Several ways to do it, but Times Fiber (which used to be allied
with Times Microwave [the former did 75-Ohm CATV stuff and
the latter 50-Ohm cables]) uses a device (triaxial, I believe) which
actually measures the transfer impedance and then there's a formula
which converts that into dB of shielding. Belden has a triaxial device
called a SEED (Shielding Effectiveness Evaluation Device) and
the TEM, G-TEM and/or Crawford Cell (all variations on a
similar concept) could be used too.
This is going on memory, so I apologize for any errors. It's been too many
years since I used these devices (and especially the SEED) to make tests.
Of course, as has been pointed out, Andrews and others make coaxial
cables (e.g. Radiax{TM}) designed to "leak." It is used in the Holland and
Lincoln Tunnels (and I think the Brooklyn Battery and Queens Midtown
tunnels too) to allow motorists tp listen to their AM/FM radios (especially
for traffic reports).
73,
Bob AA0CY
aa0cy@contesting.com
----------
From: John D. Farr[SMTP:johnfarr@ro.com]
Sent: Saturday, August 09, 1997 10:14 PM
To: PaulKB8N@aol.com
Cc: Towertalk@contesting.com
Subject: Re: [TowerTalk] "Leaky" Coax, What is it?
In their catalog (Catalog TL-10), Times Microwave Systems, Inc. provides
data on the actual leakage of various shields. Unfortunately, their data
only go down to 10 MHz, but at that frequency, shield made of a single
copper shield (95%) radiates a power that is about 50 dB down from the
signal level in the cable per foot of cable. Double braid is down about
98 dB per foot and solid sheath (hardline) is down nearly 300 dB per
foot. The curves are nearly flat for the braided shields out to about
100 MHz then increase about 10 dB per foot by 1 GHz.
They don't go into detail on how these numbers were found except to say
that the data for the braided cables were obtained through measurment
and the data for the hardline are theoretical.
John
KC4ZXX
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