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

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

"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.

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.