Richard Clark wrote:
On Mon, 21 Apr 2008 10:21:36 -0400, "J. Mc Laughlin"
wrote:

Am I missing something? It seems to me that for the purpose of choking
common mode current (on the outside of the outer conductor of coax) from 3
to 25 MHz one should prefer the use of type 31. This is a receiving
application with a long run of coax.


Hi Mac,

I can't say that I have any familiarity with type 31, but certainly
the rest and others. My reference is a 13 year old hard-copy that
does not have this material listed, so it is hard to make side-by-side
comparisons for like-sized beads. Looking at the other charts it
would seem that type 31 would have less "bulk" Z (it would take more
beads at any particular frequency to equal other formulations).


I don't know about that... the 31 material is around mu=300-800 in the
HF area, which is higher than the venerable 43, and a lot higher than
the 61.

Another advantage of the 31 material is better performance at higher
temperatures.

K9YC has written up a 50 odd page handbook on RFI suppression, choking,
etc., with a whole raft of test data on actual chokes (bead baluns,
toroids, etc.) made of various materials.
http://www.audiosystemsgroup.com/RFI-Ham.pdf



As for your application, and if it is a particularly long run along
ground, or underground, ground itself may provide sufficient snubbing
of Common Mode currents. Except, of course, 60Hz which could be
particularly vicious and I would recommend running a parallel bare
ground wire to the remote ground. In that regard, you may even need
tri-ax.

73's
Richard Clark, KB7QHC