Displaced Element Dipole and feed question
 

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.

Displaced Element Dipole and feed question
 

On Saturday, February 21, 2015 at 10:35:53 AM UTC-6, 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 wouldn't feed with one wire of the ladder line longer than
the other. I'd split it at the end long enough to feed both
elements from the middle of the two elements. I don't know
how far apart you are talking about, so couldn't say about
how it would work. You could model it and get a fairly decent
idea I would think. If they are not that far apart, I don't
think it would act much different than any other dipole.
Particularly if you use the spread apart wires of the ladder
line to feed, as those portions of the ladder line will act like
part of the dipole elements.

Displaced Element Dipole and feed question
 

"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?

Displaced Element Dipole and feed question
 

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.

What happens highly depends on what the offset is in wavelengths.

If it is small engough, nothing happens you could likely measure.

As for feeding it, you connect the ends with another wire and feed it
in the middle of the connecting wire.


--
Jim Pennino

Displaced Element Dipole and feed question
 

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...


________________
| _________________
| |
| |
| |
feed

Displaced Element Dipole and feed question
 

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...


________________
| _________________
| |
| |
| |
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.

_________________
/ _________________
/ _\
/ /
| |
| |
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
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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...


________________
| _________________
| |
| |
| |
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.

_________________
/ _________________
/ _\
/ /
| |
| |
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.

Displaced Element Dipole and feed question
 

On 2/21/2015 12:12 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.

What happens highly depends on what the offset is in wavelengths.

If it is small engough, nothing happens you could likely measure.

Okay.

As for feeding it, you connect the ends with another wire and feed it
in the middle of the connecting wire.


Good suggestion. Thanks.

Displaced Element Dipole and feed question
 

On 2/21/2015 10:51 AM, wrote:
On Saturday, February 21, 2015 at 10:35:53 AM UTC-6, 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 wouldn't feed with one wire of the ladder line longer than
the other. I'd split it at the end long enough to feed both
elements from the middle of the two elements. I don't know
how far apart you are talking about, so couldn't say about
how it would work. You could model it and get a fairly decent
idea I would think. If they are not that far apart, I don't
think it would act much different than any other dipole.
Particularly if you use the spread apart wires of the ladder
line to feed, as those portions of the ladder line will act like
part of the dipole elements.


Why didn't I think of modelling it? Duh!

Thanks.

Displaced Element Dipole and feed question
 

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.


--
Jim Pennino