Antenna pattern from two antennas
 

If I have two antennas hooked up to a transmitter, is there an easy way to
very the phase to one or both of the antennas
to be able to change the pattern around electronically?

Thanks

"John Smith" wrote in message
...
If I have two antennas hooked up to a transmitter, is there an easy way to
very the phase to one or both of the antennas
to be able to change the pattern around electronically?

Sure, drive the second antenna through a section of feedline from the first
line.
Look for description on direction finding antennas with cardioid patterns.

Dave VanHorn wrote:
"John Smith" wrote:
If I have two antennas hooked up to a transmitter, is there an easy way to
very the phase to one or both of the antennas
to be able to change the pattern around electronically?

Sure, drive the second antenna through a section of feedline from the first
line.

Not as easy as it sounds. It's a juggling act between phase shift and
impedance matching. Reference: "The Simplest Phased Array Feed System
.... That Works", Roy Lewellen, W7EL, ARRL Antenna Compendium, Vol 2.
--
73, Cecil http://www.qsl.net/w5dxp



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Not as easy as it sounds. It's a juggling act between phase shift and
impedance matching. Reference: "The Simplest Phased Array Feed System
... That Works", Roy Lewellen, W7EL, ARRL Antenna Compendium, Vol 2.

http://users.rcn.com/dmurphy.nai/mar...unt/gnvrdf.pdf
I've used this design a fair bit, it works nicely.
Look under "Well, how do I get a directional bearing"

"...easy way ..."? To change the phase, yes.... To change the pattern,
Probably not. The impedance changes with the phase relationship. Antennas
feed power to each other. Try searching for "Phased arrays".

--
Steve N, K,9;d, c. i My email has no u's.
"John Smith" wrote in message
...
If I have two antennas hooked up to a transmitter, is there an easy way to
very the phase to one or both of the antennas
to be able to change the pattern around electronically?

Thanks

Steve Nosko wrote:
"To change the phase, yes...To change the pattern. Probably not."

Certainly changing just the phase of the signal between two identical
driven elements makes an enormous difference in radiation pattern.

My newest copy of the ARRL Antenna Book is the 19th edition, but most
editions will have a figure similar to Fig 11 on page 8-8 of my copy.

The double-page figure is arranged into (8) vertical columns of
radiation patterns. Each column is for a different physical spacing
between the two parallel elements. All of the (5) patterns in a vertical
column have the same physical space between elements.

The only change between patterns in a vertical column is the phase angle
between the two elements. The top pattern is for 0-degrees between the
elements; they are fed in-phase. Other patterns are given for: 45, 90,
135, and 180-degrees electrical phasing between the two elements. It is
obvious that the pattern changes every time the phase between the
elements changes.

There are several ways to get the desired phase change. Roy Lewallen has
written an article in QST on the subject and has entered suggestions in
the ARRL Antenna Book. You can find them for yourself. Commercial arrays
often use a special T-network.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
The double-page figure is arranged into (8) vertical columns of
radiation patterns. Each column is for a different physical spacing
between the two parallel elements. All of the (5) patterns in a vertical
column have the same physical space between elements.

The only change between patterns in a vertical column is the phase angle
between the two elements.

Are they assuming equal currents into each element? That's the real trick.
--
73, Cecil http://www.qsl.net/w5dxp



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"Richard Harrison" wrote in message
...
Steve Nosko wrote:
"To change the phase, yes...To change the pattern. Probably not."

Certainly changing just the phase of the signal between two identical
driven elements makes an enormous difference in radiation pattern.

Obviously I was not complete in my response. I was focusing on the
"simple" part. Where I was going here was that simply paralleling the two
feeds with different coax lengths to set the phase difference won't do it.
(perhaps too much assumption on my part regarding the OPs desired patterns
and definitino of simple) The job of combiming the two feeds is non-trivial.
If you drive two antennas with a given power ratio (say, equal) but
different phase, the patterns are easy to calculate. However, you can't
just parallel the two lines. For equal powers in the antennas, I believe
the patterns are well known. What about the coupling effect between
antennas?

Richard, Are the patterns in the handbook all equal power division? I
don't think I have a recent handbook...

In a case of a broadcast antenna pattern some years ago, it turned out that
to get one of the desired patterns, one of the antennas had to actually
absorb power. There was a negative resistance term that fell out of one of
the equations and the original engineer had problems desiging the network.
An associate of mine dug into it and figured it out.
'guards,
--
Steve N, K,9;d, c. i My email has no u's.

On 23-Mar-2004, "Steve Nosko" wrote:

Richard Harrison" wrote in message
...
Steve Nosko wrote:
"To change the phase, yes...To change the pattern. Probably not."

Certainly changing just the phase of the signal between two identical
driven elements makes an enormous difference in radiation pattern.

Obviously I was not complete in my response. I was focusing on the
"simple" part. Where I was going here was that simply paralleling the two
feeds with different coax lengths to set the phase difference won't do it.
(perhaps too much assumption on my part regarding the OPs desired patterns
and definitino of simple) The job of combiming the two feeds is non-trivial.
If you drive two antennas with a given power ratio (say, equal) but
different phase, the patterns are easy to calculate. However, you can't
just parallel the two lines. For equal powers in the antennas, I believe
the patterns are well known. What about the coupling effect between
antennas?

Richard, Are the patterns in the handbook all equal power division? I
don't think I have a recent handbook...

In a case of a broadcast antenna pattern some years ago, it turned out that
to get one of the desired patterns, one of the antennas had to actually
absorb power. There was a negative resistance term that fell out of one of
the equations and the original engineer had problems desiging the network.
An associate of mine dug into it and figured it out.
'guards,
--
Steve N, K,9;d, c. i My email has no u's.

I believe that Roy Lewellen published an article in the ARRL Antenna Compendium titled something
like: "The simplest phasing method - That works" in which he goes into the difficulties of getting
the desired phase delays to coupled antenna elements using transmission lines fed from a common
source. The coupled energy has an effect on the feedpoint impedance which affects the phase of the
feedpoint current.

Ken, KO6NO

Cecil, W5DXP wrote:
"Are they assuming equal currents into each element? That`s the real
trick."

The subscript says:
"The two elements are assumed to be thin and self-resonant, with
equal-amplitude currents flowing at the feed-point."

If everything is symmetrical, the self-impedances and the mutual
impedances of the two elements should be equal, producing equal powers
into each element.

Roy Lewallen may have been the source of a caution on phasing errors
which appears on page 8-13 of the 19th edition of the ARRL Antenna Book.
Roy is named on page 8-12.

Kraus says on page 284 of the 1950 edition of "Antennas":
"It is important that the antenna power W be considered
constant.---Until the antenna power was considered constant by G.H.
Brown (Proc. I.R.E., January 1937) the advantages of closely spaced
elements were not apparent. Prior to this time the antenna current had
usually been considered constant."

Kraus took a G.H. Brown idea and ran with it producing the W8JK antenna.
He had a lot of trouble gettinng the W8JK antenna story published due to
naysayers. But it works despite its low impedance.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
If everything is symmetrical, the self-impedances and the mutual
impedances of the two elements should be equal, producing equal powers
into each element.

Unfortunately, it is not that easy except under special circumstances.
The element with leading phase often has a different feedpoint impedance
than the element with lagging phase. For instance:

Given two 1/4WL monopoles, 1/4WL apart, and fed 90 degrees apart with
one amp each will exhibit a gain of 3 dBi in one direction. However,
the feedpoint of one element is 20-j20 and the feedpoint of the other
element is 50+j20. The feedpoint voltages are obviously not equal so
to equalize the current magnitudes takes some juggling. That' what
Roy's BASIC program does - finds a solution if one exists.

I once had a BASIC program that calculated the mutual impedances given
the feedpoint impedance of one element alone and the feedpoint impedances
of the two elements during operation but I seem to have misplaced it.
--
73, Cecil http://www.qsl.net/w5dxp



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Steve Nosko wrote:
"Are the patterns in the handbook all equal power division?"

The subscript says:
"The two elements are assumed to be thin and self-resonant, with
equal-amplitude current flowing at the feed-point."

I think the "Antenna Book" authors were familiar with Kraus` Fig. 11-11
on page 290 of the 1950 edition of "Antennas". Kraus` Fig. 11-11 is
similar to the "Antenna Book" Fig 11 on page 8-8 of the 19th edition.
Kraus makes a point of G.H. Brown`s equal power observations.

I am familiar with the negative-resistance tower occasionally found in a
broadcast array. John E. Cunningham says in the "Complete Broadcast
Antenna Handbook":
"In an array of four towers or more, the resistive part of the
driving-point impedance of one or more of the towers often has a
negative value. This means that the tower obtains its energy through the
mutual impedance between it and the other towers of the array. This is a
confusing situation, but if it is carefully thought out, it will cause
no serious problems. We know the following things concerning the
negative tower:

1. The tower must carry a current of the proper magnitude and phase.
2. The direction of this current is 180-degrees out of phase with what
it would be in a tower having a positive base resistance.
3. We need some method of controlling the magnitude and phase of the
tower current.

The simplest, although not the most efficient way of handling the
negative-resistance tower is to terminate it through a matching network
to a resistor, as swhown in Fig.11-15. The energy that the negative
tower actually gets from the other towers is thus dissipated in the
resistor. The magnitude and phase of the current may be controlled by
the parameters of the network. Naturally, this isn`t a very efficient
arranngement, particularly if the negative tower handles a substantial
amount of current.

The preferred way to handle a negative tower is to feed the energy back
to the power divider, where it will be passed back into the feeder
system again. In this way, all of the energy is radiated rather than
some being dissipated in a resistor.

Figure 11-16 shows an arrangement for recovering power from a
negative-resistance tower.----"

Since I can`t do diagrams, I suggest finding a copy of the book. It`s a
good one.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
....
The preferred way to handle a negative tower is to feed the energy back
to the power divider, where it will be passed back into the feeder
system again. In this way, all of the energy is radiated rather than
some being dissipated in a resistor.

This makes sense, but I wonder if this condition can be made to hold
true
over the bandwidth of the transmitted signal. Would this scheme result
in
a system that had such a high Q that it would quickly degrade the
further away from the carrier frequency you got (i.e. mismatch at the
sideband frequencies)?

regards
L

Not sure I understand. Why not use sections of different impedance coax
to raise the effective antenna impedance of each antenna to 100 Ohms,
then use different lengths of 50 Ohm coax from the "Tee" adapter to
these matching sections. The delay difference will cause the pattern to
be steered in some direction. I assume you will be using crossed
dipoles. When the two 100 Ohm antennas are connected together through
the Tee, it will result in a system impedance of 50 Ohms. As long as
you use resonant dipoles, current and voltage will be in phase in its
respective dipole, so the impedance will not change. You are only
providing a phase difference between the two dipoles by changing the
path length the signals travel from the transmitter to each antenna. It
should work. We basically did this when I worked at the U.S. Navy ELF
transmitter site.


Steve Nosko wrote:
"...easy way ..."? To change the phase, yes.... To change the pattern,
Probably not. The impedance changes with the phase relationship. Antennas
feed power to each other. Try searching for "Phased arrays".

acepilot wrote:
As long as
you use resonant dipoles, current and voltage will be in phase in its
respective dipole, so the impedance will not change.

This isn't always true. The feedpoint impedances of two resonant
dipoles when phased together are not usually resistive. For instance
using EZNEC:

A 33 ft. dipole at 66 ft. has a feedpoint impedance of 70+j0 ohms
on 7.298 MHz.

Two 33 ft. dipoles at 66 ft. spaced 33 ft. apart and fed 90 degrees
out of phase have the following feedpoint impedances on 7.298 MHz:

109+j34

29-j33

Please describe how you would achieve equal magnitude currents 90
degrees apart into those dipoles given those feedpoint impedances.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:
acepilot wrote:

As long as you use resonant dipoles, current and voltage will be in
phase in its respective dipole, so the impedance will not change.


This isn't always true. The feedpoint impedances of two resonant
dipoles when phased together are not usually resistive. For instance
using EZNEC:

A 33 ft. dipole at 66 ft. has a feedpoint impedance of 70+j0 ohms
on 7.298 MHz.

Two 33 ft. dipoles at 66 ft. spaced 33 ft. apart and fed 90 degrees
out of phase have the following feedpoint impedances on 7.298 MHz:

109+j34

29-j33

Please describe how you would achieve equal magnitude currents 90
degrees apart into those dipoles given those feedpoint impedances.

I just ran this matching problem through Roy's (W7EL) SIMPFEED BASIC
program downloadable from http://www.eznec.com

Using 300 ohm twinlead, the program said that one feedline should be
27 degrees and the other should be 168.5 degrees. For 7.298 MHz and
VF=0.9, that's lengths of 9.1 ft. and 56.78 ft. I plugged them into
EZNEC and it worked great - a phased dipole array with 10.5 dBi gain
in one direction. The above lengths ensure that equal currents flow
in both elements and gives a 50 ohm SWR of about 2:1 at the junction
of the two 300 ohm feedlines.

Incidentally, there were no solutions using 50 or 75 ohm coax.
--
73, Cecil http://www.qsl.net/w5dxp



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Oops, my bust I guess. Damn computer programs! ;) Well, I probably
should have stated that this should be "tried at your own risk". I
guess that my point was that it should be possible to steer a signal by
using crossed dipoles. We do it at ELF frequencies for submarine comms,
so it should work at any other frequency as well...just takes a little
dinkin' around...but hey, that's what autotuners are for ;)

Scott
N0EDV



Cecil Moore wrote:
Cecil Moore wrote:

acepilot wrote:

As long as you use resonant dipoles, current and voltage will be in
phase in its respective dipole, so the impedance will not change.



This isn't always true. The feedpoint impedances of two resonant
dipoles when phased together are not usually resistive. For instance
using EZNEC:

A 33 ft. dipole at 66 ft. has a feedpoint impedance of 70+j0 ohms
on 7.298 MHz.

Two 33 ft. dipoles at 66 ft. spaced 33 ft. apart and fed 90 degrees
out of phase have the following feedpoint impedances on 7.298 MHz:

109+j34

29-j33

Please describe how you would achieve equal magnitude currents 90
degrees apart into those dipoles given those feedpoint impedances.


I just ran this matching problem through Roy's (W7EL) SIMPFEED BASIC
program downloadable from http://www.eznec.com

Using 300 ohm twinlead, the program said that one feedline should be
27 degrees and the other should be 168.5 degrees. For 7.298 MHz and
VF=0.9, that's lengths of 9.1 ft. and 56.78 ft. I plugged them into
EZNEC and it worked great - a phased dipole array with 10.5 dBi gain
in one direction. The above lengths ensure that equal currents flow
in both elements and gives a 50 ohm SWR of about 2:1 at the junction
of the two 300 ohm feedlines.

Incidentally, there were no solutions using 50 or 75 ohm coax.

And one sidenote...

Cecil, I think you were implying that the dipoles you modeled were
parallel to each other, correct? Our ELF antennas were dipoles that
were perpendicular to each other. In theory, there should be minimal
interaction between them because of the nulls off of each end of the
antennas, correct? Somebody else mentioned that the antennas, when
driven, feed power into each other. Placing them at 90 degrees to each
other should minimize interaction, would it not? More stuff to digest!

Anyhow, whoever wants to give it a try, have fun and let us know how
well it works!!

Scott
N0EDV

acepilot wrote:

Oops, my bust I guess. Damn computer programs! ;) Well, I probably
should have stated that this should be "tried at your own risk". I
guess that my point was that it should be possible to steer a signal by
using crossed dipoles. We do it at ELF frequencies for submarine comms,
so it should work at any other frequency as well...just takes a little
dinkin' around...but hey, that's what autotuners are for ;)

Scott
N0EDV



Cecil Moore wrote:

Cecil Moore wrote:

acepilot wrote:

As long as you use resonant dipoles, current and voltage will be in
phase in its respective dipole, so the impedance will not change.




This isn't always true. The feedpoint impedances of two resonant
dipoles when phased together are not usually resistive. For instance
using EZNEC:

A 33 ft. dipole at 66 ft. has a feedpoint impedance of 70+j0 ohms
on 7.298 MHz.

Two 33 ft. dipoles at 66 ft. spaced 33 ft. apart and fed 90 degrees
out of phase have the following feedpoint impedances on 7.298 MHz:

109+j34

29-j33

Please describe how you would achieve equal magnitude currents 90
degrees apart into those dipoles given those feedpoint impedances.



I just ran this matching problem through Roy's (W7EL) SIMPFEED BASIC
program downloadable from http://www.eznec.com

Using 300 ohm twinlead, the program said that one feedline should be
27 degrees and the other should be 168.5 degrees. For 7.298 MHz and
VF=0.9, that's lengths of 9.1 ft. and 56.78 ft. I plugged them into
EZNEC and it worked great - a phased dipole array with 10.5 dBi gain
in one direction. The above lengths ensure that equal currents flow
in both elements and gives a 50 ohm SWR of about 2:1 at the junction
of the two 300 ohm feedlines.

Incidentally, there were no solutions using 50 or 75 ohm coax.

Scott, N0EDV wrote:
"Anyhow, whoever wants to give it a try, have fun and let us know how it
works!!"

I don`t have EZNEC but it seems to be very useful.

Crossed dipoles are often used as quasi omnidirectional antennas.
Broadcasters call it a "turnstile".

If the two dipoles are fed in phase, a figure-eight pattern results.
This is similar to the pattern of a single dipole but at 45-degrees to
the crossed dipoles.

The usual practice is to excite the dipoles 90-degrees out-of-phase.
This produces a nearly circular pattern in the plane of the turnstile.

One could get a choice of patterns with crossed dipoles by feeding them
in-phase, out-of-phase, and in quadrature for an omni pattern. Then
using the dipoles one at a time, two more diirectional patterns are
available.

Best regards, Richard Harrison, KB5WZI

acepilot wrote:
Cecil, I think you were implying that the dipoles you modeled were
parallel to each other, correct? Our ELF antennas were dipoles that
were perpendicular to each other. In theory, there should be minimal
interaction between them because of the nulls off of each end of the
antennas, correct? Somebody else mentioned that the antennas, when
driven, feed power into each other. Placing them at 90 degrees to each
other should minimize interaction, would it not?

Yep, that's true, and a turnstile is an example. But for a phased beam,
one needs maximum interaction. The dipoles in my example are 1/4WL
apart, parallel, and in the same horizontal plane.

Incidentally, one of the disadvantages of Roy's SIMPFEED program is
that one needs to know the mutual coupling impedance between the
elements. For a two-element system, with identical elements, there
is a way to use EZNEC to calculate (estimate) the mutual coupling
impedance, Rm +/- jXm.

For two identical (resonant) elements, the feedpoint impedances reported
by EZNEC will be of the form, (Rs +/- Xm) +/- jRm, where Rs is the
resonant resistance of a single element alone (second element
open-circuited).

For instance, in my earlier example of two 33 ft dipoles, 33 ft apart
at a height of 66 ft, fed 90 degrees apart - the feedpoint impedances
a

109+j34 and 29-j34

That makes Rm = 34 ohms and makes Rs (109+29)/2 = 69 ohms, which
makes Xm = -j39 ohms. Those Rm and Xm values can then be plugged
into Roy's SIMPFEED program to obtain the length of the feedlines.

Note that two phased 20m dipoles work just fine as a beam on 17m.
All it takes is different phasing of the feedlines.
--
73, Cecil http://www.qsl.net/w5dxp



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"Richard Harrison" wrote in message
...
Steve Nosko wrote:
"Are the patterns in the handbook all equal power division?"

The subscript says:
"The two elements are assumed to be thin and self-resonant, with
equal-amplitude current flowing at the feed-point."

I see in another post there is also talk of equal power vs equal
currents. This nicely skirts the issue of HOW do you get them to be equal.
I should keep my nose out of these complex discussions, but I think this
supports my original comment that in general, it is not "easy".

Also, Good description of the antenna problem, Richard. Good
refresh of memory. I wouldn't have been able to do it justice from memory.


Since I can`t do diagrams, I suggest finding a copy of the book. It`s a
good one. Best regards, Richard Harrison, KB5WZI

If I had a burning desire to get into this subject I would, but I don't.
Thanks,
--
Steve N, K,9;d, c. i My email has no u's.

I would say yes. This certainly makes sense. The techniques mentioned are
used in fixed frequency broadcast. Any power or phase changes would affect
the pattern and any power matching - dividing network most certainly will
have frequency dependence.

"aa6lk" wrote in message
...
Richard Harrison wrote:
....
The preferred way to handle a negative tower is to feed the energy back
to the power divider, where it will be passed back into the feeder
system again. In this way, all of the energy is radiated rather than
some being dissipated in a resistor.

This makes sense, but I wonder if this condition can be made to hold
true
over the bandwidth of the transmitted signal. Would this scheme result
in
a system that had such a high Q that it would quickly degrade the
further away from the carrier frequency you got (i.e. mismatch at the
sideband frequencies)?

regards
L

Yikes! Double Yikes!! the latest on this thread.

Agreement and a non adversarial discussion of the technology. Good example
fellas. Keep up the good work.
--
Steve N, K,9;d, c. i My email has no u's.

"Cecil Moore" wrote in message
...
acepilot wrote:
Cecil, I think you were implying that the dipoles you modeled were
parallel to each other, correct? Our ELF antennas were dipoles that
were perpendicular to each other. In theory, there should be minimal
interaction between them ....

Yep, that's true, and a turnstile is an example. But for a phased beam,
one needs maximum interaction. ...
! =-----

Steve Nosko wrote:
I see in another post there is also talk of equal power vs equal
currents. This nicely skirts the issue of HOW do you get them to be equal.
I should keep my nose out of these complex discussions, but I think this
supports my original comment that in general, it is not "easy".

It's "EZ" if you use W7EL's SIMPFEED.ZIP stuff :-) downloadable from:
http://www.eznec.com I have modified one of his programs to take the
feedpoint impedances predicted by EZNEC as the inputs to Roy's feedline
phasing program. It's actually "EZ". :-)
--
73, Cecil http://www.qsl.net/w5dxp



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Steve Nosko wrote:
Agreement and a non adversarial discussion of the technology. Good example
fellas. Keep up the good work.

When both sides are technically correct, there can be no valid argument. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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I'll take that as a compliment! I never see a reason to get all bent
out of shape and start name calling anyway. Guess I'm a "Live and let
live" pacifist ;)

Scott
N0EDV



Steve Nosko wrote:
Yikes! Double Yikes!! the latest on this thread.

Agreement and a non adversarial discussion of the technology. Good example
fellas. Keep up the good work.

Again I say, thanks for the compliment! I am by no means an expert on
this topic or antennas in general. I just love antennas! Plus, I was
trying to recall, from my (shorter by the day) memory how our ELF
antennas worked. I haven't worked there for just shy of 9 years...and I
can't even remember what I had for dinner last night ;)

I've often thought of giving the crossed dipoles (I guess you call it a
turnstile antenna) a try on 75M since a 2 element yagi or quad there
might be a bit unwieldy :O Now if I could only remember the math
formula to figure the direction of steering by knowing the phase
difference between the feeds of each antenna. I do remember it was
fairly simple and used sine, cosine, or tangent. Maybe I'll have to ask
a buddy still working there if he can dig the info up in the books
there. Actually, he worked for the company who built the site in the
1980's so he might even be able to pull it from memory. I don't think
that information would be classified :)

Scott


Cecil Moore wrote:

Steve Nosko wrote:

Agreement and a non adversarial discussion of the technology. Good
example
fellas. Keep up the good work.


When both sides are technically correct, there can be no valid argument.
:-)

Scott, N0EDV wrote:
"O Now if I could only remember the math formula to figure the direction
of steering---."

All the simple options are bidirectional except the omni which results
from 90-degree phasing between the two dipoles.

You know that used separately, maximum radiation is broadside to the
energized dipole.

Fed in-phase or out-of-phase, the crossed dipoles have lobes at
45-degrees and 225-degrees, or at 135-degrees and 315-degrees. The
figure-8 pattern is the same as from a single dipole but shifted plus or
minus 45-degrees, depending on in-phase or out-of-phase feed of the two
dipoles.

Patterns of the crossed dipoles are similar to the oscilloscope display
of the same signal fed to both sets of defllection plates but with a
variable or selected phase angle between the plates.

Just by selecting one dipole or the other you could have a north-south
or east-west pattern.

By quadrature feed of the two dipoles you get a near circullar pattern
from the crossed dipoles.

From two dipoles and a 90-degree delay for a non-directional pattern,
and with some switching you get 5 radiation patterns. That`s pretty
versatile.

To get the 90-degree phase shift, a T-network with equal reactances in
all branches is often used. For a 90-degree lag, coils X1 and X2 are in
series with the load. Capacitor X3 connects between junction of the
coils and the other side of the line.

For a 90-degree lead, replace the coils with capacitors, and replace the
capacitor with a coil in the T-network.

It`s easy to remember the reactance values.

X1=X2=X3=Zo= sq rt (ZinZL)

The reactances may well be 50-ohms if we have a match to the usual load
impedance.

To adjust the phase lag of the T-network by as much as plus or minus
15-degrees without significantly affecting the magnitude of the
shifter`s output, X1 and X2 are often ganged variable inductors.

An imperfection in the phase shifter may result from uncertainty about
its input and output impedances. Nevertheless, many T-network phase
shifters are in use.

Best regards, Richard Harrison, KB5WZI

acepilot wrote:
I've often thought of giving the crossed dipoles (I guess you call it a
turnstile antenna) a try on 75M since a 2 element yagi or quad there
might be a bit unwieldy :O Now if I could only remember the math
formula to figure the direction of steering by knowing the phase
difference between the feeds of each antenna.

A turnstile has a fixed 90 degree relationship between the two dipoles.
This makes it somewhat of an NVIS antenna good for satellite communications.
On 75m, a low dipole does approximately the same thing. If you are thinking
of changing the phasing away from 90 degrees, it would technically not be
defined as a "turnstile".
--
73, Cecil http://www.qsl.net/w5dxp



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It's possible to get exact self and mutual impedances from EZNEC. I'll
explain the method for two identical elements.

Excite the two elements with equal, in-phase currents. Record the
feedpoint impedance of either element (they should be the same) as Z0
(= R0 + jX0). Then change the phase of one of the currents to 180
degrees, so the elements are fed exactly out of phase. Record the
feedpoint impedances with this excitation as Z180 (= R180 + jX180).

The mutual impedance Zm = (Z0 - Z180) / 2

The self impedance can also be found as Zs = (Z0 + Z180) / 2

For example, use the Cardioid.EZ EZNEC example file. Change the phase of
the second source to zero, click Src Dat, and note the element impedance
Z0 = 56.11 - j14.22. Change the phase of the second source to 180, click
Src Dat again, and note the impedance Z180 = 16.54 + j16.37. The mutual
Z is then (56.11 - 16.54) - j(-14.22 - 16.37) = 19.8 - j15.3. The self Z
is 36.3 + j1.1. These values can be used in SIMPFEED program Lewall1.

As it turns out, you can also calculate the exact self and mutual
impedances from the feedpoint impedances of two elements fed 90 degrees
out of phase. For identical elements fed with equal magnitude 90 degree
phased currents, where Z1 is the feedpoint impedance of the leading
element (that is, element 2 is fed at -90 degrees relative to element 1)
and Z2 is the feedpoint impedance of the lagging element,

Rm = (X2 - X1) / 2
Xm = (R1 - R2) /2

and

Rs = (R1 + R2) / 2
Xs = (X1 + X2) / 2

Caution: Don't think that because the self impedance is the average of
the two feedpoint impedances in the above two special cases, that it's
always true. It isn't.

Going back to the Cardioid model as it comes with EZNEC, note that
Z1 = 21.03 - j18.71 and Z2 = 51.61 + j20.86 when the elements are fed at
90 degrees. So

Rm = (20.86 - -18.71) / 2 = 19.8
Xm = (21.03 - 51.61) / 2 = -15.3
Rs = (21.03 + 51.61) / 2 = 36.3
Xs = (-18.71 + 20.86) / 2 = 1.1

exactly the values calculated before. Note that the values of mutual
impedance are very close to the values from the graph in Chapter 8 of
the ARRL Antenna Book.

The equations for these special cases are derived from the more general
equations which can be found in Chapter 8 of the ARRL Antenna Book, and
numerous other references. In the 20th Edition of the Antenna Book,
they're Eq 20 and 21 on p. 8-19. Equations can easily be derived for two
dissimilar elements from feedpoint impedances with in-phase and
out-of-phase excitation with equal currents. And although it's possible
to derive equations for self and mutual Z from the feedpoint impedances
of more complex arrays, it requires more "measurements" in order to have
enough equations for the increased number of unknowns.

Roy Lewallen, W7EL

Cecil Moore wrote:
acepilot wrote:

Cecil, I think you were implying that the dipoles you modeled were
parallel to each other, correct? Our ELF antennas were dipoles that
were perpendicular to each other. In theory, there should be minimal
interaction between them because of the nulls off of each end of the
antennas, correct? Somebody else mentioned that the antennas, when
driven, feed power into each other. Placing them at 90 degrees to
each other should minimize interaction, would it not?


Yep, that's true, and a turnstile is an example. But for a phased beam,
one needs maximum interaction. The dipoles in my example are 1/4WL
apart, parallel, and in the same horizontal plane.

Incidentally, one of the disadvantages of Roy's SIMPFEED program is
that one needs to know the mutual coupling impedance between the
elements. For a two-element system, with identical elements, there
is a way to use EZNEC to calculate (estimate) the mutual coupling
impedance, Rm +/- jXm.

For two identical (resonant) elements, the feedpoint impedances reported
by EZNEC will be of the form, (Rs +/- Xm) +/- jRm, where Rs is the
resonant resistance of a single element alone (second element
open-circuited).

For instance, in my earlier example of two 33 ft dipoles, 33 ft apart
at a height of 66 ft, fed 90 degrees apart - the feedpoint impedances
a

109+j34 and 29-j34

That makes Rm = 34 ohms and makes Rs (109+29)/2 = 69 ohms, which
makes Xm = -j39 ohms. Those Rm and Xm values can then be plugged
into Roy's SIMPFEED program to obtain the length of the feedlines.

Note that two phased 20m dipoles work just fine as a beam on 17m.
All it takes is different phasing of the feedlines.

acepilot wrote in message

I've often thought of giving the crossed dipoles (I guess you call it a
turnstile antenna) a try on 75M since a 2 element yagi or quad there
might be a bit unwieldy :O

I've used turnstiles on 75m off and on for quite a few years. If you
are looking for gain over a dipole, don't bother. You won't see it.
You will only have the gain of a dipole when fed in phase, or 180
degrees out. "Changes direction".
The gain when fed 90 degrees out will be less than a dipole, being the
pattern becomes nearly omnidirectional. Don't let this totally scare
you away though..
Myself, I think the turnstile is one of the best NVIS antennas you can
use on the lower hf bands. If you want gain over a dipole, run them
parallel, and change phase. You'll see more gain over a dipole, if
thats what you want. MK