[Top] [All Lists]

Topband: Rx 4square

Subject: Topband: Rx 4square
From: "LEE STRAHAN" <>
Date: Sun, 02 Sep 2007 13:58:58 -0700
List-post: <>
Roger said:     I'm not an engineer and am having a hard time understanding 
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

Topband mailing list

<Prev in Thread] Current Thread [Next in Thread>
  • Topband: Rx 4square, LEE STRAHAN <=