RadioBanter - View Single Post - Dual-Z0 Stubs

Tom Donaly · May 8th 09, 07:43 PM posted to rec.radio.amateur.antenna

wrote:
Still here, and still trying to strip away the personal attacks to try
to understand the various underlying technical opinions!

When I model a "bugcatcher" plus stinger using EZNEC, I see
significantly less current at the top of the coil compared with the
bottom of the coil. I also see a slight increase in current part way
up the coil before it then decreases towards the top.

In reply to an earlier question of mine, Roy assured me that this
current distribution was "real" and not some problem that EZNEC has
modelling this arrangement.

I have two questions if I may:

1) Cecil: I believe I understand how the Corum transmission line model
accounts nicely for the reduced current at the top of the coil. Does
it also account for the slight increase in current a short way from
the bottom?

2) Jim, Tom, Roy (and any others): It appears you think the Corum
model is flawed, or not appropriate to the "bugcatcher". What I've not
yet understood is what alternative model you are advocating which
would match the EZNEC results more closely than the Corum model. I've
heard at least Roy say that a lumped-inductor model is inappropriate,
but I don't think I've yet heard any other model proposed. Perhaps I
missed it?

Regards,
Steve G3TXQ

EZNEC _is_ a model. If you can accept that, then you've got your model.
If you want to know why current is less at one end of the bugcatcher
coil than at the other, you'll have to study electromagnetic theory.
(Try adding a large top hat to the bug catcher and see what that does
to the current in the coil.) If you're satisfied with Cecil's work,
fine. Whatever works for you. Rules of thumb and such sometimes work
pretty well. The late Reg Edwards knew that and worked out some programs
that relied on such. You might want to see if you can find some of his
work on the web.
73,
Tom Donaly, KA6RUH