I am a ham radio operator teaching myself as much about elctronics as I can as a hobby. A dispute has come up with one of my friends:

Is a choke a form or type of transformer?

I thought so, but my friend says not. He says:

"A choke impedes alternating current (which includes RF),
and a transformer induces it onto the secondary winding.

How can two things that work so differently
be the same?"


I SAY IT IS A TRANSFORMER... JUST A DIFFERENT TYPE which uses induction to choke common mode current.

Context / Application = We are both ham radio operators and use 1:1 "balun" chokes to tame common mode noise on transmission lines, among other things, and I say a 1:1 feed line choke is a form of transformer, but he says not. These are usually wound on ferrite toroid cores or (occasionally) made with clamp on ferrite beads.


Can you settle the argument?

Thank you for your courtesy. Happy days - James - K8JHR -
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A Choke is designed to have a high Reactance at a particular frequency, where as a Transformer is designed to conduct a magnetic flux to induce a voltage in the secondary windings.

 

The Wiki on baluns shows a perfect example of a balun used as a transformer. If I may quote Wikipedia, underlining what I think is important,

"Generally a balun consists of two wires (primary and secondary) and a toroid core: it converts the electrical energy of the primary wire into a magnetic field. Depending on how the secondary wire is done, the magnetic field is converted back to an electric field.

This clearly says a balun is a transformer, and the point that caught my attention is that one-winding thing called an auto-transformer. A transformer does not need two windings to be a transformer, it needs to do a conversion using a magnetic field.

I (personally) have used a transformer as a choke by ignoring one of the windings. It still looks like a transformer, but it isn't acting like a transformer. It is only creating a frequency dependent impedance.

If I have not addressed your question, feel free to elaborate.

 

Oh... darn. It appears you understand the very crux of the matter.

So, if I understand what you are saying, it appears I am mistaken. A choke does an inductance/impedance thing, while a transformer does an electrtical-magnetic-transformation thing. I suppose that accounts for the "transform" part of the word "transformer." Thus, while a balun often looks like a transformer, once again looking like a duck does not always make it a duck.

Man, I hate losing bets. ;-(

Thank you for your assistance and courtesy. Looks I may have to buy the beer this time...

Happy trails. --- James -K8JHR ---

 

Here's another quote and a link to the page:

Sometimes, in the case of transformer baluns, they use magnetic coupling but need not do so.

and another:

To avoid feed line radiation, baluns are typically used as a form of common mode choke attached at the antenna feed point to prevent the coaxial cable from acting as an antenna and radiating power.

http://en.wikipedia.org/wiki/Balun

A balun might be a transformer and it might not. It depends on how you are using it.
You might be buying the beer and you might not. It depends on the exact wording of the bet. If you said baluns are always transformers, make mine a Coca-cola.

 

A inline coax "balun" is a choke.

Due to skin effects at RF you have three conductors instead of two. The outside of the shield due to skin-effect, the inside of the shield and the center conductor. The common-mode signal is coupled to the "outside","inside" and center and is mainly canceled between the "inside" and center conductor generating the same polarity field (no net potential energy) from the common signal. The energy on the "outside" is 'choked' by the coil stopping it from flowing past.

 

OK... you guys are right on the point of our debate. We ARE talking about "choke baluns" at it were, where the one under discussion is a 1:1 "balun" and is intended to thwart common mode noise (which I suppose must be different from differential mode noise... I am a novice at all this...) and the idea is to allow the signal to pass, free from external EMI/RFI by using a 1:1 choke "balun".

I recall a local ham (a retired radio engineer for NASA and the Air Force) telling me that a 1:1 choke balun is a form of transformer, but I am NOW wondering if he was giving me the 5th grade level answer, and not the college grade level answer. Perhaps the context was different in some material way, but you guys seem to fully comprehend the exact application we are contemplating.

IF I can say the 1:1 choke "balun" is a transformer in some way, I can at least salvage some face... but it appears not.

-- Weak Attempt at Damage Control --

You see, I was thinking that even a 1:1 choke "balun" did some sort of impedance transformation as it applied a (high?) impedance to remove the unwanted common mode current that is carried on the outside of the coax shield. Or... even if it does that, is not not a transformer because it does not transform the RF carried on the inside of the coax shield and on the center conductor inside the central dielectric?

If the "balun" ALSO acts as a transformer, as in my 4:1 balun used on an Off Center Fed Dipole also transforms impedance, then it is, I suppose, both an inductor (i.e. the choke function) AND a balun-transformer (i.e. the transformer function) ??

You cannot imagine how helpful this is to me !!!

Thanks!

----------------------- James - K8JHR --------------------

 

You may think of a choke as a special case of a transformer, where a transformer is defined as 1 or more windings on a common core such that they share their magnetic field.

To make a transformer you start out by first making a choke when you add the first winding. (Nitpickers please ignore bifilar primary and secondary ;-) )

The distinction between a choke and a transformer is essentially arbitrary, so to solve your dilemma I suggest a rousing round of rock-paper-scissor-lizard-Spock to pick the winner.

 

A choke (inductor) and a transformer both have winding(s) on a magnetic core (or perhaps an air core for very high frequencies). The difference is basically in usage. A choke uses the inductance as a primary characteristic to affect a signal. In a transformer the inductance is secondary and is just to establish the magnetizing current with the transformer's primary purpose being to transform one signal level (or impedance) to another.

A common-mode choke is a sort of hybrid where the inductance is used to suppress common-mode signals, but the transformer action allow normal-mode signals to readily pass.

 

A inline coax "balun" is a choke.

...The energy on the "outside" is 'choked' by the coil stopping it from flowing past.

So this begs the question, how then does the coil selectively attenuate just the energy riding on the outside surface of the coax shield and not impact the signal on the inside of the shield or the center conductor? I ask because all three surfaces are coiled since they are part of the same cable.

Would they not all be impacted by the inductance of the coil? I could understand if the energy riding on the outside of the shield was at a different frequency than the energy on the inside of the shield or the center conductor because the inductance of the coil being equal to all 3 signals, obviously different frequencies would be impacted differently. Is common mode voltage/current a different frequency by virtue of it's path? Just curious? Thanks,
Ωhm

 

The inner currents balance in terms of their net flux. They produce no net flux in that balun core. Only the external current produces flux in the core. Creating a high impedance return for the external current is the only criteria. If the coax were of a length equivalent to an odd integral value of quarter wavelengths at the operating frequency, the external current would be miniscule or non existent. But one is rarely afforded that luxury.

You might find the attachment of interest.

 

Not sure what is meant by inner currents and external currents.

Common-mode currents are equal currents in the inner conductor and outer shield that go in the same direction, so they generate a net flux in the core. This causes an inductive impedance to these currents and attenuates them.

Normal-mode currents go in opposite direction in the inner core and outer shield so generate no net core flux and thus no inductive impedance to their conduction.

 

The ("inner") equal magnitude / phase opposing currents flow on the inside of the coaxial cable - on the inner core conductor surface and outer shield inside surface. The "external" current flows (when it exists) on the outside of the shield. For a coax directly driving a balanced antenna (such as a center fed dipole) the current flowing on the outside of the shield accounts for the imbalance in the radiator element currents. The attachment in my previous post seeks to explain this in greater clarity and as the rationale for the need of the balun. At the point of attachment to the corresponding radiator element the current leaving the inner surface of the coaxial shield divides into two parts, with some going to the radiator element and some flowing back down the outside of the coaxial cable to ground. So the only flux generated in the balun core is due to the aforementioned current flowing on the outside of the coax. The coaxial inner core current induced magnetic flux is balanced by the flux due to the current flowing on the inside of the coaxial shield. In the typical discussion of the problem the differentiation of the RF currents flowing on the shield inner and outer surfaces seems to find its justification in the skin effect.

Crutschow - perhaps we are writing about the same thing but using different terminology. I'm simply following what seems to be conventional terminology used by ham operators.

 

OK. I'm not that familiar with RF and was describing the currents at low frequencies where skin effect is not that significant.