In article ,
Cecil Moore wrote:

How do they avoid the dissimilar metal problem? If I
need to connect copper to aluminum, can I use that
center conductor with copper on one end and copper
scraped off the other end to avoid the dissimilar
metal problems?

My understanding is that the copper/aluminum issue is only a problem
in the presence of oxygen and an electrolyte. A bond between the
metals, which excludes these factors doesn't have galvanic corrosion
problems. There's apparently a high-pressure crimping system which
creates a cold weld between aluminum and copper wire which is
considered permanent and corrosion-free. I suspect that for LMR-400
and similar cables, the copper layer is deposited on the aluminum via
electrolysis.

LMR-400 is usually terminated to an N connector, using a pressure
crimper to attach the center pin... soldering is also used sometimes.
The recommended connectors come with heat-shrink tubing to seal the
coax-to-connnector junction. When properly assembled, these
connectors are moistureproof. The copper/aluminum junctions should
thus be well sealed away from anything which would cause them to
corrode.

LMR-400 does have another characteristic which some people feel make
it unsuitable for certain applications - its foil-and-braid shield.
It's nigh-on impossible to guarantee a good electrical bond to the
foil itself, since it's aluminum and cannot be soldered easily... you
have to depend on the pressure of the crimp, and on contact between
the foil and the braid. People who build repeaters seem to consider
this construction prone to become noisy over time.. Allegedly,
current flow between the braid and shield can create micro-arcing and
increases the transmitter's broadband-noise output, especially if the
cable is moving e.g. due to wind hitting the tower. This can
apparently result in enough broadband noise at the repeater's input
frequency to cause desense problems or scratching sounds when the
cable moves.

This doesn't seem to be a significant issue for simplex applications,
though.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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