On 2/27/2011 5:14 PM, Christopher E. Brown wrote:
> I think there is a terms issue here.
YES!
> I did refer to a common mode choke as a filter once, but the important
> part is _common mode_. And it does have band-stop, high-pass or
> low-pass behavior depending on the core and windings. 32turns on a
> 2.4" #31 toroid is a common mode only high pass. High common mode
> impedance on 160, decent on 80M and dropping from there. Very little
> impact on say 20M given the environ it is normally installed in.
More accurately, the chokes you are describing are actually parallel
resonant circuits of very low Q (typically on the order of 0.4). A
32-turn choke on a 2.4-in o.d. #31 toroid is likely to be resonant
between 1 and 2 MHz and have an impedance at resonance on the order of
30K ohms. So yes, they are band-stop filters in the common mode circuit.
You can see measured data for some chokes with small diameter wire wound
on 2.4-in toroids of #31, #43, #61, #77, and #78 in
http://audiosystemsgroup.com/RFI-Ham.pdf
There is also a development of the approximate equivalent circuit. When
you wind a cable pair or coax around the core, that parallel resonant
circuit shows up in series with the common mode circuit, so it
essentially functions to kill common mode current by brute force. :)
The tutorial also shows why resistance is FAR better than inductive
reactance at suppressing RFI.
If your primary interference source is on 160M, it makes sense to get
that resonance close to that range. If the interference source is higher
in frequency, the choke should be tuned (by nr of turns) higher.
Multiple chokes can be used in series to provide higher choking Z or to
cover a wider frequency range. If 160M and 80M are the main problem,
I'd tune the choke to about 2.5 MHz.
Another point about 160M. 160M is a weak signal band -- you need a lot
of power and great antennas, both for TX and RX. We're trying to work
halfway around the world just above the AM broadcast band! There are
guys who try to run daily skeds between Maine and VK! Most folks who
are serious about 160M run max legal power, and some people of low
morals go beyond that. No serious 160M operator (and few 80M contesters)
are going to accept a solution that only works for operation at 100W.
Some other thoughts. This has been an excellent discussion, and I think
we're all learning from it. I certainly am.
Gordon noted that common mode chokes are only effective if near the STB
(set top box). This strongly suggests that the STB is the weak link. If
the common mode choke makes a big dent in the RFI, I'd bet the major
component of the coupling is something as simple as a Pin One Problem in
the STB. The Pin One Problem is discussed in the RFI tutorial,
previously cited. Now, that does NOT mean that there couldn't be other
means of ingress -- poor balance of the input circuit for the balanced
cable, poor strongly signal handling of that input stage, poor shielding
of the STB, or even ingress somewhere else in the system. The nature of
Pin One Problems is that they tend to be quite severe, so fixing them
makes the biggest dent. But once you've killed them, the other coupling
routes are still there, usually at a lower level.
73, Jim Brown K9YC
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