ttenuation As A Function Of Material, Freq., etc. ?
Robert11
New at this.
I guess radio waves can go through wood just fine, as I can har signals from
my Discone that's in my attic.
But, there is, I imagine, some attenuation as a function of type, thickness
(of the wood) and the frequency involved.
Are there any good links explaining Attenuation as a function of frequency
for different materials, etc. ?
thanks,
Bob
Richard Clark
wrote:
Hi Bob,
Yes. I've even calibrated products that measure the percent moisture
content of wood using RF probes. However, most of what you might find
is in terms of nearfield (much like my probes); or as forest. Of
course, the attenuation of 2x4s falls somewhere in between, but I
would hazard to guess, it doesn't amount to much.
Nearby trees, on the other hand, do represent a significant wavelength
of wetter wood (dry wood is about 15-22% water, and standing wood is
significantly wetter). We have, in the past, found postings here on
how to drive a nail into a tree for a feedline connection, lay out
radials, and use the tree as a radiator for HF. I suppose it was as
efficient as a 160M antenna the size of two shoeboxes. That is to say
it is not so amazing that it works, as it is to say that someone tried
it.
When you consider how much bulk wood is in a house, and how much
volume the house occupies, the loss would seem to be inconsequential
at HF. As you mention a Discone in the attic, I must expect you are
more concerned with VHF/UHF. This will alter the loss landscape, and
I doubt there is a lot of accessible data about wood's loss in those
bands as it is so easy to avoid (putting an antenna near timbers, that
is).
73's
Richard Clark, KB7QHC
K7ITM
The actual calculation for a practical situation isn't simple--or
perhaps I should say isn't easy for a human to do. It's the sort of
thing we'd most likely let a computer do these days. But you can at
least get an idea about what's going on. The characteristics of a
material that will interact with radio waves--electromagnetic
radiation--are its permeability, permittivity, and resistivity.
Permeability relates to magnetism, and for non-ferromagnetic
materials, you probably don't have to worry much about it. As a
start, you might have a look at Wikipedia for permittivity, or do a
google it. The Wikipedia article I found about it looked pretty
reasonable.
Yours is a good question. Some people make a career out of studying
that sort of thing and becoming experts in it. You'd probably need to
study "electricity and magnetism" up through a good understanding of
Maxwell's equations and then get into the details of the physical
properties of materials to thoroughly understand it. Even then,
there'd be lots of puzzles to figure out, if you wanted. In other
words, you can dive pretty deeply into it if you want. But you're
absolutely right, the properties change with frequency, and the
effects you'd see at different frequencies would be different even if
the properties were to stay the same.
Cheers,
Tom
David G. Nagel
Bob;
Attenuation can be a problem but not until frequencies reach the Ghz
range. Then it is a case of radiated signal being reflected not absorbed
by the intervening material. i.e. wood will absorb, radome plastic will
transfer.
As for a source of information, all I can suggest is GOOGLE.
Dave WD9BDZ