Interesting.
Our first table was just an ordinary table, where the table top was of
chipboard which is a mixture of woodchips and glue.
That was quite bad, and when changing it we thought it was best to get a
table as good as one can get. We also test upto 18 GHz and styrofoam
table has proved to be a good one.
Ari Honkala
________________________________
From: [email protected] [mailto:[email protected]] On Behalf Of
ext Taylor, Michael
Sent: 24. tammikuuta 2008 17:40
To: [email protected]
Subject: Wood vs. Plastic table
Hello All, just a brief comment on this wood table business.
There are 2 issues at play here.
#1, the absorption of water into the wood which changes the
permittivity of the structure and the subsequent effects at microwave
frequencies.
#2, the minerals trapped in the interstitial boundaries
surrounding the cellulose fibers which impact the Permittivity of the
material, also impacting higher frequencies.
Some time back I conducted extensive experiments on wood tables,
I replaced my "specially" constructed wood table with Styrofoam blocks.
I made measurements using a well characterized broad-band noise source
that I have used for years in my chamber. The data from measurements
performed from 30Mhz to 6 Ghz with both the table and the foam blocks
will virtually overlay each other.
The differences in the measurements were so minor as to be
considered well within the limit of the Uncertainty of the system, less
than 1.2db (p-p) for repeated measurements made one after another of the
same point source radiator. Extreme care was exercised in assuring the
placement & orientation of the radiator remained the same for both
tests.
The "Special" table was made of triple dried Ash - mortis &
tendon / glue construction - and sealed with 2 coats of Spar Varnish.
A review of the properties of wood as listed in the "Wood
Products Handbook" shows a wide range of mineral content and
dramatically different water absorption properties for different woods.
The Hardwoods of "Slow-Growth" trees have higher fiber density
and lower mineral content than the softer woods.
They have dramatically different water absorption
characteristics also.
When the correct hardwood is fully dried and then sealed with a
natural spar varnish the resultant wood structure is almost RF
Transparent below 6 Ghz.
This cannot be said of some of the plastics & fiberglass
materials currently available, many of which have poor microwave
properties.
I'm sure this posting will elicit a rash of responses.
No threats of neutering or dismemberment please - I suggest
everyone conduct there own study and arrive at their own conclusions of
which materials work best for them.
As for me - I will continue to use a Hardwood table - properly
constructed & sealed against moisture and avoid all the hassle of a
Styrofoam. If I need something tested above 6 Ghz, I'll send it out to
another lab.
Just one guy's opinion
Michael Taylor PE, NCE
Chief EMC / RF Engineer
Hach Company
Loveland, Colorado
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