On Jun 28, 4:51*pm, Owen Duffy wrote:

I had a look at your model and couldn't make head and tail of it.

Owen, maybe the following description will help with understanding my
model.

The antenna is an OCF Dipole in a Vee configuration. Wires 1 and 3 are
the short and long sides respectively. Wire 4 is just a short
connector at the apex. The antenna is fed with 100 feet of 450 ohm
line which transitions through a 4:1 transformer to 5 feet of 50 ohm
coax and then to the source. Wire 6 is an attempt to simulate the
unbalance radiation from the 450 ohm line. Wire 7 is an approximate
80m 1/4 wave RF "sink" on the ground side of the source end of the
model and is not meant to be a main element radiator so its position
relative to the other elements is immaterial.

Also, as you can see, I have wire 6 grounded which isn't actually the
case in the physical world. It is meant to simulate radiation from the
450 ohm line which actually ties into the 200 ohm (ungrounded high Z
side) of the transformer. I guess I figured grounding it was the right
thing to do since the EZNEC Help file implies the need for that to
simulate coax radiation. I do have wire 6 positioned correctly
relative to the other antenna elements, so its effect on the antenna
pattern should be accurate.

Don't let short wire 5 which looks out of position throw you. It just
serves as a termination junction between the 50 ohm side of the
transformer and the short run of coax. There are a couple of other
short "junction only" wires in the model.

A look at the wires, transmission lines, transformers, and sources
tables should aid in understanding the model.

I by no means mean to imply I've modeled everything correctly. I just
wanted you to know what I was attempting when I built the model.

Thanks for the Carolina Windom model. Good reference for comparing to
what I have done.

Also, thanks for looking at this. Any advice much appreciated.

73

Dykes (AD5VS)

dykesc wrote in news:6c2bff9b-583d-4eb5-90fb-
:

You have two parts to your model, and they are disconnected (apart from
conductor coupling effects). You have complicated your model with your
counterpoise concept and it doesn't work.

Try a simple model with not more elements than necessary, then when you
have it working, current look rational etc, extend the model.

Hint: the TL element models the differential mode of the transmission line,
and NOT the common mode. To answer your original question, you do not need
to model differential mode TL, just put the source(s) up in the centre of
the dipole.

You do need to know the common mode impedance of your 4:1 transformer to
include it in the model. If it is a voltage balun, you can probably treat
it as zero ohms without introducing great error, but if it is a current
balun (as your mention of choke implies), you need to know its impedance at
the model frequency.

Owen