[Elecraft] 4:1 balun

[Paul Christensen]

> How does modeling with 450 Ohm window lead work out?  That would suggest
> a 9:1 balun.

Someone earlier (K2VCO, I believe) cautioned against falling into the trap 
of believing that the characteristic Z of the line is always the impedance 
actually present at the end of the line.  The Z seen at the line end is 
determined by several factors, including the magnitude of the line-to-load 
(mis)match, type of feedline, and feedline length.  For multi-band wire 
antennas, the line input Z can range from less than 10 ohms with some short 
antennas to over 5K ohm.  These numbers appear at the line input.  Just 
because it may be "450-ohm line" does not mean that's the target Z to tune 
and match.  The only time 450-ohm line presents a 450-ohm impedance at the 
line input regardless of distance, is when the characteristic Z of the line 
equals the load Z and line loss is small.

In the case of the folded dipole for mono-band operation, the antenna 
feed-point Z can easily be made 300 ohms with little or no reactance with a 
bit of antenna length pruning and perhaps slight height change.  Feeding it 
only with a 300-ohm characteristic Z line makes best sense, since the Z at 
the line input, regardless of line length is going to be near 300-ohms at 
the cut operating frequency, with little reactance.

Paul, W9AC