On 2/21/2015 11:35 AM, John S wrote:
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.
Then, what about feeding them from a balanced line?
The questions a
* How will the antenna impedance and current distribution suffer, if at
all?
* What feed system is required? 90 degrees to the axis is preferable,
but that means that the twin lead has one side longer than the other.
Is this interesting? Or, has it been discussed before? What was the
outcome, if any?
Thanks, Guys.
Hi, John,
I finally got a chance to give this the consideration it deserves.
A lot is going to depend on how far offset the two halves are displaced.
If they're less than 0.1 or 0.2 wavelengths, you aren't going to notice
the difference. Too many other things will affect the operation -
trees, wire fences, etc.
What will affect it the most will be the feeding of the antenna. My
suggestion would be to feed it from the middle of the difference, i.e.
________
|
|
* *
|
|________
Where the '*'s are the feed point. Both halves are 'L' shaped (at the
feedpoint, anyway), with one going up and one going down. The vertical
pieces are of equal length, as are the horizontal pieces. This will
keep the antenna pretty close to being balanced (as much as any dipole
in real life, anyway).
Also, best would be to run the twinlead perpendicular to the feed -
straight towards or straight away from you in the above picture.
The further you have to displace the two halves, the less it will act
like a dipole. However, what would be more of a concern to me is the
capacitive effects of the ground when the distance of each half is at a
significantly different, yet close to the ground. This could also
unbalance the antenna, throwing off the pattern and the impedance.
Checking each half at the feedpoint with an antenna analyzer (treating
them as a 1/4 wave antenna). But unless the antenna is close to the
ground (in wavelengths) and the two are significantly separated
vertically in wavelengths, I doubt it will be a problem.
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Jerry, AI0K
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