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January 30th 05, 02:33 PM

On Fri, 28 Jan 2005 05:26:09 -0000, (gudmundur)
wrote:

Hi folks,

I needed to produce some referance power levels that were 'down' from
my calibrated 0DbM generator. (It's output power is auto leveled, and
non-adjustable) 9445mhz, center of one of the maritine radar bands.

I built two little aperature plates that look like a waveguide cover,
but have a hole drilled in the exact center. A .185" hole provides about
45db of reduction, and a .310" hole provides about 25db of reduction.

There are some differances of exact gain reduction depending on where
in the waveguide system I place these plates. (Close to source, or close
to receiver) Perhaps up to about 3 db differance, not a big worry for me.

Do the numbers sound correct? Should I have attenuated in a differant
way? Like perhaps a slot, instead of a round hole??

According to the one reference I can lay my hands on at the moment
(MIT Rad Lab Report No.43, Feb. 1944), a round aperature in a W/G is
equivalent to a shunt inductance.

The reference gives the data in graphical form. I put a few data
points into Excel and let it solve for the normalized susceptance v.
iris diameter.

For WR-90 guide (0.4" x 0.9") at 9.445 GHz, the formula is:

-Y' ~ 0.2838 * (d/0.9) ^ -3.1635

Where Y' is the normalized susceptance and with the iris centered in
the W/G "d" is the iris diameter in inches.

The reference doesn't specify a thickness of the iris plate, but I
would assume it's for plates lambda.

After writing the foregoing, I remembered Steve Adam's book,
"Microwave Theory and Applications" (I should have remembered it
sooner, he signed it for me).

He references H. A****er's, "Introduction to Microwave Theory" for the
following equation:

Z/Zo = j (2 * pi * d^3) / (3 * a * b * lambda_g)

Where for WR-90:

Zo = W/G impedance
a = 0.9
b = 0.4
d = iris diameter in inches
lambda_g = guide wavelength.

Whether these two references give the same answer will be, as they
say, left for an exercise for the reader.

What you're getting with these reactive elements is mismatch loss. If
you want true loss in your W/G, you must introduce resistance.

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