This is in response to Jim's comment on the full wavelength dipole comment.
Again, it is correct to point out the potential danger in using this antenna
but it is wrong to insinuate it is not a good antenna.
INDEED, THIS IS ONE OF MY FAVORITE ANTENNAS!
(Of course you must match it efficiently for it to work well.)
We are all familiar with the radiation pattern of a half wavelength
horizontal dipole.
It has two major lobes, perpendicular to the wire, as well as two deep nulls
off its ends.
The full wavelength dipole has 4 major lobes, kind of like a 4-leaf clover.
The lobes are centered on about 40 degrees either side of the wire.
Of course it has 4 nulls instead of 2 but these are not quite as deep as the
nulls off the end of a dipole.
Here in Europe, if I string it exactly East-West, then my four lobes are
approximately NE, NW, SE and SW. From Central Europe:
- NE is Japan
- NW is stateside
- SW is long path to VK
- SE is towards Middle East countries
This is a great dipole for DX contests because of its more omni-directional
coverage than a halfwave dipole.
A Net Control Station on an 80m or 40m net would probably find this better
than a halfwave dipole.
THE PROBLEM: The antenna has a feedpoint impedance of about 2500 Ohms. You
must know how to deal with this. You MUST use openwire, not coax. If you
are only interested in one band (the 2nd harmonic), then the simple solution
is to use an odd multiple of a quarter wavelength of feedline.
Often that doesn't work, for one reason or another, but there are other
ways.
An old Johnson Viking matchbox is a good candidate here and there are many
other ways of attacking the problem. Too many to go into here. There are
several good articles on matchbox designs by people who have tackled and
licked the problem.
There are also several BAD suggestions for matching this, which result in
fried baluns, burned up matchboxes, or even damaged transceivers.
CAUTION: If you jump in and just try to use it with whatever matchbox you
happen to have, you might burn up your matchbox. DO NOT TRY TO MATCH THIS
WITH THE BUILT-IN TUNER IN YOUR TRANSCEIVER.
AND...DO NOT TRY TO FEED THIS USING ONLY A 4:1 BALUN BETWEEN THE FEEDLINE
AND 50 Ohms.
I have used this type of antenna with power levels up to 1kW for at least 30
years. I burned up a lot of stuff in learning how to do it right. In the
meantime there is sufficient information available that tells how to do this
safely.
This Ten-Tec group is probably not the right forum to discuss this stuff in.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Rob
Atkinson
Sent: Monday, October 19, 2015 11:51 PM
To: tentec@contesting.com
Subject: [TenTec] Open Wire Line
>Re-read my sentence. I am not objecting to the use of open wire line when
it is matched to the antenna, but to the use of it as a band-aid for badly
matched antennas.
I suppose it gets down to the meaning of "badly matched." If by that
you mean for example a 1/4 wave dipole fed in the center then I'd say
nothing should feed such a load because it's pretty close to hopeless.
This is an antenna that will probably have an impedance of just a few ohms.
I have heard of hams building balanced matching networks capable of handling
20 or 30 amps for such an antenna but I'd put up a bigger antenna.
Most all medium wave broadcast stations have abandoned their open wire
unbalanced line. KFBK in Sacramento still has it as far as I know.
They may be the last. That line worked okay in the days of tube rigs but
the solid state boxes would kick off if a bird flew into the line and got
across the center and one of the outer lines. they had to go to coax below
grade. I am not against coax; I use it with unbalanced loads.
73
Rob
K5UJ
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