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

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

Ian White GM3SEK wrote:
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.

Could a model be created for each band? What would be the
VF of the wire when 50% of the current was common-mode
and 50% of the current was differential mode?
--
73, Cecil http://www.w5dxp.com

In article , Ian White GM3SEK
wrote:

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.

Hello, Ian. You would use the physical length for all wires that are
interconnected and/or separated by free space. After all, that's what
we're trying to model. You still must decide how many electrically-small
segments would constitute, say, a 1 foot length of conductor. The higher
the frequency, the more segments you will need. If transmission line is
to be connected between segments, NEC has tools for doing that. BTW, my
experience is with LLNL's NEC-4 (FORTRAN-77 source code) rather than the
commercially-available packages. Sincerely,

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

Last things first - I just read John's later posting, and rescued this
message from the out-tray. I hope this message will supply the extra
detail you need, John.

Just one final thing:
I trust this is not another one of those situations where there is an
attempt by vendors to "reinterpret" Maxwell's equations (or explain
things that Maxwell "left out").

Oh no. On that topic, I am an ironclad hardliner!

If you remember where we came in, Roy was mentioning a few types of
antennas that it is acknowledged cannot be modeled with NEC-based
programs. Cecil then inquired if the Lattin was one of those... and,
subject to correction, I think it may be (if you require one model that
covers all frequencies).

But every one of this small number of exceptions is for a clear and
understandable reason, so they don't change the big picture, which is
that "almost" all types of wire/rod antennas CAN be modeled accurately
by NEC. If anyone thinks NEC doesn't work for their own pet antenna, the
burden of proving that is entirely on them.


We now hand you back to the original reply...

J. B. Wood wrote:
In article , Ian White GM3SEK
wrote:

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.

Hello, Ian. You would use the physical length for all wires that are
interconnected and/or separated by free space. After all, that's what
we're trying to model.

Certainly... but most of this antenna consists of pairs of parallel
wires that are physically interconnected, but are *not* separated by
free space - the wires that are part of the twin-lead.

You still must decide how many electrically-small
segments would constitute, say, a 1 foot length of conductor. The higher
the frequency, the more segments you will need. If transmission line is
to be connected between segments, NEC has tools for doing that. BTW, my
experience is with LLNL's NEC-4 (FORTRAN-77 source code) rather than the
commercially-available packages. Sincerely,

Sorry, that model still wouldn't work (unless I've misunderstood the
principle of this antenna).

The whole point of modeling a multiband antenna is to get one model that
is good for all its operating frequencies. That allows us to check that
the SWR dips at all the right places, and to find out what's really
happening in the supposedly "non-operative" parts of the antenna.

AIUI, the central part of the Lattin antenna is a half-wave dipole at
the highest operating frequency - call it 30MHz, so the wavelength is a
nice round number, 10.0m. Outside each end of this 5m long dipole is a
quarter-wave stub made of twin-lead. These stubs are resonant at 30MHz,
so they cut off the rest of the antenna (much like a trap) leaving just
the central half-wave dipole as the only functional part at of the
antenna.

The normal differential-mode velocity factor of the twin-lead applies to
this stub, so its correct physical length is not a quarter-wavelength
(2.5m) but about 0.8*2.5m = 2.0m.

Moving to the next lower operating frequency, there will be another pair
of quarter-wave resonant stubs isolating the ends of a half-wave
resonant dipole. But part of the physical length of this longer dipole
is the 30MHz stub. If you model it at its true physical length of 2.0m,
this will be correct for the lower frequency, but if you ignore the
differential-mode velocity factor, the stub won't be resonant at 30MHz
any more.

So the question remains: how can we model this "simplest" case of a
two-band Lattin antenna, in a way that will be accurate at both
frequencies? If we can solve that one, then extending it to the full
5-band Lattin should be child's play :-)




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

On Tue, 3 Oct 2006 13:33:21 +0100, Ian White GM3SEK
wrote:

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?

Hi Ian,

This is simply a veiled expectation for EZNEC not being able to model
the "special attributes" of the antenna.

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.

The answer is even easier than that. The Lattin antenna has a basic
rationale behind it that does not demand two different lengths: stub
tuning which is an electrical quality (not physical). What acts like
a stub, acts like a stub for any wire mesh modeling a stub. The
Lattin antenna does not exhibit this action to any correlation to
frequencies attributed to it. It is THAT simple. Appeals to physical
size relate only to the far field radiation characteristic. Even here
the Lattin is noted for being un-notable.

You don't need to worry about velocity factor, or dielectrics when the
basic rationale calls it a stub and it doesn't work as a stub for bare
wire. The Franklin antenna employs some of the same geometries and
nowhere makes a desperate grab for theoretical underpinnings called
stubs. Yet the Franklin delivers as promised if or when you add
dielectrics. The Franklin's simple distribution of currents (which
works for every antenna) works without having stray wires tacked on
like Irish Pennants. There are more apologists for this design than
working Lattins flying their tuning wires (in their notorious
disregard for the rationale of the design).

The fact of the matter is that modeling lays bear the myth.

73's
Richard Clark, KB7QHC