John S wrote:
On 2/22/2015 4:09 PM, wrote:
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.
I modeled this in free space starting with the elements offset by 0.01
lambda in the Z axis and the length adjusted for resonance.
I then stepped the offset up to 0.1 lambda.
As the offset increased, the gain stayed the same to within a fraction
of a dB but the pattern rotated off the Y axis.
The only effect of resizing the elements for the 0.1 lambda section was
to bring the reactance back to zero; nothing else changed by any
significant amount
This might be usefull if one had only E/W supports and wanted to skew the
pattern from N/S.
Great, Jim. Thanks for that info. That pretty much nails it.
Well, not quite as it turns out.
I did it again in free space looking at the 3D pattern.
What happens is that as the separtion increases, the familiar doughnut
shaped 3D pattern start to tilt along the axis of the doughnut.
At 0.01 lambda the tilt is about 2 degrees; at 0.1 lambda about 24 degrees;
at 0.2 lambda 56 degrees.
I then put it over real, average ground with the lower element at .5 lambda.
What happens is difficult to describe in a few words and your really
should download the demo version of EZNEC to see it for yourself.
Essentially what happens is the pattern beging to favor one direction,
the elevation angle of the lobes go up, and multiple lobes start appearing.
The bottom line is that below about 0.02 lambda (about 16 inches at 20M)
you will not see any practical difference in the real world.
Above that an the pattern gets so weird I see no application for it.
--
Jim Pennino
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Displaced Element Dipole and feed question
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