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I completed agree with Sergio. I found even for
return losses below 5 dB the antenna still work.
with a lot of inefficiency of course but works.
However, the antenna has to be resonant, otherwise
even with the best matching will not radiates. It looks
paradojic since one of the fundamental parameters
used for Ham radios guys( I was one of them) is the
VSWR, but happens that the ham radios always uses
known resonant antennas. For some arbitrary structures like
the one that can be build by HFSS or CST,etc the
"antenna" can have a better VSWR than a dipole
for example and still have a very bad radiation efficiency.
In other words the "antenna" accept a
good amount of energy buy it is not capable of
transform that in FAR FIELD energy, consequently
most of the energy is confined to near field, losses
due to dielectric, copper, etc. or split in LHP and RHP