Cecil Moore wrote:
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?

That would be two part-models that don't join up to make a complete one.


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

On Tue, 03 Oct 2006 17:14:00 GMT, Cecil Moore
wrote:
How does EZNEC handle that?
Try using the excellent online help manual.

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?

Ian White GM3SEK wrote:

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 :-)

Hello, Ian and I think the problem here is with the dielectric in the
transmission line. As I said earlier, the presence of dielectric
material in the structure does complicate things. NEC AFAIK was never
intended to handle this situation. I wouldn't say that NEC couldn't
model this antenna but it would be a challenge. And as you point out a
NEC wire model of this antenna not accounting for dielectric effects
would be incomplete at a given frequency. I have never tried to model
an antenna with NEC that included dielectric material nested between
wires. Sincerely,

On Tue, 03 Oct 2006 17:46:13 -0400, "J.B. Wood"
wrote:

I have never tried to model
an antenna with NEC that included dielectric material nested between
wires.

Hi John,

EZNEC has the capacity to model wires with insulation. I presume that
is a legacy of NEC, but I will await tutoring on that point from other
posters. Carry that a bit further, it has at least "some" capacity to
model wires with material nested between them. After all, the
difference is in degree, not in concept, and the degree is hardly
remarkable. When I observe common window line, it is not all that
different from two insulated wires. Further, there is nothing
remarkably different to the degree that the Lattin analysis is so
entirely thrown off as to be wholly useless. For that matter, I
haven't observed any postings here on any Lattin analysis other than
my own. If this all hinges on TV type twin lead, then too much credit
is being given to too little plastic.

73's
Richard Clark, KB7QHC

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?



I built one of these, and it didn't seem to actually resonate very
sharply anywhere. It certainly did not work as advertised.

I ended up with a lot of ladder line pieces to make 2m J poles with
though, so not a great loss.

Anyone need a J pole?

tom
K0TAR

Richard Clark wrote:

EZNEC has the capacity to model wires with insulation. I presume that
is a legacy of NEC, but I will await tutoring on that point from other
posters.

Although it's a feature of NEC-4, it's not part of NEC-2. The insulated
wire capability of EZNEC was developed independently from other sources.

Carry that a bit further, it has at least "some" capacity to
model wires with material nested between them. After all, the
difference is in degree, not in concept, and the degree is hardly
remarkable.

No, they're different things. The insulated wire feature slightly
modifies the field from a wire, and is valid only for thin insulating
layers. Insulation between conductors has a considerably larger effect
on the field and consequent coupling between them. Adding insulation to
a parallel wire line gives you a model of something like an
air-insulated ladder line made with insulated wire.

When I observe common window line, it is not all that
different from two insulated wires.

It's enough to drop the differential mode velocity factor down to
somewhere around 0.91 - 0.95 (from various sources - I haven't measured
any), which indeed isn't very different from the common mode velocity
factor of insulated wire. Whether or not the difference is significant
depends on the application.

Further, there is nothing
remarkably different to the degree that the Lattin analysis is so
entirely thrown off as to be wholly useless. For that matter, I
haven't observed any postings here on any Lattin analysis other than
my own. If this all hinges on TV type twin lead, then too much credit
is being given to too little plastic.

You could probably make a model with EZNEC which would be fairly close,
then manually adjust it to optimize performance. A real antenna would
have similar performance if optimized for the type of line it's
constructed from, although the final dimensions would be a bit different.

Roy Lewallen, W7EL

On Tue, 03 Oct 2006 17:33:14 -0700, Roy Lewallen
wrote:


No, they're different things. The insulated wire feature slightly
modifies the field from a wire, and is valid only for thin insulating
layers.

Hi Roy,

And insulated wire is different from wire insulated by window line
insulation?

Insulation between conductors has a considerably larger effect
on the field and consequent coupling between them.

? Insulated wire HAS insulation between conductors. Air certainly
qualifies to some degree, the insulation on the wire another.

Adding insulation to
a parallel wire line gives you a model of something like an
air-insulated ladder line made with insulated wire.

That makes sense only in that it repeats the obvious. How is
insulated parallel wires (air-insulated ladder line made with
insulated wire) different from window line? Or twin lead? Except by
degree?

When I observe common window line, it is not all that
different from two insulated wires.

It's enough to drop the differential mode velocity factor down to
somewhere around 0.91 - 0.95 (from various sources - I haven't measured
any), which indeed isn't very different from the common mode velocity
factor of insulated wire. Whether or not the difference is significant
depends on the application.

This is not a very compelling argument for how Lattins WORK (seeing as
most reports suggest they do not). It is not a very compelling
argument for very remarkable differences in where they do work
(however, few seem to be offered in that regard either).

Quite simply, velocity factors do not explain away the lack of
resonance ANYWHERE near the intended frequency. What you suggest is
percentages where actual performance misses the target, not just the
mark and as a multiband structure is so wildly useless as to be a
product of chaotic, random doodling.

Further, there is nothing
remarkably different to the degree that the Lattin analysis is so
entirely thrown off as to be wholly useless. For that matter, I
haven't observed any postings here on any Lattin analysis other than
my own. If this all hinges on TV type twin lead, then too much credit
is being given to too little plastic.

You could probably make a model with EZNEC which would be fairly close,
then manually adjust it to optimize performance. A real antenna would
have similar performance if optimized for the type of line it's
constructed from, although the final dimensions would be a bit different.

Having modeled more than a few Lattins (and there are so many as to
beg the definition), any claim to resonance associated with a stub
dimension FOR ANY "ELECTRICAL LENGTH" is a fantasy of the first order.

The inability to model a working Lattin has no basis in these
arguments about the shortfall of EZNEC/NEC insulation issues. The
antenna design fails quite abysmally for bare wire when designed to
the purported rationale of trapping by stub construction. To think
the design can be resurrected by unmodelable insulation tricks is
based on hope and charity.

73's
Richard Clark, KB7QHC

Hi John,

EZNEC has the capacity to model wires with insulation. I presume that
is a legacy of NEC, but I will await tutoring on that point from other
posters. Carry that a bit further, it has at least "some" capacity to
model wires with material nested between them. After all, the
difference is in degree, not in concept, and the degree is hardly
remarkable. When I observe common window line, it is not all that
different from two insulated wires. Further, there is nothing
remarkably different to the degree that the Lattin analysis is so
entirely thrown off as to be wholly useless. For that matter, I
haven't observed any postings here on any Lattin analysis other than
my own. If this all hinges on TV type twin lead, then too much credit
is being given to too little plastic.

73's
Richard Clark, KB7QHC

Thanks for that clarifcation, Richard. We (Navy) have modeled Franklin
arrays but the short-circuited 1/4 wave sections did not contain any
dielectric material. 73s from N4GGO,

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