This is a follow-up to my notes with subject "Newbie needs help modeling
a fan dipole."
Bob, W3IDT, and Guy, K2AV, suggested that, when modeling parallel,
closely-spaced wires, they must have the same segment lengths. Since
it's not possible in NEC to control segment length directly (the segment
length must be a whole integer division of the total wire length), the
work-around was to compose the longer wire of three segments, the middle
one exactly matching the higher frequency one in length, with the same
number of segments, and the two outer ones making up the rest of the
length of the lower frequency dipole, with similar, but not necessarily
the same segment length.
I created a new card deck and processed it. The starting point was
similar to the card desk in my note of 23 July. The only difference was
that I changed the distance between the dipoles, and therefore the
length of the transmission line, to 0.01m. All other dimensions should
have been the same.
Then, I modified the deck to create the three-section lower frequency
dipole as noted above. I've pasted that deck, and attached it, below.
The final analysis was:
Res.F. Gain Impedance Freq Gain Impedance
1 sec: 7.116MHz 1.7 86.0-j7.6 14.280 1.4 71.7-j1.9
3 sec: 7.134MHz 1.5 73.0-j6.1 14.514 1.4 68.9-j9.3
As you can see, the resonate frequency changes by about 0.25% in the
40m band, and 3.0% in the 20m band between the two models. The other
numbers show similar, small changes.
However, this begs the question: Which numbers are the 'right' ones?
Since this is a mathematical model, no one can build this antenna to
these specifications (ideal ground, no interfering structures, etc.) to
check which of the mathematical answers comes closest to the 'real'
answer. Is there any way to tell which of the two models is most accurate?
Thanks for your thoughts and help. I really appreciate all of your help
getting me started in antenna modeling.
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