[Elecraft] Toroids frequency range and baluns

[Jim Brown]

On Fri, 19 Sep 2008 10:21:44 -0700, Ron D'Eau Claire wrote:

>The balun doesn't have any effect on the efficiency of the antenna unless it
>involves lots of coax at high SWR such as the big coil of coax some Hams use
>for a "choke balun". (They're FB as long as the SWR is low but, like any
>coax, the losses go up with the SWR.) 

WRONG! There are three fundamental cases of ferrite "baluns" or chokes. 1) A 
choke wound with coax  2) a choke wound with parallel wires (bifilar)  3) a 
transformer (voltage balun). 

Case #1: Coax contains 100% of the differential power (transmitter feeding 
antenna) within the dielectric. The ferrite core of a COAX CHOKE sees ONLY 
the common mode voltage and current. If the impedance of the choke is high 
enough, the current is very small, so the dissipation in the choke is very 
small. 

Case #2: A choke with a bifilar winding (that is, two parallel wires, NOT 
coax) puts a significant fraction of the transmitted power in the core. This 
is "leakage" flux from the bifilar winding (really a short length of parallel 
wire transmission line). This leakage flux is typically 30% of the 
transmitted power, and is NOT related to common mode current. 

Case #3: A VOLTAGE balun is VERY different -- it puts 100% of the transmitted 
power in the ferrite. 

All of this is discussed in the tutorials, previously referenced. BTW -- 
earlier work and publications by W7EL, W2DU, and W1JR are all very good, but 
they were done 30 years ago. My work builds on theirs, and is considerably 
more advanced. Also, the #31 material that is so useful for HF chokes was 
developed only a few years ago. 

73,

Jim K9YC