[TowerTalk] VSWR Heating in Chokes and Baluns

[Joe Subich, W4TV]

Balun/common mode choke what is the difference?  Neither
should be magnetizing the core unless you are talking
about a magnetically coupled transformer like the isolation
transformer in an FCP system.

73,

    ... Joe, W4TV


On 2020-09-03 10:30 PM, David Gilbert wrote:
> 
> 
> I was referring to a balun with that statement.
> 
> Dave   AB7E
> 
> 
> On 9/3/2020 4:53 PM, Joe Subich, W4TV wrote:
>> On 2020-09-03 5:52 PM, David Gilbert wrote:
>>
>>> 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.
>>
>> Where would that "high current in the winding" come from?  VSWR is a
>> transmission line effect and, as such, the currents in the winding
>> would be balanced (parallel wire choke) and/or contained on the
>> *inside* of the shield (coaxial choke).  There would be no current
>> contribution and thus no magnetic field from the transmission mode
>> currents at any level of VSWR.
>>
>> Further VSWR does not on its own cause common mode (unbalanced)
>> currents - thus no increase in common mode current with VSWR.  In
>> any case, with sufficient choking impedance there should be very
>> little common mode current (and no heating).
>>
>> The most likely cause(s) of heating are:
>> 1) thermal heating due to I^2R losses in the transmission line -
>>   e.g. a parallel wire balun wound with small gauge wire in
>>   a high power system
>> 2) excessive common mode current - e.g. a balun designed for
>>    14-30 MHz at the feedpoint of a tribander with insulated
>>    elements at the top of shunt fed tower on 160 meters.  In
>>    that case the 5-10K Ohms of choking impedance at 14-30 MHz
>>    becomes maybe 1K a 1.8 MHz but the "circuit" impedance at
>>    that point on the tower can be 10-20K meaning the balun
>>    does nothing to prevent (reduce) the common mode current
>>    through the balun/core.
>> 3) possibly poor thermal design (e.g. closed case).
>> In all cases the issue isn't choice of core material, it is
>> proper system design.
>>
>> 73,
>>
>>    ... Joe, W4TV
>>
>>
>> On 2020-09-03 5:52 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.
>>>
>>> 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.
>>>
>>> Dave   AB7E
>>>
>>>
>>>
>>> On 9/3/2020 11:54 AM, Ron WV4P wrote:
>>>> *Dave,
>>>> *
>>>> *Is that not something you should do yourself ? I recall the advice 
>>>> you gave me...
>>>> *
>>>> *
>>>> *
>>>> /"Neither is expecting/
>>>> /someone else to do all the work give you all the answers.
>>>>
>>>> Dave AB7E"/
>>>>
>>>> *Perhaps this is some testing you should do on your own, or at least 
>>>> the Google search to find other people's work ? *
>>>> *
>>>> *
>>>> *Thanks again for  your help when I had questions on Baluns.. *
>>>> *
>>>> *
>>>> *Ron, WV4P*
>>>>
>>>> On Thu, Sep 3, 2020, 1:14 PM David Gilbert <ab7echo at gmail.com 
>>>> <mailto:ab7echo at gmail.com>> wrote:
>>>>
>>>>
>>>>     Can anyone point me to a technical reference that discusses
>>>>     dielectric
>>>>     heating (not magnetic flux related) due to high VSWR in the
>>>>     ferrite core
>>>>     of a balun or common mode choke?
>>>>
>>>>     Thanks much,
>>>>     Dave   Ab7E
>>>>
>>>>
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