[RFI] shielding question

[Jim Smith]

Ooohhh Yuckkk - I didn't want to hear this.  A while ago I bought a 500 
ft spool of Belden 9501 single pair with Beldfoil aluminum polyester 
shield with drain wire and used it to run every conceivable audio line 
(except mic) in the shack to a jack field. As all the lines are mono I 
just tied the ends of the two conductors together to give me single 
conductor plus shield. Sounds like I may have wasted a little money and 
a lot of time.

Could you explain the mechanism (or point me to a source) whereby the 
sheld is rendered useless by the presence of a drain wire at HF?

73 de Jim Smith    VE7FO

Jim Brown wrote:

>  
>
>2. If the shield on your cable is a foil with a drain wire, it is worse
>than no shield at all for frequencies below about 20 MHz. That is
>because any current flowing on the shield in this part of the spectrum
>flows almost entirely on the drain wire, which usually has the same
>twist ratio (lay, in cable-speak) as the twisted pair, and lies closer
>to one conductor than the other. In this condition, the current flowing
>on the shield will be converted to a differential mode voltage on the
>signal pair!  Above roughly 20 MHz, the current distributes more evenly
>over the shield, and this becomes less of an issue. 
>