On 7/22/2011 5:04 PM, Jim W7RY wrote:
> Jim, this is also from your white paper. Cables such as RG-223, RG-141 and
> RG-400. All double shielded and the size of RG-58.
>
> Q: I have a high power, multi-transmitter station. Can I use RG58 or RG8X
> between my transceiver and amplifier?
>
> A: Smaller coax cables like RG58 and RG8X provide less shielding than larger
> coax, thanks to the higher resistance of their shields. To minimize
> inter-station interference, use coax with a beefy copper shield for all
> cables.
It's a matter of the shield resistance, as well as the density and the
uniformity of the shield. First, as I noted early on, RG-numbers have
been next to meaningless for nearly 50 years. We have to study the spec
sheets for the cables we are considering. Look at the resistance of the
shield and compare it to larger cable with a beefy shield.
While I didn't discuss it, there's an standard engineering spec for coax
called the transfer impedance. It's the ratio of voltage induced on the
center conductor by current flowing on the shield. The lower limit on
the transfer impedance is the resistance of the shield. It's the
standard specification for the quality of the shield, and the leakage
from the coax (and to the coax). The lower the transfer impedance, the
better the shielding. Obviously, the transfer impedance can vary with
frequency.
73, Jim Brown K9YC
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