On Thu, 19 Nov 2009 11:48:12 -0800, Jeff Liebermann
wrote:

Thanks. 50 ohm coax does not make it a matching section to a 200 ohm
folded dipole.

Perhaps not an optimal one (which would be the geometric mean of the
source/load) but an effective one - if it were designed so (I don't
think it was ever intended to be one).

However, the ferrite beads are a good way to simply
block the reflected power from the folded dipole so that it looks like
it's matched. In any case, that reflected power is lost (converted to
heat) in the ferrite beads. So much for efficiency.

In fact the beads won't do that at all. They see only the common mode
circuit. Reflected power is going to reside in the transverse mode
circuit where the beads are invisible.

When I change
the characteristic impedance of the model from 200 ohms to 50 ohms,
the VSWR climbs to 5.5:1. Yech. (Note that the radiation efficiency
is 75% with or without the mismatch).

My model shows a more benign mismatch to a 72 Ohm load.

73's
Richard Clark, KB7QHC