On 10/6/2011 10:48 AM, Grant Saviers wrote:
> What is good practice for mounting the standard transmission line balun
> (DXE, Array Solutions, etc. PVC box with toroid balun inside)? Can they
> be mounted directly to a metal plate? Is a standoff needed to minimize
> coupling if the plate is aluminum? What distance?
I suggest that you study my RFI tutorial, which includes a rather
detailed section on common mode chokes wound with coax through ferrite
cores. So-called "current baluns" are really common mode chokes.
http://audiosystemsgroup.com/RFI-Ham.pdf
Note that the choke is really a parallel resonant circuit, with very low
Q and capacitance typically on the order of 2-5 pF. The choke is
optimized for the frequency by selecting the core material, the size of
the core(s), the number of turns and spacing With such small
capacitance, it doesn't take much more to shift the resonance down.
As to an enclosure -- the resistive component of the choke will
dissipate heat if the common mode voltage is high and the resistance is
too small. Putting a choke inside an enclosure reduces air circulation,
which makes the choke hotter, and more likely to fail. Ferrite
materials do not absorb water, but they could be cracked by freeze-thaw
cycles. Living five miles from the Pacific ocean, I don't have that
problem. I first published my work on this about five years ago and many
hams are using them. So far, no one has told me about having problems
with freeze/thaw cycles or with failures due to overheating.
73, Jim Brown K9YC
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