Topband: FERRITE BEADS

[Tom Rauch]

> Per Amidon, type 77 material is best for amateur radio frequencies from 2
> to 30 mHz and type 43 material is best for everything above 30 mHz.

I spent a few weeks making measurements when HyGain was having problems with
beads, and some years before when MFJ was having baluns in one antenna cook
at just a few hundred watts!

In both cases the field failures or SWR drifting problems were traced to use
of 73 or 77 material strings of beads. 73 or 77 materials are good only in
low stress or low power applications. 73 and 77 mix cores add an equivalent
impedance that is mainly dissipative resistance on 1.8MHz and higher.

1/2 inch OD beads can only dissipate about 2 watts per linear inch of string
length in open air continuously, and less if you heat shrink them or enclose
them! A one foot long string of beads would overheat with 25 watts of power
dissipation in the beads, and that is easy to reach with kilowatt rigs!

If you are running only a couple hundred watts into a low Z system, 73 and
higher mixes are great. They also work OK at higher power if the system
almost doesn't  need a bead balun, because small amounts of common mode
voltage won't cause unacceptable current to flow through the bead
resistance.

The cheapest way to make a good balun is still an air core winding of coax,
assuming you have the room and weight isn't a problem. If you want to save
space or weight, something like a 65 material with multiple turns is the
next choice.

The poorest way (cost-wise) to make a balun is to pass the coax through the
core one time, like a string of beads does. You can get away with using a
very high permeability core, but the tradeoff is the balun will overheat
with fairly low common mode voltage across the balun.

73 Tom