[TowerTalk] VSWR Heating in Chokes and Baluns

[jimlux]

On 9/3/20 2:52 PM, David Gilbert wrote:
> 
> I've done a BUNCH of Google searching using (among others) the search 
> phrases "dielectric properties of ferrites", "heating effects in 
> ferrites", "VSWR failures of baluns", etc and so far not found a clear 
> discussion of the topic.  That first search phrase alone generated 
> 1,230,000 hits and while I obviously didn't check them all I did check a 
> LOT of them.
> 

Yes - you want to look for "permittivity" - that cuts down the hits a lot.



> I remember, though, previous discussions here on TowerTalk where more 
> knowledgeable people than either you or I pointed out that damaging core 
> heating in a typical ferrite balun or common mode choke is far more 
> likely to be the result of dielectric losses due to high VSWR than it is 
> due to eddy current losses due to high current in the winding.  I just 
> hoped that one of those people would know of a technical link that more 
> rigorously describes the condition.
> 
> What I do know is that ferrites are basically an iron-based ceramic with 
> other materials (usually other metals) included as dopants to get 
> various desired properties, such as mu or best frequency range. Being a 
> ceramic, ferrites are basically insulators with both a dielectric 
> constant and a loss tangent that results in current flow (and therefore 
> loss and heating) under RF voltage (E-field) excitation.  That E-field 
> could logically be the result of a high VSWR ... just like the folks who 
> have stated same in the past.
> 
> As I remember, you were asking for others to do some studies that you 
> could have done yourself.  I'm merely asking for a link if anybody knows 
> of one.
> 



In a "choke" application (as opposed to a transformer application) - 
let's think of a toroid.. where would the RF current "around the core" 
(i.e. right angles to a central wire through the core) come from?

With multiple turns, I can see there being some asymmetry of the windings.

I've always thought that the heating was from magnetic loss, especially 
if the choking impedance was too low (so the "choked current" isn't 
reduced enough, so the equivalent I^2*R losses were high)