> As I mentioned earlier, I have a method to make those measurements but it's
> not practical for most hams. That's why I've been asking if anyone else has
> a less complex solution to this kind of measurement.
>
> 73, Rudy N6LF
Forrest Gehrke K2BT wrote a 6part series in Ham Radio in 1983 / 1984 on phased
vertical antennas. He came up with a technique to derive the Zparameter matrix
(self and mutual impedances 2port matrix) without using a VNA. (If you can
measure the complete Sparameter matrix between verticals using a VNA, it's
easily converted to the Zparameters [and viceversa]).
His method is described in Part 3  July 1983 issue (pg 2634) and works for
element lengths of 1/4 wave or less. If they're longer you need to come up with
a different way to decouple the elements.
1. Start with element 1. Opencircuit all the other verticals except the pne
being measured. Opencircuited quarterwave vertical couples weakly. Measure
the selfimpedance of the excited antenna element. This provides Z11 for that
antenna. Iterate for all antenna elements (providing Z11 for each).
2. Short element 2 to ground. Measure driving point impedance of element 1.
Opencircuit element 2, shortelement 3 and measure driving point impedance of
element 1. this gives Z12, Z13, and Z14 for example. Repeat for all elements.
This gives a table of measured values. For example for a 4element array, four
measurements at element 1, four measurrments at element 2, etc.  16 total
measurements. A passive antenna will give a symmetric Zmatrix, so the above
table has duplicated measurements which can be used to check for consistency.
Forrest was always amazed at how closely symmetric the values ended up being
after his measurements.
Extract the 2port values for each element pair. (Z11, Z21, Z12, Z22).
For element 1 and element 2, the math is:
Z1 = E1/I1 = Z11  (Z12)^2/Z22. This is driving point impedance of element 1
with element 2 grounded.
Then Z12 = +or SQRT(Z22) (Z11  Z1). You need to pick the correct root, but
it's usually pretty easy to guess.
My personal hunch would be that if there is spurious coupling from a tower or
other wire, then the Zmatricies would become nonsymmetric.
 Tom, N5EG
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