Hi Rick,
An answer to Rick and some number of others with inquiries off reflector....
I’ll mount a downloadable .EZ file of a basic L over FCP on k2av.com
sometime this week, along with some text, and post the URL. I'll make sure
it runs on the NEC2 engine, but will only do that in current maintenance
level V6.
For a 2 wire transmission line that's actually being used as a transmission
line one should first use the EZNEC transmission line element. And yes,
that element cannot show common mode. For a coax that's a wire of the same
thickness as the diameter of the coax shield, with insulation that matches
the coax jacket. For balanced line, two wires of same diameter and spacing
as the feedline, but matching the dielectric effect of window line's flat
PE wire support or large numbers of plastic spacers is complex.
The EZNEC transmission line element is provided because otherwise balanced
transmission lines are a royal pain, occupying tons of segments and
significantly extending compute times. You CAN do literally modeled
balanced transmission lines. But coax is impossible to literally model in
EZNEC. It's almost always the transmission line element PLUS the wire for
common mode effect.
The FCP is NOT a transmission line. It's a non-resonant single piece of
wire that's been folded up to reduce space and reduce RF fields from it at
the ground. It has to be modeled and understood as a single wire that's
been folded.
Closely spaced parallel stretches of that wire have to follow certain
rules. The very sparse spacers (every 11 feet for 160m) mean that they can
be ignored. Spacers every foot will change behavior. An FCP's
representation as a collection of shorted transmission lines does NOT show
the RF fields at ground, which is the crying necessity for an FCP in the
first place.
Luckily for this post the entire FCP and aerial wire is above ground and
can use NEC 2 with some accuracy. Not so with ground radials. There the
need for NEC4 starts to rear its ugly head.
If one has an apparent issue to solve with common mode on a balanced
transmission line, literal modeling of the transmission line is the clean
way you get to see what is going on. That is how we determined that running
QRO, we had at the antenna end of 450' of Wireman #554 a modeled 900 volts
common mode imbalance being placed across ferrite core "balun" windings for
suppression. The cores in turn were literally being burned up. We were
using a 4:1 to drop #554's actual 360 ohms down to 90 ohms resistive to a
series vacuum cap, then to an up 90 foot, out 105 inverted L over an FCP.
This was a problem in 2010, one reason that led to using a 4:1 isolation
transformer on a T400A-2 #2 powdered iron core with beefy polyimide+teflon
insulation between windings and windings to core. That event is chronicled
in the 2012 NCJ article you can find on k2av.com .
I went to using NEC4/Pro EZNEC exclusively in 2002, but you don't need Pro
to model an L over FCP, just one of the NEC2 versions that will handle the
number of segments.
I learned about the method for closely spaced wire earlier, like 1998 or
1999. I think something to do with folded dipoles and 82 ohm handmade
balanced feedline. I could have gotten it from W4RNL, or W7EL. Most likely
Roy, but the exact memory of that time has turned quite fuzzy and I'm not
sure who put me onto it But I do remember I didn't find it myself. The
EZNEC doc had those particulars at least as far back as V2. Then that was a
printable Microsoft Word dot doc file included with the distribution.
I've received several other inquiries off reflector about where that
information is, and noting their explanation of where they have looked and
not found it, I think I probably had a similar reason for "not found",
besides paper doc, but just don't remember that anymore. How to find the
stuff in current EZNEC help files follows below. In another post I will
show the evolution of this instruction.
Those with EZNEC V5 or V6, including the demo version, can find the help
reference by starting EZNEC (it's identical in V4/5/6). Click on "Help" in
the menu bar on the main window. Click on "Contents". In the User Manual
window, click on "Search" in the Contents-Index-Search tab row. In the "to
find:" space, type in "closely spaced wires" without the quotes. Click on
the "List Topics" button. In the topic list, double click "Closely Spaced
Wires".
You will note at the bottom of the text that the minimum spacing between
wires is "several wire diameters". Dictionary.com defines several as "being
more than two but fewer than many in number or kind". I translate that to
1/4 or 1/3 inch for #12 wire. The 4 inch (100mm) spacing of the FCP folds
is nowhere close to violating that. The "several wire diameters" is also in
the V2 doc. 24 years ago. Not new.
A general piece of advice to anyone using EZNEC, Roy Lewallen's EZNEC help
files are probably the best single "book" you can get on the ugly and
necessary details of modeling. The helps are heavily indexed and best of
all they are searchable. :>) The challenge is that it's quite large,
excruciatingly detailed, and you have to know what something is called. But
if one is going to do it seriously, and especially if one is going to dole
out advice, RTFM is required, seriously. And if that nasty bit of hard work
is beyond resources or emotional tolerance (I do understand that, I was
close to giving it up at one time), maybe it's best to leave it alone and
let others.
I will also post to this reflector a piece that shows the increasing
warning over time of consequences for not following the help files'
"closely spaced wires" dictum. Being the semi-hoarder I am, I have
everything from V2 onward. I doubt many have that laying around. Very
interesting stuff.
73, Guy K2AV
On Fri, Jan 8, 2021 at 7:22 PM Richard (Rick) Karlquist <
richard@karlquist.com> wrote:
> On 1/8/2021 10:41 AM, Guy Olinger K2AV wrote:
> >
> > Hi Rick,
> >
> > There is no problem modeling an FCP. You do have to do the parallel
> > wires a certain way that’s documented in EZNEC since 1997 (version 2).
> >
> > What do you perceive to be the issue?
> >
> > 73, Guy K2AV
> >
> > On Fri, Jan 8, 2021 at 11:10 AM Richard (Rick) Karlquist
> I am vaguely aware that for a 2 wire line, EZNEC recommends that you
> replace it with the EZNEC transmission line element. It also says
> that EZNEC will not model the effect of the line in common mode, AFAIK.
> In any event, the FCP consists of 3 parallel wires. I have no idea
> what to do with such a structure. So perhaps there is no problem
> modeling an FCP; it's just that I haven't got a clue. I would
> be happy to learn how to do it. Could you explain what to do or post an
> EZNEC file?
>
> 73
> Rick N6RK
>
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