My "stack" consists of an LP at 100 feet and a TH-6 at about 47 feet with
separate feed lines to the base of the tower. Certainly not the ideal
"stack". After correcting for velocity factor, baluns, and effective feed
point in the LP I "equalized" the feed line length as closely as possible.
I also inserted a phase control box which inserts coax sections in 2 foot
increments from 2-14 feet. Checking with AO using 3 element yagis, and
confirmed in practice with my combination, I find that the "optimum" phase
curve is quite broad at the top, with a fairly narrow and very deep notch at
the null. I therefore look for the null which is relatively easy to fine and
then go about 180 degrees one way or the other. With my system at least, I
find the "optimum" stack situation makes little enough difference that in a
contest I don't feel I'm giving much up to leave the TH-6 at 47 feet pointed
at the Carib/SA and doing the rest with the LP at 100 feet. This also puts
the lower radiation angle on the longer paths. Since the LP in on a
relatively slow turning prop pitch motor and the TH-6 is on a much faster
Ham IV, in the late afternoon I'll have the LP on JA and can relatively
easily pick up the stray AF or EU with the TH-6. Similarly, when the LP is
on EU or AF I can pick up ZL and VK with the TH-6.
Gene / W2LU
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <towertalk@contesting.com>
Sent: Thursday, August 01, 2013 12:33 PM
Subject: [TowerTalk] Antenna Arrays, Phase, and Polarity
On 8/1/2013 7:09 AM, Cqtestk4xs@aol.com wrote:
BIP/BOP makes a difference.
Phase is a continuously valued function, it has the units of degrees, and
it increases linearly with frequency. BIP/BOP (both in phase, both out of
phase) is a simple-minded view of things, and confuses PHASE with
POLARITY. Polarity is what we're talking about when we reverse the wires
feeding a circuit element. The difference is VERY important.
Interestingly, the pro audio world figured this stuff out almost 40 years
ago, thanks to the prodding and teaching of the late Dick Heyser, a really
sharp engineer working in communications at JPL, and whose hobby was
audio. He wrote and taught prolifically, and his work is well worthy of
study.
To understand what's going on with our antennas, we must first understand
that there are many sources of phase shift in a system. One is the
transmission lines feeding the antennas, which varies linearly with
frequency, AND is a function of the source and terminating impedances.
There's a discussion of this in the ON4UN book -- look for the discussion
of the "Christman feed" to a 2-element vertical array. Another important
source of phase shift is in the radiation pattern of the antenna, and that
will be different for every antenna. There is also the contribution of
the ground reflection.
As an example of how the difference between phase and polarity matters --
let's say that we produce a 180 degree phase difference by reversing the
feed to one element of an array. That will result in a 180 degree
difference at EVERY frequency. But if we do it with a length of coax, or
by space between elements of the array, or with a reactive network (L and
C), the phase change will vary with frequency.
The radiation from ANY array is FAR more complex than adding the
magnitudes of the components. It's the COMPLEX (magnitude and phase)
addition that occurs, and depending on all of those phase relationships,
can put peaks and nulls in the pattern (both in the horizontal and
vertical plane). Combine this with wildly varying propagation, which
causes signals to arrive at varying vertical angles, and even to take
skewed paths, and it's obvious that a null can do quite a lot of damage.
:)
73, Jim K9YC
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