Topband: "return" current - what is it?

Tom W8JI w8ji at
Sun Aug 5 02:25:33 PDT 2012

> None of the NEC sources violate Kirchoff's laws, but the implementation
> of the SI and SV sources isn't exact and doesn't exactly approximate a
> real source (for which NEC doesn't allow you to see the actual current
> at the ends of the source).  But why bother, most people know the answer
> anyway.  The problem is just understanding what the numbers mean that
> the program is giving.

The fact modeling programs allow perfect sources right at the wire also 
allows building antennas in models that cannot be built in real life. This 
has happened several times with antennas.

One case I recall was a receiving antenna that used two very short 
horizontal elements very close together fed out-of-phase, which was 
hypersensitive to the feed system, construction, and balance. Antenna gain 
was far into negative numbers and it was hypersensitive to common mode on a 
balanced feed. Another was a 44-foot or 88-foot long dipole that had well 
over 100:1 SWR on the feeder on the lowest band.

I always wonder if the 43-foot vertical didn't come from that dipole turned 
on its side.

This is why any unproven design based on models should be "tested" in the 
model for sensitivity to changes, include feedlines, and why we really need 
to confirm in the real world.

Without matching and feedline losses, and without common mode current 
problems, we can design some pretty unique antennas. :-)

73 Tom 

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