On 9/3/2013 8:28 AM, K7LXC@aol.com wrote:
OTOH the potential problems with out-of-phase antennas can be
disastrous. Bob Heil's , K9EID, dramatic demonstration of out-of-phase speakers
(another propagation device) will give you pause to maybe pursue more precise
phasing. (The sound disappears when they are out of phase. Imagine your
antennas canceling each other out.)
While the principle at play in Bob's demonstration is important, he used
the wrong word to describe it. The correct word to describe the reversal
of a pair of wires carrying signal, and the inversion a a signal in a
gain stage is "polarity," not "phase." Polarity is a two-valued
function -- either positive or reversed -- and it is independent of
frequency. Phase is a continuously valued function and has the units of
degrees or radians.
Phase can be described only as the difference between two signals of the
same frequency, and for any method that we use to shift the phase,
increases linearly with frequency. For example, the signal from two
ideal sources that are at different points in space will have a
different phase relationship with each other at every frequency, and at
every point in space (because the travel time to every point in space is
different). When we shift the phase between two signals by means of a
network, or by adding a length of transmission line to one of them, the
phase shift will be different at every frequency. But when we reverse
the polarity of the feed to one of the radiators, the phase does not
change, but their fields are opposite at all frequencies.
The difference is critically important in understanding how things
work. In the field of pro audio, we learned to appreciate and understand
this principle more than 30 years ago, and to use the right words to
describe what we're talking about. It's long past time for RF engineers
to learn them.
Now, getting to the original question -- how to calculate the needed
length of delay lines for stacked antennas when those antennas are
different from each other. If I were faced with this problem, I would
build an NEC model of both antennas, with the geometric relationship
between them as close as I could get to how I could mount them, add the
feedlines to the model, and tweak the line lengths until I got the far
field patterns I wanted. Why? Two very important reasons. First, the
amplitude and phase of the field from the two antennas will be different
at every vertical and horizontal angle, and the far field pattern will
be the result of the complex addition (magnitude and phase) at every
angle. Second, the phase shift in the delay lines will be dependent
upon the complex impedance at the feedpoint of the antenna. There's a
discussion of this second effect in the ON4UN book -- look for Christman
feed in the index. It's in the chapter on phased vertical antennas.
73, Jim K9YC
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