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[Antennaware] Fan dipole: similar or identical segment lengths?

Subject: [Antennaware] Fan dipole: similar or identical segment lengths?
From: Kevin Zembower via Antennaware <>
Reply-to: Kevin Zembower <>
Date: Mon, 27 Jul 2020 11:14:48 -0400
List-post: <>
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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