In article , Ian White GM3SEK
wrote:

Google for "Lattin antenna". (Too many "lentils", Cecil :-)

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 (you wouldn't
want to tackle this with just pencil and paper).

Once the individual segment currents are found (the time-consuming part)
It is relatively straight-forward for NEC to find the radiation pattern
shape, antenna gain and driving point(s) impedances. As with any
modelling program the trick is to make sure the wire segment model
adequately represents the actual/planned structure. Besides segment
length, there are a few other rules imposed by NEC that must also be
adhered to in order to obtain the correct results.

Roy is absolutely right in a previous post that an antenna vendor is most
likely blowing smoke by proclaiming that his/her antenna can't be modelled
by a method-of-moments program like NEC. (My favorite antenna "myth
busters" using NEC are Drs. John Belrose and Gerald Burke). Sincerely, and
73s from N4GGO,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337