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