On Tue, 15 Sep 2009 21:25:29 +0100, "christofire"
wrote:

Well, the moment of a section of a dipole is proportional to the average net
current on it and it's the integral of the moments at a point of inspection
that yields the radiation pattern.

Hi Chris,

I have already offered that what you say above is not disputed. I
merely add that it is not the only perspective and says nothing of the
"absence" of current throughout the entire radiator.

In my simplistic way of thinking, if the
moment of the end sections is zero, or as close as makes no difference, then
there's no contribution to the pattern from there, so there's no radiation
from there.

That was the logical basis for Art's claims of length efficiency:
those portions that did not support current (read contribute to
radiation) were thus ancillary (redundant as the Briticism would go)
and unneeded. Art would then expand this logic to perform his Ritual
Antenna Bris and lop off a portion to reduce the length (increase the
efficiency). I've already commented on this reductio ad absurdum.

Far field patterns are created from the phase relationships and time
relationships, and distance relationships (all the same thing,
mathematically) from all points of the radiator to any single point of
the characteristic lobe. In the teachings of radiation as light, a
wave front can be considered to be an infinite number of points of
radiation along a curved line (that front).

Interference (with its product being the shape of a lobe) is the
combination of all their phases, distances, and times.

Someone else who posted here a while ago used the term
'unopposed' current which is useful because it's the basis of why twin-wire
transmission line, driven differentially, is a poor radiator - put another
way, the moment at any point is close to zero. Alternatively, if there's no
radiation from a 'source' then there can't be any unopposed current there.

This is not the same sense of current in a single wire that gives rise
to a structure known as a "standing wave antenna."

If you ran a twin line up into the air to an open connection, then you
would have two closely space radiators. The open would enforce a both
a longitudinal and transverse standing wave. They would both radiate
like twin fire hoses. The key point here is that in the distance of
their separation, that distance is an incredibly small fraction of the
wavelength they are radiating. Their two currents (the standing waves
on each wire being immaterial) impose an 180 degree relationship
throughout their entire length. Both waves' phases, distances, and
times cancel to within the degree of that space of separation. This
is very easy to demonstrate by observing how they become efficient and
productive non-canceling radiators as you draw them apart to form the
V antenna. The only thing that has changed is the distance which
imparts a phase (or time, or distance - all the same thing
mathematically) shift apparent at a great distance. They will still
have the same SWR along their length, and the same currents (apart
from what is imposed through the radiation resistance).


I wouldn't contradict what you say about there being a collection of charge
at the ends of a dipole during each cycle, especially when it has added
capacitance (e.g. a 'hat' or the top of a 'Tee'), but the current in a
symmetrical hat is fully opposed and, as I noted before, the current at the
end of the conductor must be zero - by the definition of conduction.

Well, to this point there has been no discussion of end loading.
Doesn't matter, all the key issues are discussed above.

I believe there is danger in trying to relate radiation to voltages rather
than currents, arguing that displacement current causes radiation.

This is an engineering shorthand. It works with great precision. But
the simple fact of the matter is there is no current without a
potential gradient. Radiation could as easily be described by it.
Without regard for patterns, radiation is a function of Ohm's law and
we have three variables there. You cannot ignore any element or
assess some distinction of one at the cost of the other(s).

Therein
lies the fallacy of the CFA, E-H antennas, and associated efforts at
re-writing of Maxwell's equations, which are all being demonstrated as bunk.
Also, this appears to be the basis of Mr. Bialek's lecture series. If you
wish to argue 'that superposed voltage nodes also define the (pattern of
the...sic) far-field pattern' then I won't stand in your way ... but I
probably won't believe you.

So I gather. It is merely a shift in perspective of conventions, not
an up-ending of them. You may note that none of my discussion above
demands any new physics, nothing new in math, no novel methods. I've
used only two wires both close together and drawn farther apart under
the most simple of terms to reveal on one hand a transmission line,
and on the other hand a V antenna. The math of phase, distance, and
time is drawn from NEC; or rather, NEC leans heavily upon it and drew
it from Optics and I state my case in the strict terms of a method of
moments.

To cut to the chase: The full length of the radiator contributes to
radiation and the evidence of this is found in any characteristic lobe
displayed in the far field.

73's
Richard Clark, KB7QHC