I am designing a 1.25-wavelength dipole antenna with the 4nec2 software. What I noticed is that the input impedance of antenna is changing greatly when I change the number of segments.
How many segments should I choose?
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$\begingroup$ If your dipole is modeled as a single length of wire, which is common for a simple dipole antenna, then you will want an odd number of segments if you are choose a center feed via the excitation card. $\endgroup$– K7PEHNov 5, 2015 at 21:13
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Generally in NEC2, 10 segments per wavelength is good, so 12 or 13 segments depending on where you want your feed.
But you are doing better than just following the rule, investigating the effect of the number of segments!
I'd suggest trying 4, 6, 8, 10, 15, 20, 30 segments per wavelength and comparing the results. The impedance should be pretty constant from 10 to 30. If not,
Watch out for all the other rules too. Segment length to diameter - they must not be too fat; Length to length ratios - joining long and short segments; radius step changes, joining at too narrow an angle so they overlap too much; small loops are difficult in NEC, etc etc.
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1$\begingroup$ Also, the excitation point (driving the antenna) is usually chosen by naming the segment number on a wire of your geometry. To properly place the excitation point where you want it, you need to be mindful the number of segments and lengths of segments. NEC2 uses the middle of the segment number specified to insert the excitation. If this is not properly done then the resulting impedance can change in ways that you may not expect. Even though you may be using 4nec2, you should be familiar with the geometry cards and excitation as described in the NEC2 user guide. $\endgroup$– K7PEHNov 5, 2015 at 18:17