[TowerTalk] VSWR Heating in Chokes and Baluns

Fri Sep 4 10:36:08 EDT 2020

On 9/4/20 7:13 AM, Richard (Rick) Karlquist wrote:
> A common mode choke is one form of balun.
> A different device is the voltage balun,
> which is commonly seen in ham radio catalogs.
> It is flux coupled and does involve
> "magnetizing inductance".  Actually, it is
> simply an RF transformer packaged for
> antenna applications.

But in that case, would permittivity (i.e. tan d term) feature as a 
significant loss source? (the OP, Dave's original question) I would 
think the dominant (by far) loss is the loss term of the permeability 
(mu'').

The second likely source would be IR loss in the conductors.

> 
> For completeness, I will mention that a third
> device that will function as a balun is the 180 degree hybrid.
> Not so common in ham radio antennas, but found
> in many solid state amplifiers.  It is different
> from either a voltage balun or a current balun.
> 
> Unfortunately, "balun" is a confusing term of art.
> 
> Rick N6RK
> 
> 
> On 9/4/2020 5:04 AM, Joe Subich, W4TV wrote:
>>
>> 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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>>>>>>
>>>>>>
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