J. B. Wood wrote:
One of the first hits is http://www.g3ycc.karoo.net/lattin.htm which
shows a good sketch. The antenna is made from sections of 300-ohm ribbon
or tubular feeder, configured as a string of quarter-wave stubs that
progressively make the dipole shorter as the frequency increases.

The modeling challenge is that the ribbon operates in two different
modes at the same time: a radiating common mode with a velocity factor
of say 0.95; and a non-radiating "stub" mode with a VF of about 0.8. The
problem is to model both modes simultaneously, for the whole string of
stubs, without changing the physical dimensions of the real antenna. I'm
not sure if NEC can do this, but maybe Roy can comment?

Hello, and Roy will probably want to weigh in here. What I can say is
that if you can create a wire model of the antenna consisting of
interconnected segments (ideally about 1/20 wavelength each) then NEC
will find the currents in each by considering all the interactions
(conductive, capacitive, inductive) between the segments. NEC doesn't
care about the geometry or "modes" of the antenna - it just sees a
bunch of interconnected segments distributed in 3-D space. There is no
magic here as NEC is merely applying text-book electromagnetic theory

That isn't a complete model of this particular antenna. The missing part
is the velocity factor of the twin-lead when acting as a stub, which
means that the electrical length of the stub is different from the
physical length. Which of those two lengths would you use in the NEC
model?

The answer is easy for a single-band model; but it's not so easy to
create one NEC model that will be valid for all the bands this antenna
is designed to cover.


--
73 from Ian GM3SEK

http://www.ifwtech.co.uk/g3sek