[TowerTalk] VSWR Heating in Chokes and Baluns

[David Gilbert]

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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>>
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