To: The Microwave Group
From: Dick, K2RIW 10/03/03.
Re: Waveguide Dummy Load Info.
I still feel that with some care you can make a FB 10
GHz termination from a tapered wooden dowel. When Bruce, N2LIV and I did
this work in 1999, we were making terminations that had a measured S11 as
good as -35 dB (a VSWR of 1.04:1). We were making these dummy loads as a
test fixture for testing the insertion loss and quality of our 3/4 inch
copper pipe wave guide that we were using in the fabrication of "Button
Hook" feeds for our 10 GHz parabolic dish antennas.
that there might be some minor differences required when a dummy load is
being constructed for WR-90 wave guide, instead of 3/4 inch circular wave
guide. But, I see no reason to believe that the same techniques wouldn't
work equally as well.
For the dummy load we merely bought 3/4
inch wooden dowels that had been sitting in the Home Depot store on Long
Island for a couple of months. Long Island, New York, USA is a reasonably
high humidity area that is surrounded by sea water. I'm convinced that
the moisture content within wooden dowels helps make them into a lossy
material that makes them behave as a nearly ideal RF absorber. Remember
that the Loss Tangent of the water content of your food is the major
cause of the heat that you get from your Microwave Oven. If you place
something in your Microwave Oven that contains no moisture, it is
possible that the Oven will think it is empty, and the Magnetron may
become damaged by the high RF reflectivity.
If the material that
you use for a dummy load has a high or a low: dielectric constant, or
number of dB's per inch; that only affects the number of inches required
for the tapered section (for a good VSWR), and the number of inches of
the material that's required to obtain a given number of dB's of
absorption. We discovered that if any reasonable material has a
sharp-tipped taper that is gradual enough, the RF wave never knows that
anything is happening, and there will be virtually no reflection from
that load material.
I've even heard a crazy-sounding story of a
crafty Microwave Amateur who used bicycle tire material as very effective
wave guide dummy load material -- Hi. With a little care, I believe that
almost any material from slight lossy, to very lossy, can be made to
function as a wave guide dummy load.
Here is a repeat of the
appropriate paragraph that N2LIV and published on the Microwave Reflector
in July 3, 1999:
"7. DUMMY LOADS -- In circular WG are quite
easy to construct. Simply sharpen a 3/4" broom stick handle and force it
into the 3/4" copper pipe. About 3" of taper and 2" of non-taper is FB.
The usual moisture in the wood makes a great "slow absorber", which makes
it more forgiving of errors. The main difference between a -35 dB S11
dummy load (VSWR = 1.04, [sharp tip]) and a -20 dB S11 (VSWR = 1.22) seems
to be how sharp the point was at the tip of the broom stick handle and was
the taper too abrupt (too short). There may be some variations caused by
knots in the wood, but we didn't seem to have that problem."
hope this material is helpful.
73 es Good VHF/UHF/SHF/EHF Optical