On 7/13/2012 10:12 AM, Paul Christensen wrote:
> What I think is missing from the discussion is that any benefit from using a
> CM choke between shack hardware can be undermined by necessary parallel
> paths. Consider a CM choke placed on the RF cable between a transmitter and
> amplifier. Not only is the shield of the coaxial cable connected between
> the equipment, but so is the AC ground conductor, and shielding of the PTT
> key line and ALC line (if used).
NO, unless the coax shield is improperly terminated (the Pin One
Problem, or a badly installed connector). Remember -- a signal path
through a transmission line looks to the source like a resistance equal
to Zo, while ANY other return path includes the inductance of the loop,
which is FAR greater than Zo. Thus, if we provide a proper transmission
line path, there is ZERO RF current on those other conductors. ONLY at
DC, or near DC, does any return current divide to those parallel paths.
I witnessed an excellent demonstration of this at an IEEE EMC workshop
many years ago when I was still living in Chicago. An audio generator
fed RG58 with a load on the other end, and there was large buss bar
running in parallel. There were current probes on both the buss bar and
the shield. At very low audio frequencies (60 Hz, for example), all the
current was in the buss bar -- it divided between the two paths by Ohm's
Law, and the inductive reactance was near zero. At the shield cutoff
frequency (roughly 1 kHz), the current began to shift to the cable
shield, and by 10 kHz nearly all of the current was in the cable shield.
Think about this in the context of band switching in a multi-band
bandpass filter (like the ICE 419), or the RF path for T/R switching
through a power amp. The GOOD ones don't depend on the chassis for
signal return, it's an all coax path from input and output connectors
through the switching relays. Ditto for antenna switching networks like
those used to assign antennas in multi-transmitter stations. A properly
built switching or relay box will either use coax or will have a proper
ground plane carefully spaced so that return currents can follow a path
directly under the "hot" path through the switch. Any inductance in that
path is a common impedance that couples signal into other circuits
(crosstalk).
As Henry Ott has observed, we must always be aware of where the current
is flowing, and that includes ALL of the current, and it includes the
return path -- what he calls the invisible schematic hiding behind the
"ground" symbol.
73, Jim K9YC
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