qha antenna

Can we trust on the analysis results of CST MWS in simulating QHA? I have simulated and analyzed QHA by CST MWS and also NEC software. In comparison with measurement results of this antenna in one journal article, the results of NEC had better compliance.

It is always healthy to look at simulation results with caution.

Well I can tell you one thing.
one of my friend designed crossed dipoles in NEC & he got perfect 1 VSWR in simulation.
Is it really possible or rather simple to get this in practise?

For helix you can trust NEC for sure.
Only take care of practical stuff such as proper turns,angles,proper soldering etc.
they will change result a lot.