Roger said: I'm not an engineer and am having a hard time understanding
this.
It's my understanding that, if a cable is terminated in it's
characteristic impedance, the phase shift will be equal to the
electrical line length regardless of source impedance. If this is
true, then the phase shift through the antenna matching system,
whether it be active or passive, would be irrelevant as long as
all were stable and equal.
Roger you are correct however what some have missed is that most people
cannot assume that their load impedance on their cable is stable or correct
to keep a constant cable phase shift. If you make it so you are fine. Some
are advocating simply the use of a transformer with a shortened element
which is a highly reactive source. Some, not all 4-square systems simply
connect 2 phases in parallel to sum signals which makes the cable load
impedance consist of the other antenna and its cable in parallel with the
impedance of the system you are driving. Usually a highly reactive
termination. Of course adding terminated amps or matching devices would
correct the mistermination. Yes, you can build arrays with these transformer
coupled elements and cables, however one should not just simply consider
them equal replacements without a complete system design review. I was
unable with misterminations to keep consistent phase shifts for my design
goals when switching 8 slightly different antennas with 8 slightly different
cables into slightly different load impedances for 8 different directions.
Not a good situation for predicting the phase shift as the loads change. In
my system I drive the antenna/cable combinations into Magic Tee combiners.
These combiners input impedance is also controlled by what the load
impedances are on the other input port as well as the load impedance on the
output port. After chasing phase shifts around and around these circuits for
the better part of 6 years off and on I found one could more easily assume
the shifts were irrelevant only as long as one could maintain the load
impedance through all the environmental changes or in my case make it 75
ohms and forget it. One of the things we must also remember that we are
talking in generalities here. Four-square systems and other smaller arrays
are pretty tolerant of phase and amplitude changes whereas we with more
complicated and miniaturized arrays need to be more concerned. It might not
indeed make any measurable difference on a 4 square with misterminations
however as dimensions changed each and every one system would be different.
The first time I built one of these complicated arrays the phasing was
so far off I nearly gave up. I had to eliminate a lot of assumptions I made
before it started to work. For instance a simple toroid transformer in my
system causes over 2 degrees of phase lag at 160 meters. At some cable
lengths and when they are misterminated the rate of change of phase as the
load varies can be compared to a resonant circuit. Often too large of phase
changes for my 8 element array could be seen with small changes in load as
the antennas were switched. With a terminated or near terminated system this
rate of change becomes less dynamic and very linear with cable length. It is
a lot easier to make an array that needs no cable length compensation for
each cable when you are making arrays with more elements.
Each season as I put up the array now I never worry about getting the
right antenna with the right cable or Hi-Z amplifier. Every thing is matched
to 75 ohms and indeed the eight phase shifts equal and irelevant as you say.
73es Lee K7TJR
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