Tom,

Firstly, I'm guilty of a "sloppy" choice of words. Whenever I've been
using the phrase "drop off in current" I've meant the current at the
top of the coil as a percentage of the current at the bottom. So when
I've quoted 70% the current will have reduced by 30%. Apologies!

Secondly, you're testing the limits of my understanding with the
overall current distribution from base section, through the coil, to
the top section. However I think the point is that you can't simply
"add electrical degrees" through the various sections when the
characteristic impedances of the sections are so disparate. That was
Cecil's point in the very first posting. We also know that, as
expected, summing the "degrees" for the three sections gets nowhere
near a total of 90 degrees, so clearly you can't assume a cosine
distribution that is contiguous across all three sections.

I'll investigate what happens with a "top hat".

73,
Steve G3TXQ



On May 11, 6:56*pm, K7ITM wrote:

Hi Steve,

OK, so I suppose you are assuming that the current distribution will
follow a cosine along electrical degrees of your antenna, with a
maximum at the base/feedpoint. *If that's the case, then would you not
account for the bottom 10 feet of wire, about 20.5 electrical
degrees? *If I do that and assume 1 amp at the feedpoint, I should see
about .9367 amps at 20.5 degrees and 0.5101 amps at (20.5+38.83)
electrical degrees. *0.5101/.9367 would then be the ratio of currents
between the ends of the coil, and that's 0.5446, only a 45.54 percent
fall-off.

In fact, it seems to me that the idea of cos(38.83 degrees) = .779
would imply a fall-off of 22.1%... and that tells me that perhaps I'm
still not understanding your model very well. *Maybe you are NOT
assuming the current along the electrical degrees of the antenna, up
from the feedpoint, will have a cosine distribution. *At this point, I
have to say that I'm just not at all sure what your model really is.
Perhaps you are making different assumptions about the current
distribution...

Also, if you still have the model around, try adding a top hat to the
upper wire. *For simplicity, you can just use a simple "T" structure,
where the top horizontal wire is, say, five feet long total. *With
such a configuration, what's the current distribution along the
radiating element going to be?

Of course, what I'm suggesting here is that one must be careful to
test ones models at corner cases before putting too much faith in
them, and even then, one must always be wary of cases where the model
may go awry.

Cheers,
Tom