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Re: [RFI] Paying for information

To: "Jim Brown" <jimbrown.enteract@rcn.com>,"RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Paying for information
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Tue, 23 Dec 2003 14:20:33 -0500
List-post: <mailto:rfi@contesting.com>
Hi Jim,

I'm not trying to be argumentative about this but....

> But the simple fact is that my work has shown that some of our
> assumptions about how cable behaves are incorrect. How much are they
> wrong?  Braid-shielded cable (Belden 8412 and equivalent) has SCIN
> performance that is roughly 30 dB better than foil/drain shielded cable
> (Belden 8451 and equivalent) in the AM broadcast band!

Here is where you can help. Correct me if I'm wrong.

30 dB is a ratio between the two cables you tested, but if one cable
is -140dB based on some outside test to "no shield" and the other cable
is -110, who cares?

Let me make a statement that is true as an example. Clean shiny
pressure-compacted woven braid without an extreme weave (like a cable
shield) has about 7 times more loss than a solid wire the same dimensions
and material. If I lost one watt in a unit length of solid wire and
measuring the same length shield at 7 watts, it would be 8.5dB more loss.

7 times nearly nothing, however, is meaningless. 1000 times nearly nothing
can be meaningless. But the ratio between the nothings can be 8.5dB.

Saying the common mode problem increases 30dB doesn't mean a thing to me.
Unless we know much more. It falls into the category of useless specs people
worry about like "shielding effectiveness" of coaxial cables. Ask almost
anyone how shielding effectiveness is measured and what it means, and they
don't have a clue. That's why so many people waste money on extra shields
they don't need.

Also, measuring ratios in nulls is a pretty difficult task. There was a
"radiation from a Beverage drop wire" attempt to measure the depth of nulls,
and there were obvious problems with the test method.

There are many things that are almost impossible to measure accurately, let
alone are meaningless ratios. I'm not saying you are right or wrong. I'm
only saying without knowing how the measurement was done, the results really
don't mean anything....and I'm not willing to pay money to make someone
else's statement have meaning.

73 Tom

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