On Fri, 28 Nov 2008 18:55:26 -0800, Roy Lewallen
wrote:

Jeff Liebermann wrote:
. . .
Incidentally, since the top 1/4 wave element represents something
close to perhaps 50 ohms, it would be interesting to measure the
amount of RF that isn't radiated and actually gets to the top section
of the antenna. If my analysis of the antenna is correct, the first
section (near the coax connector) radiates 1/2 the power. The next
section 1/4th. After that 1/8th, etc. By the time it gets to the top
of the antenna, there won't be much left. However, that's theory,
which often fails to resemble reality. It would interesting if you
stuck a coax connector on the top, and measured what comes out.

I'm intrigued by this, and would like to know what "theory" it's based on.

I just knew this would create a problem. I'm open to corrections and
explanations. I'm still learning and tend to make some rather
disgusting fundamental errors.

It's an observation based upon my measurements with a field strength
meter on similar UHF colinear antennas (using 1/2 wave stubs for
phasing). Also on a center fed 2.4GHz Franklin sector antenna of
similar construction. Most of the voltage peaks were at the base of
the antenna, tapering off as the field strength meter was dragged to
the top of the antenna. Since the current through the antenna is
constant, I assumed that the bulk of the power came from the lower
elements of the antenna. My explanation was a geometric decrease in
radiatated power starting at the feed point.

I've also seen a similar effect with relatively high gain (10dbi)
2.4GHz omni antennas in WISP applications. Any blockage of the lower
sections of the antenna, had a much bigger effect on the range and
measure signal strength than covering roughly an equal amount near the
top of the antenna.

The field radiated from a conductor is proportional to the current on
it. You'll see from either modeling or measurement that the currents on
all sections of a collinear array, or a long wire antenna for that
matter, are nearly the same. So in those directions in which the fields
reinforce, each section is contributing about the same amount to the
total field as any other.

I can see that on some models. I never could successfully model an
antenna using coax cable sections as elements. Using a wire model,
the current distribution is constant along the length as you describe.
However, my field strength measurements show more RF towards the feed
point. It's difficult for me to tell exactly how much more RF because
my home made meter is not calibrated. I don't recall the exact
numbers but I can dig out the FSM and make some measurements on some
of the antennas I have hanging around on the roof this weekend.

Although the logic is sound for this particular situation, it can't be
used in general to assign particular amounts of radiated power to
particular parts of an antenna. The fields from two parts of the antenna
might partially or fully cancel in some directions, even though both are
producing large fields. Any part of the antenna which is carrying
current is involved in the radiation process, and the total field is the
vector, not algebraic, sum of those fields.

The models all show the total pattern produced by all the elements
combined. I haven't found a way to show the contributions by
individual elements, thus making it difficult to model my observation.

So if you have a valid method of determining how much of the total
radiated power comes from each part of an antenna, I'd be very
interested in learning more about it. References would be welcome.

Nope. I'll give in easily on this one as it's highly likely I'm
wrong. However, I will double check my measurements on the roof
tomorrow and see if they're reproducible. I may have simply goofed
and/or drawn the wrong conclusion.

Incidentally, I've been offering this observation for several years
and you are the first to question it.

Roy Lewallen, W7EL

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558