Hi Kevin.
There are a lot of "gotcha" situations in antenna modeling where the math
and the method do not deliver correct results (or worse) if those gotcha's
are not avoided. The reasons are in the computational scheme and the
algorithm particulars. Ghastly complex, subtle and confusing stuff for the
most part, often only clearly understood by the developers. The "why" would
take a book.
Roy Lewallen's EZNEC, which is a shell for NEC2 and the pro NEC4, has an
excellent built-in help system with many of these situations explained.
One of those has to do with restrictions on how to model closely spaced
parallel wires. Not following the rule for a given gotcha can introduce
errors, some rather severe, while if the restrictions are followed,
everything is peachy. This information seems to be poorly specified in the
NEC2 derivatives other than EZNEC and I personally do not know of a
compilation to make things easy for someone just starting out.
In NEC2 and NEC4, closely spaced parallel wires MUST have equal sized
segments, and they MUST be aligned directly across from each other. Failure
to do that can sometimes introduce large errors in the results. For that
and other reasons, I always use 1 foot segments on my 160 models. The wires
can be as close as 3 or 4 wire diameters.
I have often found it necessary to directly model transmission lines, and
particularly in those cases not having oppositely positioned and equal
segments can render results from the next planet.
You can download the free demo version of EZNEC which contains the full
help function, and search on "closely spaced wires" in the help. That
article has an illustration or two. The closely spaced wires stuff applies
to *any* NEC2 derivative program, not just EZNEC.
73, Guy K2AV
On Fri, Jul 24, 2020 at 8:46 AM Kevin Zembower via Antennaware <
antennaware@contesting.com> wrote:
> Hi, Bob, thank for responding again. I have some comments on your
> responses in-line, below.
>
> On 7/22/20 2:03 PM, Bob, W3IDT wrote:
> > Kevin,
> >
> > [I sent you a copy of a 4NEC2 input file a while ago as an example, but
> > now realize that 4NEC2 has capabilities well beyond xnec2c.]
> >
> > I am NOT an expert on NEC2 control "cards". [Not the issue of "cards"
> > themselves - I am a very old hand at computing, started programming in
> > 1957 with punched paper tape, then punched cards.]
> > I let 4NEC2 generate the NEC2 deck from its pretty nice user front end.
> >
> > That said, my look at your geometry suggest at least the following
> > problems:
> >
> > 1. NEC2 has inherent problems with closely spaced wires - such as your
> > two dipoles.
> > To get anything resembling good results the segments in the closely
> > spaced wires must match exactly.
> > So, your 40 meter dipole must consist of (at least) three wires:
> > - on outer left side,
> > - a middle section, exactly the same length as your 20m dipole, and
> > - an outer right side.
> > And the 40m middle wire and the 20m wire must have exactly the same
> > number of segments.
>
> Thanks. I didn't find this information anywhere I searched. What I did
> find was a recommendation that all elements of the antenna have the same
> size segment length. That's why I made the 40M dipole twice as long as
> the 20M dipole, with almost twice the number of segments. Is getting the
> exact same segment length critical? I'll try this way next.
> >
>
> > 2. You have specified .15mm as the RADIUS of the wires.
> > This is equivalent to about AWG #28 or #29 (DIAMETER of about .3mm).
> > Probably much to thin for a 40m dipole with a 20m dipole hanging below
> > it. Try a RADIUS of 1mm (equivalent to about #12 wire, DIA of 2mm)
> >
>
> Yeah, I was modeling #28 AWG wire. I'll move up to 1mm wire. Again, I
> didn't think this would be that significant.
>
> > 3. The transmission line you have specified (when re-imported into
> > 4NEC2) appears to have
> > - an impedance .02 ohms ("2.00000E-2")
> > - a length of 0 ("0.000000E+00")
> > - an "end 1" of .02 ("2.00000E-02")
> > - an "end 2" of 50 ("5.00000E+01")
> > Surely not what you intended.
> > [4NEC2 offers "open" or"closed" as the options for the ends of a
> > transmission line.]
> >
>
> I was trying for a short length of 50 ohm coax. Since the units are
> Seimens, I took 1/50 ohms = 0.02 mho. I think I just forgot end 2. In
> the instruction for the NEC2 TL card, it says if the length was blank,
> it would use the length between the two attachments points. However,
> xnec2c won't let you enter 'blanks' so I thought zeros might work.
>
> > With changes to the geometry as per (1) and for (3) a 100 ohm
> > transmission line 1 meter long, open at both ends, gives a very nice swr
> > / reflection coefficient at 7.1 and 14.0. Good for a start.
> >
>
> Let me try to implement what you described in xnec2c over the next
> couple of days, and see what I get.
>
> Thanks so much, again, for all your help and advice.
>
> -Kevin
> KC3KZ
>
> > Changing the wire RADIUS to something more reasonable, say #12 (RAD
> > ~1mm) moves both resonances lower (as expected).
> >
> > The 4NEC2 generated card deck is attached.
> >
> > Welcome to the world of antenna modeling.
> > It can be "all consuming".
> >
> > Bob, w3idt
> >
> > ............................
> > . Robert F. Teitel, W3IDT???? .
> > .???????????????????????????????????????????????????? .
> > . W3IDT@arrl.net?????????????????????? .
> > . W3IDT@comcast.net???????????????? .
> > ............................
> >
> > On 7/22/2020 12:07 PM, Kevin Zembower via Antennaware wrote:
> >> Hello, this is my first post on this list. The folks on TowerTalk
> >> suggested I ask my modeling questions here.
> >>
> >> I'm using xnec2c on a Ubuntu Linux desktop to perform the modeling. Is
> >> anyone here also using this software?
> >>
> >> I'm trying to model a 40/20M dipole. My two dipoles are connected by a
> >> 6 inch (0.1524m) transmission line. When I use the deck below, I don't
> >> get an error, but do produce a VSWR of 999 on all the swept
> >> frequencies from 7-14.35MHz.
> >>
> >> I have these questions:
> >>
> >> 1. What am I doing wrong in this model?
> >>
> >> 2. Using either the xnec2c interface, or the nec2c engine, is it
> >> possible to sweep more than one frequency range? I'm only interested
> >> in 7-7.3MHz and 14-14.35MHz, and don't care about the frequencies
> >> between the bands.
> >>
> >> 3. Can xnec2c, or the nec2c engine, produce 2D azimuth and elevation
> >> charts? The 3D illustrations that xnec2c produces are pretty, but hard
> >> to read.
> >>
> >> 4. Can I eliminate any of the cards in this deck? My starting point
> >> was the deck of cards that xnec2c produces when you ask for a new
> >> file. Are all of these needed?
> >>
> >> 5. Do you all have any suggestions for me to improve this model? Is
> >> there anything I'm overlooking. As I said, this is my first time
> >> trying to model an antenna, and the details can overwhelm me.
> >>
> >> Thanks so much for your suggestions and advice. I look forward to
> >> hearing from you.
> >>
> >> -Kevin
> >> KC3KZ
> >>
> >> (Deck also attached, to prevent email formatting errors)
> >>
> >> CM --- NEC2 Input File created or edited by xnec2c 3.6.1-beta ---
> >> CM 40/20M fan dipole, 10m elevation over 'normal' ground.
> >> CM Dipoles connected by short transmission lines.
> >> CM Used wire diameter of #28 AWG, 0.0126in, 0.00016002m radius
> >> CM Kevin Zembower, KC3KZ
> >> CE --- End Comments ---
> >> GW???????? 1?????? 41???? 0.00000E+00 -1.01370E+01?? 1.00000E+01??
> 0.00000E+00
> >> 1.01370E+01?? 1.00000E+01?? 1.50000E-04
> >> GW???????? 2?????? 21???? 0.00000E+00 -5.28700E+00?? 9.84800E+00??
> 0.00000E+00
> >> 5.28700E+00?? 9.84800E+00?? 1.50000E-04
> >> GE?????? -1???????? 0???? 0.00000E+00?? 0.00000E+00?? 0.00000E+00??
> 0.00000E+00
> >> 0.00000E+00?? 0.00000E+00?? 0.00000E+00
> >> EX???????? 0???????? 2?????? 11?????????? 0?? 1.00000E+00??
> 0.00000E+00?? 0.00000E+00
> >> 0.00000E+00?? 0.00000E+00?? 0.00000E+00
> >> TL???????? 1?????? 21???????? 2?????? 11?? 2.00000E-02?? 0.00000E+00??
> 2.00000E-02
> >> 0.00000E+00?? 5.00000E+01?? 0.00000E+00
> >> FR???????? 0?????? 51???????? 0?????????? 0?? 7.00000E+00??
> 1.47000E-01?? 1.43500E+01
> >> 0.00000E+00?? 0.00000E+00?? 0.00000E+00
> >> NH???????? 1???????? 1???????? 1?????????? 1?? 0.00000E+00??
> 0.00000E+00?? 0.00000E+00
> >> 0.00000E+00?? 0.00000E+00?? 0.00000E+00
> >> NE???????? 1???????? 1???????? 1?????????? 1?? 0.00000E+00??
> 0.00000E+00?? 0.00000E+00
> >> 0.00000E+00?? 0.00000E+00?? 0.00000E+00
> >> RP???????? 0???????? 9?????? 37???? 1000?? 0.00000E+00?? 0.00000E+00??
> 1.00000E+01
> >> 1.00000E+01?? 0.00000E+00?? 0.00000E+00
> >> GN???????? 2???????? 0???????? 0?????????? 0?? 1.20000E+01??
> 1.00000E-02?? 0.00000E+00
> >> 0.00000E+00?? 0.00000E+00?? 0.00000E+00
> >> EN???????? 0???????? 0???????? 0?????????? 0?? 0.00000E+00??
> 0.00000E+00?? 0.00000E+00
> >> 0.00000E+00?? 0.00000E+00?? 0.00000E+00
> >>
> >> _______________________________________________
> >> Antennaware mailing list
> >> Antennaware@contesting.com
> >> http://lists.contesting.com/mailman/listinfo/antennaware
> >>
> _______________________________________________
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