"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

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

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

"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.

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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