Displaced Element Dipole and feed question
On 2/21/2015 1:09 PM, Jeff Liebermann wrote:
On Sat, 21 Feb 2015 12:22:48 -0600, John S
wrote:
On 2/21/2015 11:34 AM, gareth wrote:
"John S" wrote in message
...
Suppose I have a dipole where the two quarter-wave elements are not
exactly aligned. That is, the axis of, say, the left element is displaced
by some amount from the axis of the right element.
In other words, the long-established V-Beam?
No. I will try to use ascii to represent my thought...
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feed
It will work, but you have a potential problem with having the
balanced line becoming a radiator. Any imbalance in the antenna or
imbalance in the reactance of the feedline to ground, is going to
cause the feed lines to become unbalanced and therefore radiate. The
most common version of this problem is the OCF (off center feed)
antenna, which has very different currents in each feed wire. Another
is mounting the dipole perpendicular to a hillside or building, where
the capacitance to the ground or building is different for the two
wires.
However, everything is a question of degree. If the frequency is
fairly low, and the offset shown in your drawing is a small fraction
of a wavelength, the effect will be minimal or negligible.
You can also reduce the effects by simply forcing the two feed wires
to be equal length. Something like this except that I can't draw the
feed wires in 3D. Just cut the wires the same length.
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feed
Also note that a log periodic antenna has staggered (displaced) dipole
feeds and works just fine.
http://upload.wikimedia.org/wikipedia/commons/f/f7/Schwarzbeck_UHALP_9108_A.jpg
Gotcha. Thanks.
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