much cheaper home brew versions. They are essentially
insensitive to ground. I sold my BP and got an Alex, and it works at
least as good as the BP and probably a little better, comes in a small
canvas carrying case, sets up on a light tripod about 2 meters high,
QSY's 40 to 10 in an instant with the turn of a knob. Setup/teardown
takes about 5 mins max. Several of the NA SOTA group have them and find
them very effective.
73,
Fred K6DGW
- Northern California Contest Club
- CU in the 2013 Cal QSO Party 5-6 Oct 2013
- www.cqp.org
On 1/28/2013 11:03 AM, N1EU wrote:
Fred, in terms of angle of radiation versus height, it will behave similar
to a dipole. If you can get it up a half wavelength or higher in the air,
great, but it will work well only a quarter wave off the ground (with higher
angle of radiation). If the ground slopes down in the direction of
propagation, it doesn't have to be high at all. I used it once on a
mountain summit about 6ft off the ground and it worked like gangbusters.
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