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

J. B. Wood wrote:
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,

The problem that EZNEC has with this antenna, as I understand it,
is that the same pair of parallel wires has common-mode current
in them on some frequencies resulting in a high VF and differential
mode current in them on other frequencies resulting in a low VF. I
don't know how to model changing VF's with EZNEC without changing
the physical length of the wires as frequency is changed.
--
73, Cecil http://www.w5dxp.com

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

In article , Cecil Moore
wrote:


The problem that EZNEC has with this antenna, as I understand it,
is that the same pair of parallel wires has common-mode current
in them on some frequencies resulting in a high VF and differential
mode current in them on other frequencies resulting in a low VF. I
don't know how to model changing VF's with EZNEC without changing
the physical length of the wires as frequency is changed.

Hello, Ian, and I guess I don't see how this is a problem provided you
have described the antenna geometry correctly and have chosen the
appropriate number of segments at the evaluation frequencies of interest.
Velocity factor and other electromagnetic phenomena are implicit in the
solution for the current distribution on the structure. Now, if
dielectric material is distributed in the structure that does complicate
things a bit. Can you point me to some further info on the antenna in
question? 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").

I would encourage frequent users of NEC to subscribe to the mailing list
at . Lots of practical discussion there IMHO.
Sincerely,

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

"Cecil Moore" wrote in message
om...
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

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

Richard Clark wrote:
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.

Consider a folded dipole made from Wireman #562. The radiating
part of the antenna has a VF of 0.95 and the feedline has a
VF of 0.8. Identical wires with 19% different VFs. How does
EZNEC handle that?
--
73, Cecil http://www.w5dxp.com

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

Ian White GM3SEK wrote:
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?

A different model for each band that takes the varying
VFs into account?
--
73, Cecil http://www.w5dxp.com