First let Me say this passive 8 circle antenna is a great antenna. I am
sure
it is outstanding compared to most receiving systems.
However, It seems to me a moot point to be discussing the merits of minutia
in terms of the pattern from this 8 circle antenna. If you model this antenna
with
all the transmission lines and transformers etc., you will find that the
pattern
changes dramatically form 1.8 to 1.9 MHZ. Yes, it is perfect at the design
center
frequency. And this is modeling without the effects of the ground radials often
used in the system. They will only make the following problem worse. I do not
have the NEC 4 engine so I cannot evaluate the radial influence.
So someone correct me if I am wrong, but here is why this happens. It is
incorrect to expect the delay in a transmission line to be as calculated unless
one or both ends of it are terminated in its characteristic impedance. Also
unless it is multiples of 1/4 wave which we do not have here.The delay line in
this design is 37.5 ohms and the two elements feeding it are 37.5 ohms, a
match only at the design frequency. If you look at the output side of the delay
line it is driving the output 18.75 ohms output transformer and it is also
seeing
the other two elements paralleled (37.5 ohms) so the load on the line is 12.5
ohms. The delay line is not terminated on the load end for sure. And not on the
source end off the design frequency as the frequency of the array changes. The
output impedance of the elements change naturally with frequency, so unless
you are at the design frequency, source impedance will be off 37.5 ohms and
the delay will be incorrect in the delay line. The delay varies with frequency
bottom line.
A combiner with a little more complicated impedance matching would help
this problem.
Lee K7TJR
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Topband Reflector
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