In regard to Tom's comment that measurements of base impedance are not all
that indicative of ground loss or efficiency.
It turns out that that effect has been well known in theory as well as
practice from the work of James Wait and others since the late 50's. What
their work showed is that there is an interaction between the radiation
resistance, as well as the loss resistance, as the number and length of
radials changed. The effect gets larger the poorer the soil. What can
happen is that as you add more radials, the ground loss resistive component
goes down gradually but the radiation resistance may actually drop as well,
even going below the accepted "36 Ohm" value, and then rise again. When you
measure base impedance you are getting the sum of the two plus of course any
conductor loss. Sometimes when you think you've made a substantial
improvement in the ground loss resistance you may have actually depressed
the radiation resistance, so the improvement in efficiency is not as great
as you think. Since the efficiency is the ratio of Rr to Rr+Rg, you can see
how misleading things can get if both of these are dancing around.
If anyone is interested, contact me off the reflector and I'll give you the
references.
For those who would like to have a vector network analyzer but can't afford
the profession instruments. You can build up an N2PK VNA for a fraction of
the prices of even a used professional instrument. I have now have one and
have been comparing it to my HP3577A. The difference between the two is
insignificant for antenna and array measurements. The N2PK is truly a
wonderful instrument but it is a challenge to populate the board which is
all SMD! I have also been testing the TenTec/TAPR and IW3HEV VNA's.
They're not even close in performance and are of questionable value for
antenna measurements in the field.
What is generally not appreciated is that a VNA can do a lot more than
measure impedances. It can also directly measure the relative current
amplitudes and phases in a completed array and feed network without having
to excite the array with anything more than the VNA itself.
73, Rudy N6LF
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