Ok Bob, now I understand what you are doing. I am familiar with using a
quarter wave stub for matching. What I did not understand is the type of
error message you were getting.

I use a different version of NEC 2 (Nittany Scientifics' NEC-Win Pro). It
does not return an error message when attempting to use a single segment
wire, with a source, to feed a transmission line model. I don't think you
need worry about the message, since NEC will return the correct solution.
You can always experiment with the "TL" card by placing a known load at one
end, and observing the input impedance to see that you get the expected
value.

I have attempted modeling a double extended zepp, with a source at one end,
and the results are completely wrong so guess the error message is required
to cover such events. Constructing a physical transmission line model
overcomes the double extended zepp problem.

Typical NEC code dealing with the "TL", as per L.B. Cebik, in his book
"Basic Antenna Modeling, a Hands on Tutorial" is as follows":

..
..
GW 1 21 -3.418 0 15 3.418 0 15 0.000814
GW 2 1 -0.1 0 5 0.1 0 5 0.000814
..
..
TL 1 11 2 1 75 15 0 0 0 0
..
..

73,

Frank


"Bob Bob" wrote in message
news
Hi Frank

Yes the usage of the 1/4 wave section is simple and well known. I have
been doing it for years. The loop Z varies somewhat depending on shape and
height over ground. I am modelling a triangle and it runs around 145r

When I create the "wire" in free space by istelf I get a warning;
Error: Wire 5, seg 91, Ex-src: Not allowed near open wire-end

Mind you it comes up with seemingly the correct result. If I change the
number of segments in that wire from 1 to 3 the error doesnt occur.

More feeling my way at this...

What I'd really like to do is setup a dual band single quad loop (ie one
loop inside the other) for (say) 20 and 40M with no ATU. ie direct coax
feed...

Cheers Bob


Frank wrote:

Bob, I don't know much about quad loops, so not sure what you are doing.
Is
the 1/4 wave coax just to transform the single loop input impedance to a
lower value? I think a loop has about 120 ohm input Z, so 1/4 wave 75
ohm
coax transforms it to near 50 ohm.

The transmission lines in NEC are ideal, so you will not get any loss as
with a real transmission line. You do have to have a piece of wire at
each
end. The loop end wire is the loop, and the feed point end wire must
contain the NEC source.

Do I have the right idea, or am I completely out to lunch?

Regards,

Frank