dykesc wrote in news:c0bab953-135b-455c-9a3f-
:

I'm going to 450 ohm window line to my OCF Dipole. Obviously, as
confirmed by EZNEC, without some kind of choking action at the
feedpoint, my feedline will be radiating. What kind of 1:1 choke could
I use on the 450 ohm line at the antenna feedpoint? I could build a
1:1 choke with the right toroid but not sure if I can maintain the 450
ohm line Z0 through the choke.

I differ with some of the other advice that you have.

I suggest that the "appropriate" common mode impedance is independent of
differential Zo. I refer to this as Rule 500, as it usually requires the
magnitude of common mode impedance to be 500 ohms or greater for use on a
50 ohm line.

Rule 500 is born out of a lumped circuit analysis, and is less relevant
to the antenna scenario with its coupled conductors. Like most Rules of
Thumb, it is spruiked without stating or often knowing the underlying
assumptions and is then RoT.

You could create a model of your antenna, including the common mode
current path and insert the common mode impedance as a load in the
relevant conductor in the model, and explore the effect with different
choke impedances and frequencies etc.

My article at http://www.vk1od.net/balun/W2DU/index.htm shows that done,
in that case with a W2DU balun. Several other articles on the site model
the common mode impedance of ferrite cored baluns, so they might give you
some ball park figures to try (eg
http://www.vk1od.net/balun/Guanella/G.1-1.htm).

The model also allows you to explore the importance of a shunt path to
ground outside the shack as an element of mitigating common mode current.

You may also come to the conclusion that Rule 500 is not applicable to
the scenario.


Owen