[Amps] Amps Digest, Vol 174, Issue 46

[Manfred Mornhinweg]

Hi Jim,

> You are mistaken, Manfred. I've measured chokes like that wound on #61. 
> They are NOT effective at MF or HF. #61 is FAR too high Q at HF.

I understand that your point is that a high Q common mode choke might 
resonate on the operating frequency with the system's common-mode 
capacitance, and thus increase feedline common-mode current and feedline 
radiation. I admit that this might happen, but I don't think it's very 
likely. And if it happens, the total Q will be much lower than the 
common-mode choke's Q, anyway, due to that very radiation! Still on ehas 
to be aware of this possibility. So, point taken.

> You clearly don't understand how common mode chokes work. 

I think I do! But of course there might be some things I overloooked.

> You need to study my tutorial on the topic.
> 
> k9yc.com/RFI-Ham.pdf

I downloaded it a moment ago, and seeing that it's lenghty, I'm first 
replying to this post. I will read your tutorial as I find time, and 
then come back about it.

> A good common mode choke MUST BE resistive at the operating frequency,

I don't agree with that, when we are using a common-mode choke in an 
antenna feedline. My position is that it _can_ be resistive, as long as 
the resistance is so high that not only the common-mode current is low, 
but also the resulting loss is insignificant; or it can be inductive, as 
long as the inductance is high enough to make the common-mode current 
small. That also means avoiding the creation of a resonance on a band we 
will use.

> and achieves that by making it self-resonant in that range, 

And I don't agree with that. If this was true, than a common-mode choke 
would only be usable on a single band!

We can make a common-mode choke almost completely resistive by using 
ferrite that has much lower µ' than µ" in the frequency range of 
interest. That could indeed be 31 or even 43 - but as said, I don't buy 
the argument that a common-mode choke must be resistive!

> The circuit Q of an HF choke wound on 
> #61 is on the order of 10,

In fact it's much higher. I just measured a 14 turn coax choke on an 
FT-240-61 core. On the lower HF bands the Q is too high to measure on my 
  meter, while on 20 meters it's still well above 20.

 > making it effective on a single frequency.

I disagree. Its choking impedance is high throughout the HF range.

> And since the parastic C that forms that resonance is so small, it's 
> quite difficult to measure accurately.

It's a simple matter of measuring the inductance, the resonant 
frequency, and then calculating the capacitance.

My choke resonates on 93MHz. So its self-resonance is no problem on HF. 
Of course with other capacitances connected to it it will resonate 
somewhere in the MF or LF range, and maybe on some HF frequency. If such 
a resonance falls on a band one uses, the choke won't work well there - 
but that would require a system capacitance connected to the choke 
that's only around 50pF for an 80m resonance, and much less for higher 
resonances - that's very unlikely.

> The problem with being primarily inductive is NOT that it detunes the 
> antenna, but rather that the inductance combines with the common mode 
> impedance of the feedline (it's simply a wire connected to the antenna 
> at one end and usually the TX at the other).

Yes, I do see that. The question is how often in practical use one will 
find this creating a resonance on a band one is using. Then, how bad 
things will be on that band, and then, compare this problem to the 
signal loss caused by a resistive common-mode choke, and decide what's 
the lesser evil.

> And it helps to NOT use the word "balun," which is used to describe at 
> least ten different things that are completely different from each 
> other.

I used that word because I was keeping my aim broad, including not just 
common-mode chokes but also that thing called "voltage balun" by some 
people.

 > #61 is a great material for use as the core of a TRANSFORMER
> below 10 MHz, but useless as the core of a CHOKE below about 200 MHz.

The "thing" many of us connect between our coax feed line and our 
symmetrical (balanced) antennas _is_ a transformer! If built as a 
voltage balun it's a simple, conventional transformer, and if it's a 
common-mode choke, also called a "current balun" in some literature, 
it's a transmission line transformer.

Maybe you are thinking of an entirely different application of 
common-mode chokes: That of suppressing noise radiated by cables that 
carry RF signals that are not supposed to be transmitted, such as video, 
data, etc. In fact I use lossy materials for that application, and 31 is 
great there. These cables are supposed to have a shield connected to 
ground at both ends, so there shouldn't be any end-to-end RF voltage on 
them, and any such voltage appearing and causing common-mode current 
should be suppressed by "soaking up" as much of that energy as possible, 
and dissipating it as heat. That's what lossy ferrites do very well. But 
I don't want such a choke on my antenna feedline!  That line ends up 
being connected to a dipole or quad or whatever, and if connected 
directly would have its shield connected to half the antenna voltage up 
there, and to ground at the other end. I need to suppress the 
common-mode current that would result, but I don't want to absorb and 
eliminate any of that RF energy! So I need either a common mode inductor 
there - high enough in impedance to largely suppress the common mode 
current, and at the same time having a value that does NOT 
series-resonate with the system on the band I'm using. If it 
parallel-resonates, great, but that would be impossible to do in a 
multi-band system. The alternative to a high-enough inductive reactance 
is a very high resistance, so high that the loss becomes negligible. A 
common-mode choke having this high common-mode resistance is fine, but I 
insist that an inductive one works well too.

What I do NOT want in my antenna feedline is a resistive common-mode 
choke that has only a moderately high resistance, and ends up absorbing 
a noticeable part of my TX power!

> #77 is NOT a suitable material for a transformer NOR a choke above 100 kHz.

I agree, reagrding antenna chokes. I mentioned it only because some 
people have used it, and some of them claim good-enough results, mainly 
up to 40 meters. I would not do it, since better suited materials are 
available. But for common-mode chokes intended for EMI absorption, it is 
pretty usable, specially in the higher part of the HF spectrum.

> Again, study the link I posted.

I will study it, and then post again. It might take a few days, because 
I'm busy with extreme weather and the problems it causes.

Manfred



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