On 4/22/2008 10:48 AM, Stephen Prior wrote:
I'm jumping in here without having really followed this thread, but I can
comment on this.
Bill is right that 17 mmho/cm is not equal to 17 mS/m, as you say it should
be /cm, not /m.
The unit is confusing however, and bears no relation to a per cm or per m at
all in the usual way that we think of these units.
Those of us broadcast engineers in the US who have had the (mis)fortune
to wrestle with the charts and graphs of FCC Rules Part 73 Subpart A -
formerly called "Appendix I - Standards of Good Engineering Practice" -
will remember that the map of ground conductivity used to compute the
groundwave field intensity of AM broadcast stations was found on the
infamous "Chart M-3" and the conductivity was expressed in mmho/meter.
The range was in the single digits over most of the U.S. A ground
conductivity of 8 mmho/m was the default value for "decent" soil.
Swamps and marshes (the best sites for AM stations) was up in the 20s,
and pure rock was down about 1.
Thankfully, those charts are no longer printed or stored in the online
Rules and computerized studies for AM antennas are required nowadays.
--
73 de K2ASP - Phil Kane
Elecraft K2/100 s/n 5402
VP - General Counsel & Engineering Manager
CSI Telecommunications, Inc. - Consulting Engineers
San Francisco, CA - Beaverton, OR
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