Antonio Vernucci wrote:
. . .
- with the hairpin EZNEC gives an antenna gain of 12.76 dB but, if I
just delete the three hairpin segments leaving everything else
unchanged, the gain grows up to 13.59 dB. I am surprised about that
result, as I had understood that, in EZNEC, the antenna gain has nothing
to do with the way the antenna is fed.

What you're probably seeing is a numerical problem in the NEC
calculating engine. It's very fussy about the region near a source, and
doesn't like small loops which include a source. You should run an
Average Gain check (see "Average Gain" in the EZNEC manual index), which
will reveal whether this is the problem.

The double precision calculating engine in EZNEC+ is considerably more
tolerant of small loops, but can still have problems with average gain
for other reasons.

Roy Lewallen, W7EL