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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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. ...
! =-----

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.