Topband: Antenna question
jim at audiosystemsgroup.com
Mon Mar 31 02:38:24 EDT 2014
On 3/28/2014 6:23 AM, Tom W8JI wrote:
> There is no reason why it won't work or why shielded wire won't work
> with proper transformers and wiring.
I agree with everything Tom has stated here, with one MAJOR exception.
The generic audio "rack cable" that is twisted pair with a foil shield
and drain wire as made by Belden and everyone alse is a TRAIN WRECK for
RF intrusion from the shield to the pairs. The drain wire is the bad guy
-- no problem with braid shields, with or without foil. This was first
documented by the late Neil Muncy, ex-W3WJE, in an AES paper in 1994
that also introduced the pro audio world to "The Pin One Problem." Neil
was subsequently for elected a Fellow of the AES, largely on the basis
of this very important work. Motivated by his work, which was done at
audio frequencies, I extended his work on both of these issues to RF,
showing that both of these mechanisms are major causes of RFI.
My part of this work is on my website, k9yc.com/publish.htm Look for
the tutorials about SCIN and the Pin One Problem.
The bottom line is that for cables using that foil/drain wire
construction, a shield makes matters WORSE, not better.
Another important point. Bill Whitlock (another AES Fellow) showed in a
1994 AES paper that a balanced interface should be analyzed as a classic
Wheatstone bridge, and that the ability of a the interface to reject
noise depends entirely on the balance of the entire interface -- the
source, the interconnecting cable, and the receiver, and it is the
IMPEDANCES of each element. And for good RF rejection, those impedances
include reactive elements.
Bill's paper is worthy of serious study. The essence of it is included
in tutorials at the Jensen Transformers website, which is his business.
73, Jim K9YC
More information about the Topband