Owen Duffy wrote:
Roy Lewallen wrote in
treetonline:

Owen Duffy wrote:
...
Is NEC capable of modelling the configuration shown at
http://www.vk1od.net/lost/King-22.3b.png (which is the same type of
problem as my figure b)?

A point made by King is that if the three half waves are in phase,
radiation resistance will be quite high (one third current required for
same distant field strength), around 316 ohms against 105 ohms for three
half waves not-in-phase. Presumably these figures are for free space.
. . .

I looked up the section in King, Mimno, and Wing and was pretty
disappointed. It's one of my favorite references, and I usually find the
explanations clear. But the description of that antenna is pretty vague,
with considerable hand waving ("[Operation of coaxial stubs] is much
less satisfactory than that with the open-wire stubs. . ." without
explaining why). And in the explanation of the open-wire stubs, the
authors seem to state that the wires must carry purely differential
currents. And their models (Fig. 22-4) do show purely differential
coupling from the antenna to the stubs.

I speculate that they really didn't understand how these antennas
worked, had discovered that the coaxial sleeve versions didn't work or
at least didn't work as well -- and didn't show the proper impedance --,
but didn't fully understand why. King, in particular, was and is one of
the giants of antenna theory, and leaves us a lifetime of brilliant
insight and rigorous mathematical analysis. But at least at the time
that book was published, they lacked the modeling tools we have today.

This effect is certainly observable in models using my Fig a) (though
half the respective resistances due to the vertical over perfect ground).

The feedpoint impedance looks like it might provide a hint as to whether
currents are actually in-phase.

It surely does. Given the currents on and locations of the end wires,
the modification to the center wire can be calculated from mutual
coupling considerations. And I think this is a clue that led King,
Mimno, and Wing to conclude that something was amiss with the coaxial
version.

Exploring that thought, an example (to some extent) of King's Fig 22.3b
is the W5GI Mystery Antenna (see
http://www.w5gi.com/images/w5gimster...aschematic.gif ) which claims
to be three half waves in phase at 14.2MHz. It is very similar to the
diagram above in King though I note that the phasing sections are 105° in
electrical length.

The W5GI is fed with a half wave (at 14.2MHz) of 300 ohm line, then 34'
of RG8X. W5GI reports impedance looking into the RG8X as 42+/-j18. That
suggests the load on the RG8X is 31+j2 or 70-j18. The feedpoint impedance
should be about the same value due to the half wave of 300 ohm low loss
line. Neither impedance is within a bull's roar of 316+j0, and are so low
as to question whether the three half waves are indeed in-phase. (The
highest impedance that would yeild 42+/-j18 on a short length of RG8X
would be around 80+j0, closer to the not-in-phase configuration than the
in-phase configuration).

W5GI's reported feed impedance seem inconsistent with three half waves in
phase, and questions whether the phasing arrangement works as suggested.

Thoughts?

I doubt that it does.

Roy Lewallen, W7EL