Waveguide Dummy Load Info

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.

I'll admit 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."

I hope this material is helpful.

      73 es Good VHF/UHF/SHF/EHF Optical DX,
      Dick K2RIW.
      Grid FN30HT84DC27