Felix wrote:
Cecil Moore Wrote:
Richard Fry wrote:-

Felix Meyer, HB9ABX
Felix

Felix- your confusion is based on the fact that no one will believe you
without a basis for a real comparison of your antenna with a reference
antenna, done by another person, with publication of the results and a
description of the method.
NO qso "data" will do this. In that regard, you are just another
pusher of an EH or CFA antenna.
If you are serious, you will let some independent expert make one to
your description, and test it properly. (That's what shot down the EH)
In regard to the inability of such programs as EZNEC to properly
evaluate your antenna, I have not seen a well described antenna that
could not be evaluated honestly by a person aware of antenna theory and
the modelling programs.
Good luck-Bill

Bill wrote:
I have not seen a well described antenna that
could not be evaluated honestly by a person aware of antenna theory and
the modelling programs.

The Lentine (sp?) antenna, consisting of different lengths
of radiating transmission stubs proved impossible for me
to model with EZNEC.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Bill wrote:
I have not seen a well described antenna that
could not be evaluated honestly by a person aware of antenna theory and
the modelling programs.

The Lentine (sp?) antenna, consisting of different lengths
of radiating transmission stubs proved impossible for me
to model with EZNEC.
--
73, Cecil http://www.w5dxp.com

Cecil- Obviously, you fit the qualifications I mentioned, and- just as
obviously, I did not know of that example. I need to do some homework.
Thanks-Bill

EZNEC can model radiating transmission line stubs made from either
parallel wires or coax. To do it, parallel wire lines have to be modeled
as wires, not with the non-radiating transmission line model. Radiating
coax is modeled with a combination of a non-radiating transmission line
model for the inside, and a wire to represent the radiating outside of
the coax. This technique is described in the EZNEC manual and
illustrated with the DipTL.EZ example file included with EZNEC.

There are some types of antennas which aren't possible to model with
NEC-based programs. An example is a patch antenna on a dielectric
substrate -- NEC and EZNEC have no way to model the dielectric.
Likewise, a "loopstick" antenna -- a solenoid wound on a ferrite rod --
isn't possible because of the ferrite and possibly because of the
exceptionally small dimensions (for one used at AM broadcast frequencies).

But most often when you see an antenna inventor or seller claim that his
antenna "can't be modeled" by NEC, EZNEC, or other programs, it just
means that modeling fails to show the extraordinary performance he
claims for it. That's simply a failure of the program to include the
effects of magical properties and wishful thinking in its calculations.
I've come to regard such claims as a red flag indicating a probable
exaggeration of antenna performance.

Roy Lewallen, W7EL

Bill wrote:
Cecil Moore wrote:
Bill wrote:
I have not seen a well described antenna that
could not be evaluated honestly by a person aware of antenna theory and
the modelling programs.
The Lentine (sp?) antenna, consisting of different lengths
of radiating transmission stubs proved impossible for me
to model with EZNEC.
--
73, Cecil http://www.w5dxp.com

Cecil- Obviously, you fit the qualifications I mentioned, and- just as
obviously, I did not know of that example. I need to do some homework.
Thanks-Bill

Roy Lewallen wrote:
But most often when you see an antenna inventor or seller claim that his
antenna "can't be modeled" by NEC, EZNEC, or other programs, it just
means that modeling fails to show the extraordinary performance he
claims for it. That's simply a failure of the program to include the
effects of magical properties and wishful thinking in its calculations.
I've come to regard such claims as a red flag indicating a probable
exaggeration of antenna performance.

I wish I could remember the correct spelling for the antenna
I tried to model. Something like "Lentine". It is a dipole
of sorts made from shorted and open sections of balanced
transmission line. I tried modeling it with wires in EZNEC
and got all sorts of errors. It looked something like this:

+--------+--------+--------FP--------+--------+--------+
+------ +------ +------ ------+ ------+ ------+

Anyone remember the correct spelling for that antenna?
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Roy Lewallen wrote:
But most often when you see an antenna inventor or seller claim that
his antenna "can't be modeled" by NEC, EZNEC, or other programs, it
just means that modeling fails to show the extraordinary performance
he claims for it. That's simply a failure of the program to include
the effects of magical properties and wishful thinking in its
calculations. I've come to regard such claims as a red flag
indicating a probable exaggeration of antenna performance.

I wish I could remember the correct spelling for the antenna
I tried to model. Something like "Lentine". It is a dipole
of sorts made from shorted and open sections of balanced
transmission line. I tried modeling it with wires in EZNEC
and got all sorts of errors. It looked something like this:

+--------+--------+--------FP--------+--------+--------+
+------ +------ +------ ------+ ------+ ------+

Anyone remember the correct spelling for that antenna?

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?


--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek

On Tue, 3 Oct 2006 08:43:07 +0100, Ian White GM3SEK
wrote:

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.

Hi Ian,

This "two different modes" is the magic mode factor that has not been
designed into EZNEC.

One need only look at the Lattin designs that "work" to discover they
violate the precepts of "how" they work.

Then note those that "should" work result in those don't work.

The bottom line is fairly obvious, but there are those who can 'splain
how its done (see magic mode factor).

73's
Richard Clark, KB7QHC

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

Ian White GM3SEK wrote:
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?

Thanks Ian, for the spelling and for jogging my memory on the
subject. I believe you have hit the nail on the head. EZNEC
apparently cannot "model both modes simultaneously".
--
73, Cecil http://www.w5dxp.com

If made from twinlead with insulation between the conductors (e.g.,
window line or 300 ohm flat or tubular TV twinlead), NEC based programs
can't model it exactly in a straightforward fashion for the same reason
it can't model patch antennas on a substrate -- the program can't
account for the effect of the dielectric between the conductors.

The two modes Ian speaks of can be separated by separating the two
currents into common and differential modes(*). The common mode current
is the source of all the radiation. It propagates as Ian says at a
relatively high velocity factor. This mode is modeled quite well with
EZNEC or NEC-4's insulated wire feature.

The differential mode current doesn't cause radiation. Its velocity
factor is determined by the dielectric between the wires, and NEC-based
programs don't have any way to account for the field modification which
the dielectric causes to bring this about. That's where the shortcoming is.

The net result is that the radiating properties of the stubs can be
accurately modeled, but their length which determines the "trapping" or
loading characteristics would be off. Of course, such an antenna made
from air-insulated twinlead could be modeled easily.

I've successfully modeled a folded dipole made with TV twinlead by
physically separating it into common mode and differential mode
structures. The common mode portion is a simple dipole, with diameter
equal to the effective diameter of the two conductors in parallel, and
with wire insulation. Then across the feedpoint I put non-radiating
transmission line models to model the differential mode transmission
line stubs. The lengths of these took into account the twinlead velocity
factor, and their impedances were 1/4 the impedance of the real stubs
because of the transforming property of the structure. The result was a
good model, provided that the feedpoint impedance was multiplied by 4.

A first look indicates that this approach wouldn't be practical with the
Lattin, because it would require large jumps in effective wire diameter
as you go along the antenna, which NEC doesn't handle well.

What you really need is a way to increase the differential mode length
of each of the stubs without impacting the common mode length. A few
quick sketches indicate that it just might be possible to insert a
transmission line model (which has no physical length or common mode
radiation) in series with a stub, which would accomplish the goal if its
length were made to equal the difference between electrical and physical
length of the real stub. But I don't have time to pursue it. Anyone
interested in doing so should begin with a single stub and carefully
observe its characteristics.

Whenever modeling close spaced parallel wires with NEC-based programs,
it's vital that the segment junctions be aligned on the wires. There's
more information about this in the EZNEC manual (available also with the
demo program). Look in the index under "Parallel Wires".

(*) Modeling programs don't treat the modes separately. But separating
them makes it easier to explain and understand what causes the problems.

Roy Lewallen, W7EL

Ian White GM3SEK wrote:
Cecil Moore wrote:
Roy Lewallen wrote:
But most often when you see an antenna inventor or seller claim that
his antenna "can't be modeled" by NEC, EZNEC, or other programs, it
just means that modeling fails to show the extraordinary performance
he claims for it. That's simply a failure of the program to include
the effects of magical properties and wishful thinking in its
calculations. I've come to regard such claims as a red flag
indicating a probable exaggeration of antenna performance.

I wish I could remember the correct spelling for the antenna
I tried to model. Something like "Lentine". It is a dipole
of sorts made from shorted and open sections of balanced
transmission line. I tried modeling it with wires in EZNEC
and got all sorts of errors. It looked something like this:

+--------+--------+--------FP--------+--------+--------+
+------ +------ +------ ------+ ------+ ------+

Anyone remember the correct spelling for that antenna?

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?