Topband: Chokes - ferrite vs air

[Tom Rauch]

Hi Gary,

> I've used home-brewed and commercial "QRO" ferrite choke 'baluns' before. 
> With QRO and hi-SWR, they get warm, sometimes hot to the touch: I guess
> that's the result of choking off the power on the outside of the coax
> and/or saturating the ferrite.  With QROo. & extended cooking time, I
> expect the ferrite denatures and they cease choking anything.

The problem isn't saturation, it's good old fashioned dissipation.

Most ferrites have fairly low Q on higher frequencies. 73 material 
has a Q of one around 2 MHz or so. While this doesn't hurt 
transformers with tightly coupled windings for receiving or low 
power transmitting, at higher power levels (or voltage per turn) most 
of the choking effect comes from what would be adding a "resistor" 
rather than a reactance in parallel with the voltage across the 
winding (or in series with the current, however you look at it).      

A very small amount of dissipation will overheat beads, because 
they have limited ability to get rid of heat. Most small beads used 
for baluns will only dissipate one watt or less for a few minutes 
before overheating!

A second problem is the impedance increases at the same rate as 
the number of beads. Double the beads, and you double the 
impedance.

Multiple turn inductors, even on ferrite cores, in ideal cases 
increase impedance approximately by the square of the turns 
increase. It's much less expensive to obtain a high impedance with 
multiple turns than it is by increasing the size of a string of beads!

Capacitance across the choke is NOT a bad thing, unless the 
operating frequency is 1.414 or more times the self-resonant 
frequency of the inductor. At all frequencies lower than 1.4 times 
the self-resonant frequency, stray capacitance **increases** choke 
impedance....so can actually be a good thing.

I mostly use air cores for applications where I have room, and 
different types of ferrites for other applications. At high power or in 
resonant systems, I use materials like 65 or 61. For other 
applications 73 material is about ideal for HF. 

Higher ui materials can actually have LESS impedance and have a 
higher loss tangent, so you have to be careful to look at the core 
characteristics at the *operating frequency*, not at or near near dc 
where most cores published permeabilities are determined! A 
10,000ui core can have less impedance and require more turns on 
160 meters than a 3000 ui core.

73, Tom W8JI
W8JI@contesting.com 


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