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Forum index » Science and Technology » Physics » Electromagnetics
reflectivity of wire mesh (wire screen) and parallel bars
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Wimpie
science forum beginner


Joined: 04 Jul 2005
Posts: 1

PostPosted: Mon Jul 04, 2005 5:50 pm    Post subject: reflectivity of wire mesh (wire screen) and parallel bars Reply with quote

hello,
Wire mesh or parallel bars are often used as a replacement for solid
reflectors (for example to reduce wind load).

Is there any practical formula, computer program or graph for
calculating the (plane wave) reflectivity/reflection coefficient of
wire mesh (for example chicken mesh)?

I am thinking of a formula where you have to enter cell/grid size, wire
thickness and frequency.

Why this question? A perfect large reflector looking into the sky has
a noise temperature (for micro waves) in order of some Kelvin. If the
surface has low reflectivity, the noise temperature increases because
of dissipation in the non-perfectly reflecting surface. Knowing the
reflectivity enables making a good compromise between mesh size, gain,
wind load and antenna noise.

With kind regards,

Wim
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Lorenzo
science forum beginner


Joined: 04 Jun 2005
Posts: 4

PostPosted: Mon Jul 04, 2005 10:54 pm    Post subject: Re: reflectivity of wire mesh (wire screen) and parallel bars Reply with quote

"Wimpie" <wimtel@tetech.nl> wrote in message
news:1120506650.835494.218490@o13g2000cwo.googlegroups.com...
Quote:
hello,
Wire mesh or parallel bars are often used as a replacement for solid
reflectors (for example to reduce wind load).

Is there any practical formula, computer program or graph for
calculating the (plane wave) reflectivity/reflection coefficient of
wire mesh (for example chicken mesh)?

I am thinking of a formula where you have to enter cell/grid size, wire
thickness and frequency.

Why this question? A perfect large reflector looking into the sky has
a noise temperature (for micro waves) in order of some Kelvin. If the
surface has low reflectivity, the noise temperature increases because
of dissipation in the non-perfectly reflecting surface. Knowing the
reflectivity enables making a good compromise between mesh size, gain,
wind load and antenna noise.

With kind regards,

Wim

There are formulas for the roughness of the surface on the characteristics

of the antenna. A mesh or any nonsolid surface antenna just looks rough if
the holes are small enough. However, if the mesh is less than 1/10 of a
wavelength then the antenna looks pretty smooth and the gain will be close
to that achieved with a solid surface. There will be some additional
leakage through the antenna so the back to front ratio will be a bit worse
than for a solid. If the holes are less than 1/10 wavelength we assume that
the wires themselves will be much smaller than that and will have only a
minor effect on the pattern. Satellite antennas are often made of mesh or
of wires embedded in a resin ("hi grade fiberglass"). Such antennas
characteristically consist of two antennas, one above the other with the
wires in each at right angles thus providing two polarizations.
There is a lot of information on the web or in standard antenna design
texts.

Lorenzo
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