----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <towertalk@contesting.com>
Sent: Tuesday, November 25, 2003 5:47 AM
Subject: [TowerTalk] Really elementary question
> Somehow in my 49 years as a ham I've managed to avoid practical experience
> with phased arrays, but now I'm working on an array of K9AY loops for 160m
> receiving, and realize that I lack an essential piece of information. A
> cruise through the antenna Book hasn't helped.
>
> The question is this -- when a two-element array is specified as having
one
> element 90 degrees from the other -- in modeling terms, antenna A's phase
> is zero and antenna B's is 90 -- does this mean that you feed antenna A
and
> then connect from there to B through 90 degrees of coax (1/4 WL x velocity
> factor) or is it the other way around?a
Generally neither, because a quarter wavelength of coax won't create a 90
degree phase shift unless it just happens to be terminated in it's
characteristic impedance (which no antenna, particularly one close to
another one, will be)
The best way to figure this stuff out is to draw a little picture with the
two antennas being dots the appropriate distance apart. Then sketch in a
sinewave of the appropriate wavelength along the directions of interest from
the two antennas.
However, to answer your question quickly... Assuming everything ideal.. two
isotropic radiators 1/4 wavelength apart, fed 90 degrees out of phase. Lets
assume that the phase of A LEADS the phase of B (that is, in a perfect
world, the coax to B is 90 degrees longer). Considering a wave leaving A
heading in the direction of B. It gets to B 1/4 cycle (90 deg) later. The
electrical signal to B (from the feedline) gets there exactly at the same
time, so the two signals add. This tells you that the main lobe is pointing
in the direction from A to B.
Consider a wave leaving B, heading towards A. When it gets to A, it will
have been delayed 90 degrees. BUT, at the same time, the wave coming off A
is 180 degrees farther along (90 in the air, 90 in the coax to B). They're
opposite in polarity, cancel, and that gives you the null.
What about 0 and -90? If the two
> antennas are supposed to be 180 out of phase can you connect them in
either
> sequence, as long as the line between them is an electrical half-wave?
I'd
> think "yes" but I've grown wary of my intuition on such things.
A half wave (or any multiple of a half wave) works, regardless of match,
etc.(unless the antennas are real close together, in which case you're going
to have to deal with the loss in the cable from circulating currents). For
this case, you need to connect the antenna terminals like to like (with the
180 feed line in between). Kind of like phasing stereo speakers.
-----------
90 degree phase shifts are VERY hard to get with a casual feed system. You
can get pretty close with the "Christman" approach by choosing the right
lengths of feed lines to form transformers for the impedances, but there's a
big implication that you actually know the feedpoint impedances and mutual
coupling.
Fortunately, if you screw it up, the performance doesn't suffer all that
much. Mostly, the null depth gets less. Main lobe gain doesn't change very
much.
>
> 73, Pete N4ZR
> The World HF Contest Station Database was updated October 29.
> 2469 stations are listed -- 29 new and over 100 updated.
> Are you current? www.pvrc.org/wcsd/wcsdsearch.htm
>
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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