> How many beads of what type material are needed to construct a choke
> balun, using RG303, for a balanced antenna operating 40M-10M?
It depends on how much common mode voltage you have across
A minimum rule-of-thumb is to design the balun to handle 1/4 rated
power in a test fixture with the ground side reversed on the balun
> How many beads for the same thing operating from 80M-10M?
More than most people use or recommend when running 1500
watts. As a matter of fact, it takes so many beads to handle 1500
watts on 80 and 160 it is cheaper and better to just use a solenoid
winding of coax on a 4" PVC form.
As a matter of fact, all of my baluns are made that way.
> It would appear that there are some differences of opinion (or fact)
> on the proper bead composition for this application.
> W2FMI in his book "Building and Using Baluns and Ununs" criticizes
> W2DU for the use of "lossy high-permeability beads (2500)..."(A
> permeability of 2500 corresponds to #73 material-N4OO). W2FMI does not
> specify which material he thinks would be optimum, but infers that
> something "lower" would be more efficient.
Efficiency is NOT the problem, heat is the problem. At about 2MHz
the resistance and reactance of 73 material is about equal. As we
move above 2MHz, resistance dominates the impedance more and
There is so little surface area in the beads exposed to cooling air
and the beads have such poor thermal characteristics even as little
as 1 watt per bead can overheat the beads! This is especially true if
the beads are enclosed in a poor thermal conductivity enclosure,
like PVC pipe.
With a string of 50 beads, as little as 50 watts dissipation for 20
minute or so accumulated time without long cooling-off periods
could easily cause the beads to reach curie temperature.
That would be only about 3% power loss, yet it would be
devastating to the beads. That's why I never, even if efficiency is
good, use 73 material in high power applications.
> something of the order of #43 (850) material. In another chapter W2FMI
> suggests that something higher than #61 (125) and 64 (250) is required
> to obtain the necessary choking reactance.
43 material is OK at lower frequencies if the common mode current
through the choke balun (low voltage compared to the impedance)
is low. 43 material has a very low curie temperature, and a Q of 1
around the 40 meter band. It really isn't much better than 73
You really need 61 or 65 material at high power in many
When I have limited room at 1.8 or 3.5 MHz, I use a stack of 2" 65
material cores with multiple turns of coax. When I have the space, I
use an air-wound choke. All of my antenna have aor-wound choke
baluns, only my 160-meter phasing system and my high power
matching transformers for a 450-ohm line and my T network tuners
use 65 (or 61) material core stacks.
Strings of beads may be easy, but they make poor use of the core
You do NOT want a FMI voltage-type balun. They are load
impedance sensitive, and cause unnecessary stress on the core
By the way, be sure to model the end-application. There are cases
where adding a balun increases common mode current (unbalance).
73, Tom W8JI
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