VE3PMK wrote:
I typically connect a center fed dipole is with a 1:1, but what
configuration calls for the 4:1, or even the 9:1 ??

Tongue-in-cheek mode on:
If you are matching 50 ohms to 50 ohms, use a 1:1
If you are matching 50 ohms to 200 ohms, use a 4:1
If you are matching 50 ohms to 450 ohms, use a 9:1
Tongue-in-cheek mode off:

Just trying to emphasize that one needs to know what impedance occurs
at both the balun input and balun output terminals. If one doesn't
know, a 1:1 choke (current balun) is about as good a choice as any.

The impedance transforming function of a balun involves differential
currents. The choking function of a balun involves common-mode currents.
The relationship between these two currents can be measured but is
usually unknown. For the majority of HF ham radio applications, e.g.
multi-band operation from one dipole, IMO, the choking impedance is
more important than any transforming action higher than 1:1.

An example of where a 6:1 balun might be chosen is a resonant folded
dipole fed with 300 ohm balanced line. The SWR on the feedline will
be low. The impedance seen at the balun output terminals will be
in the neighborhood of 300 ohms. 300/6 = 50 ohms.

The key to proper balun functioning is that the choking impedance
needs to be at least five times the impedance looking out from
the balun output terminals, preferably higher. In an antenna
system with reflections, the impedance seen at the balun output
terminals will never be the characteristic impedance of the
feedline. Unless the antenna is matched to the feedline at the
antenna, the transmitter end will not see the Z0 of the feedline.
--
73, Cecil http://www.qsl.net/w5dxp



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