[TowerTalk] VSWR Heating in Chokes and Baluns
Richard (Rick) Karlquist
richard at karlquist.com
Fri Sep 4 10:13:27 EDT 2020
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.
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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