Hi all,

I'm toying with the idea of an 'electronically rotatable' dipole setup.

Consider: a dipole oriented N/S
another dipole oriented E/W
They cross each other at the mid-point of each, forming a big plus
sign, like so:

|
|
----+----
|
|

If you feed the N/S dipole, you get an E/W radiation pattern.
Similarly, if you feed the E/W dipole, you get a N/S radiation patter.

If you feed the N/S and E/W dipoles simultaneously, you get a NW/SE
radiation pattern (for example), and if you flip the feed of either one
of them 180 degrees, then you get a NE/SW radiation pattern.

So far, so good. Now for the fun part. If you feed one of the dipoles
with a signal that is only 90 degrees out of phase (or 270 degrees), you
can get radiation patterns that take of in the NNW/SSE or WNW/ESE
directions, and if you feed the other dipole 90 or 270 degrees out of
phase, you get NNE/SSW and ENE/WSW directions.

What I am looking for is a circuit that can perform the necessary phase
delays, without having to use miles of coax coiled up in the shack.
Ideally, the circuit would be able to perform the phase shift on all (or
at least several) ham bands, so that it could feed into a pair of
fan-dipoles for operation on multiple bands.

Anyone have any suggestions on just how to accomplish this? In the
extreme case, you could have a continuously adjustable system, that
would permit you to point the 'virtual dipole' in any direction at all,
which might be useful when attempting to null out an interfering station
off to the side of the desired signal. However, in practice, I suspect
that the eight positions mentioned above would be sufficient to do the
same thing (albeit with the possibility of somewhat more signal loss due
to fixed aiming directions).

I seem to recall seeing a quote in the ARRL Handbook (or the ARRL
Antenna Book) from Roy Lewallen, indicating that there is no simple way
to do this. Am I mis-remembering (or mis-interpreting) what I read?

I know that the military does this sort of thing with radar systems, and
has for some time. However, their budget is quite a bit bigger than
mine, so perhaps their solution isn't 'simple':-)

Any thoughts on this, anyone?

Harry Gross

Could you describe more fully the phase-shifted case? When I simulate
it, I get a nearly non-directional pattern... that's with two
dipoles, separated vertically by a tiny amount, with a current source
at the center of each. That's as I'd expect, since that configuration
has been used for a very long time as an omnidirectional horizontally
polarized antenna for things like FM broadcast reception.

But why not just vary the relative amplitudes in the two dipoles? Then
you can rotate the pattern fairly easily. (The pattern when both
elements are contributing won't be quite as sharp as a simple dipole,
but the null could be useful on receiving.) You can use a variometer
to change the relative amplitudes. A variometer consists of two coils
in mechanical quadrature, with the center point of the two coincident.
Then at the center, there's a rotating coil. So you feed power to the
rotating coil (or feed your receiver from it), and connect the two
fixed coils to the antennas.

It's a big help that the two dipoles in that orientation don't couple
to each other (at least in an "ideal" installation).

By the way, if you Google "variometer," you'll probably find references
to just one fixed coil with a rotatable one inside it. That's not the
only kind...maybe the one I described above has a name; I tried
"quadrature variometer" with no real luck.

Cheers,
Tom

The two dipoles don't couple to each other, so you have a lot of options
for feed systems. Transmission delay lines are one. Another is lumped
circuit networks -- an L network can be designed to effect a range of
phase shifts.

I think you'll find, though, that you'll hardly be able to tell when you
rotate the array in any increment smaller than 90 degrees, and unless a
signal is pretty close to a null, even that rotation won't make a
striking difference. It's a problem that's easily modeled, so I'd start
by seeing just what I might gain from it before going to the trouble.

Roy Lewallen, W7EL

Harry Gross wrote:
Hi all,

I'm toying with the idea of an 'electronically rotatable' dipole setup.

Consider: a dipole oriented N/S
another dipole oriented E/W
They cross each other at the mid-point of each, forming a big plus
sign, like so:

|
|
----+----
|
|

If you feed the N/S dipole, you get an E/W radiation pattern. Similarly,
if you feed the E/W dipole, you get a N/S radiation patter.

If you feed the N/S and E/W dipoles simultaneously, you get a NW/SE
radiation pattern (for example), and if you flip the feed of either one
of them 180 degrees, then you get a NE/SW radiation pattern.

So far, so good. Now for the fun part. If you feed one of the dipoles
with a signal that is only 90 degrees out of phase (or 270 degrees), you
can get radiation patterns that take of in the NNW/SSE or WNW/ESE
directions, and if you feed the other dipole 90 or 270 degrees out of
phase, you get NNE/SSW and ENE/WSW directions.

What I am looking for is a circuit that can perform the necessary phase
delays, without having to use miles of coax coiled up in the shack.
Ideally, the circuit would be able to perform the phase shift on all (or
at least several) ham bands, so that it could feed into a pair of
fan-dipoles for operation on multiple bands.

Anyone have any suggestions on just how to accomplish this? In the
extreme case, you could have a continuously adjustable system, that
would permit you to point the 'virtual dipole' in any direction at all,
which might be useful when attempting to null out an interfering station
off to the side of the desired signal. However, in practice, I suspect
that the eight positions mentioned above would be sufficient to do the
same thing (albeit with the possibility of somewhat more signal loss due
to fixed aiming directions).

I seem to recall seeing a quote in the ARRL Handbook (or the ARRL
Antenna Book) from Roy Lewallen, indicating that there is no simple way
to do this. Am I mis-remembering (or mis-interpreting) what I read?

I know that the military does this sort of thing with radar systems, and
has for some time. However, their budget is quite a bit bigger than
mine, so perhaps their solution isn't 'simple':-)

Any thoughts on this, anyone?

Harry Gross

I guess I'll have to break down, buy a copy of EZ-NEC and play with it:-)

By the way, are you suggesting that 90 degree rotation is all I'd need,
simply because the dipole radiation pattern is so wide, broadside to the
radiator? If that turns out to be true, then the problem becomes vastly
easier to deal with, and could even be done mechanically by simply
switching around the feed line to one dipole (easier to do with ladder
line - the preferred feed line to a dipole - than coax, of course)
although that might not be the BEST way to do it.

Harry

Roy Lewallen wrote:

The two dipoles don't couple to each other, so you have a lot of options
for feed systems. Transmission delay lines are one. Another is lumped
circuit networks -- an L network can be designed to effect a range of
phase shifts.

I think you'll find, though, that you'll hardly be able to tell when you
rotate the array in any increment smaller than 90 degrees, and unless a
signal is pretty close to a null, even that rotation won't make a
striking difference. It's a problem that's easily modeled, so I'd start
by seeing just what I might gain from it before going to the trouble.

Roy Lewallen, W7EL

Harry Gross wrote:

Hi all,

I'm toying with the idea of an 'electronically rotatable' dipole setup.

Consider: a dipole oriented N/S
another dipole oriented E/W
They cross each other at the mid-point of each, forming a big
plus sign, like so:

|
|
----+----
|
|

If you feed the N/S dipole, you get an E/W radiation pattern.
Similarly, if you feed the E/W dipole, you get a N/S radiation patter.

If you feed the N/S and E/W dipoles simultaneously, you get a NW/SE
radiation pattern (for example), and if you flip the feed of either
one of them 180 degrees, then you get a NE/SW radiation pattern.

So far, so good. Now for the fun part. If you feed one of the
dipoles with a signal that is only 90 degrees out of phase (or 270
degrees), you can get radiation patterns that take of in the NNW/SSE
or WNW/ESE directions, and if you feed the other dipole 90 or 270
degrees out of phase, you get NNE/SSW and ENE/WSW directions.

What I am looking for is a circuit that can perform the necessary
phase delays, without having to use miles of coax coiled up in the
shack. Ideally, the circuit would be able to perform the phase shift
on all (or at least several) ham bands, so that it could feed into a
pair of fan-dipoles for operation on multiple bands.

Anyone have any suggestions on just how to accomplish this? In the
extreme case, you could have a continuously adjustable system, that
would permit you to point the 'virtual dipole' in any direction at
all, which might be useful when attempting to null out an interfering
station off to the side of the desired signal. However, in practice,
I suspect that the eight positions mentioned above would be sufficient
to do the same thing (albeit with the possibility of somewhat more
signal loss due to fixed aiming directions).

I seem to recall seeing a quote in the ARRL Handbook (or the ARRL
Antenna Book) from Roy Lewallen, indicating that there is no simple
way to do this. Am I mis-remembering (or mis-interpreting) what I read?

I know that the military does this sort of thing with radar systems,
and has for some time. However, their budget is quite a bit bigger
than mine, so perhaps their solution isn't 'simple':-)

Any thoughts on this, anyone?

Harry Gross

Harry Gross wrote:
I guess I'll have to break down, buy a copy of EZ-NEC and play with it:-)

By the way, are you suggesting that 90 degree rotation is all I'd need,
simply because the dipole radiation pattern is so wide, broadside to the
radiator? If that turns out to be true, then the problem becomes vastly
easier to deal with, and could even be done mechanically by simply
switching around the feed line to one dipole (easier to do with ladder
line - the preferred feed line to a dipole - than coax, of course)
although that might not be the BEST way to do it.

Harry,

If you feed 90- degree delay, the result is a basically omni pattern.
It has some ripples, but not much.

I think all you would ever be able to use is one antenna, the other
antenna, or both antennas together either in or out of phase.

I can't see any reason to do anything like rotate phase or control
ratio, since the effects would be so small.

73 Tom

Roy Lewallen wrote:
Another is lumped
circuit networks -- an L network can be designed to effect a range of
phase shifts.

Don't you find it strange that lumped-circuits cause phase
shifts sometimes (above) and sometimes not (loading coils)?
--
73, Cecil http://www.qsl.net/w5dxp

Get a life, Cecil.

Tom wrote, "I can't see any reason to do anything like rotate phase or
control
ratio, since the effects would be so small."

One reason to rotate the pattern (however one does it) is to use the
nulls to kill an interfering signal when receiving. Other than that,
there's certainly not much reason. Even with one-or-the-other of a
pair of crossed dipoles, at 45 degrees azimuth from max on a perfect
dipole pattern you're only down about 4dB from max, and if you add in
the feed-both-in-phase (or 180 shift), the patterns cross at about half
a dB down from max. Note though that the pattern for both fed in phase
is a bit broader than that of a single dipole.

Harry: you can simulate all this with the free demo version of EZNEC,
I'm sure.

Cheers,
Tom

K7ITM wrote:
Get a life, Cecil.

Take the impedance at a point on a transmission line
with reflections. Install a series inductor. It
causes an arc on a Smith Chart. How can an arc
on a Smith Chart equate to a phase shift of
zero degrees?
--
73, Cecil http://www.qsl.net/w5dxp

Tom, K7ITM wrote:
"I tried "quadrature variometer" with no real luck."

I found a posting from qsl net that looked interesting. You might look
at the goniometer on page 1050 of Terman`s 1955 edition.

Best regards, Richard Harrisn, KB5WZI