Thread: Dipole advice?
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Old December 17th 09, 07:26 PM posted to rec.radio.amateur.equipment
Dave Platt Dave Platt is offline
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First recorded activity by RadioBanter: Jul 2006
Posts: 464
Default Dipole advice?

What about the dipole? I have a 1:1 balun and both ladder line and coax.
Which is prefered?


If you want to work a wide range of bands with a single center-fed
doublet, I think you'll have more success using ladder-line, and
feeding directly from the balanced output of your tuner. If your
tuner has only a non-balanced (coaxial) output, I'd suggest using a
1:1 choke (current balun") as close as is practical to the tuner
output, and then transitioning over to ladder line.

If you're using coax, and a balun at the top, you may find yourself
suffering from significant losses in the coax at high SWRs. Using
ladder or open-wire line, with its higher characteristic impedance,
can reduce these excess losses quite a bit (although not entirely
eliminate them).

Are you firm about using a center-fed doublet/dipole? A number of
people I know are very positive about their "off-center-fed"
dipoles... these can work as coax-fed antennas (using a 4:1 or 6:1
balun at the feedpoint) on a set of harmonically-related bands (e.g.
80/40/20/10). Another option here is to use an off-center-fed wire,
but feed it with ladder line (no balun at the feedpoint)... gives even
more band coverage.

These OCF antennas are typically fed at a point around 2/3 of the way
along the length (e.g. one arm is around twice the length of the
other) and are cut to a length which is halfwave-resonant on the
lowest band being used.

An example is the Alpha Delta DC-OCF - 135 feet long (legs are 45 and
90 feet), 6:1 balun at the feedpoint, coax-fed, covers 80/40/20/
17/12/10 meters "No tuner required". It covers the lower end of 6
meters pretty well, and the higher frequencies with an increased SWR
("use with caution", presumably to avoid overstressing the balun and
transmitter).

Most of these doublet/dipole antennas can also be treated as a
top-loaded vertical, and fed Marconi-style (short the two sides of the
feedline together, connect to the "hot" side of the tuner, and feed
the ground side of the tuner to a good bed of radials). This can let
you work lower-frequency bands (e.g. 160) on an antenna that by itself
is too short to load up properly at such a low frequency.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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