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

[Kevin Zembower]

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.

-Kevin
KC3KZ
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CM --- NEC2 Input File created or edited by xnec2c 3.6.1-beta ---
CM 40/20M fan dipole, 10m elevation over 'normal' ground.
CM Dipoles connected by short transmission lines.
CM Used wire diameter of #12, 2mm dia, 1mm radius, instead of 0.3mm. No change.
CM Changed TL. Entered length, changed all R and I to 0. Gave response of dip below SWR 2:1 at 7 and 14MHz.
CM Kevin Zembower, KC3KZ
CE --- End Comments ---
GW     1    21  0.00000E+00 -5.19200E+00  1.00000E+01  0.00000E+00  5.19200E+00  1.00000E+01  1.00000E-03
GW     2    11  0.00000E+00 -5.19200E+00  1.00000E+01  0.00000E+00 -1.01170E+01  1.00000E+01  1.00000E-03
GW     3    11  0.00000E+00  5.19200E+00  1.00000E+01  0.00000E+00  1.01170E+01  1.00000E+01  1.00000E-03
GW     4    21  0.00000E+00 -5.19200E+00  9.99000E+00  0.00000E+00  5.19200E+00  9.99000E+00  1.00000E-03
GE    -1     0   0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
EX     0     2    11      0  1.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
TL     1    11     2     11  5.00000E+01  1.00000E-03  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
FR     0   501     0      0  6.00000E+00  1.80000E-02  1.50000E+01  0.00000E+00  0.00000E+00  0.00000E+00
NH     1     1     1      1  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
NE     1     1     1      1  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
RP     0     9    37   1000  0.00000E+00  0.00000E+00  1.00000E+01  1.00000E+01  0.00000E+00  0.00000E+00
GN     2     0     0      0  1.20000E+01  1.00000E-02  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
EN     0     0     0      0  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00