Inexpensive WR-42 Home-Brew Attenuators

To:   The Microwave Group
From: Dick, K2RIW          7/15/03.
Re:    Inexpensive WR-42 Home-Brew Attenuators.

INTRODUCTION -- In 1999 I submitted an article to the Microwave Reflector on the subject of the 10 GHz Circular Waveguide experiments that were performed by the Ten-X Microwave Group (from Long Island, New York) using 3/4 inch copper plumbing tubing. One of the components we had constructed multiple times were Circular WG dummy loads by machining a piece of 3/4 inch wooden broomstick handle and placing it inside a piece of the 3/4" copper tubing. This made a high performance "slow absorber". If the point on the broom stick handle was sharp enough, the reflection was less than -35 dB; that's a VSWR of 1.04:1.

FANCY MATERIALS REQUIRED? -- Neophyte Microwaver's have been lead to believe that very special materials are required to construct high quality WG dummy loads and attenuators. This is somewhat true, only if the performance must be obtained in the smallest possible package, and be maintained over all environmental conditions, and recommended frequencies for that particular WG. However, we microwaver's are usually not that fussy. If a Home-Brew WG attenuator is 2" long, compared to the commercial and more expensive product that's 1" long, and if the attenuation has a slight slant with respect to frequency, I doubt the crafty Microwaver would mind. He will simply calibrate it versus frequency.

With those concepts in mind, you will soon realize that many of the inexpensive materials at your disposal can be used to construct rather high quality fixed attenuators, variable attenuators, and dummy loads.

W1GHZ used bicycle tire material as a microwave absorber (see below). I believe that the small VSWR that Paul experienced would almost completely disappear, if his material had been cut into a tapered (wedge-like) shape. We used wooden dowels and broomstick handles and achieved Home Brewed dummy load VSWR's as low as could be measured, once we used a proper taper.

PAD CONSTRUCTION -- Although we didn't perform the experiment, we speculated that high performance WG fixed attenuators could be constructed by placing a wooden "bullet" in the WG that had been sharpened on both ends. This technique should work equally well on Circular WG and Rectangular WG. The attenuation can be adjusted by changing the length, width, and placement of the wooden "bullet", or by placing a number of bullets in the WG. If the desired attenuation is too large, when using a reasonable-sized wooden bullet, there are at least two alternate approaches.

(1) The bullet doesn't have to symmetrically fill the WG. A small diameter, sharp pointed, piece of wood (a dowel, or a sheet of wood) that is placed in one of the WG corners will perform admirably. The exact cross section of the attenuator doesn't matter. As long as it has a tapper that is slow enough in cross sectional change per wavelength, than there will be very little reflected energy (VSWR).

By moving the piece of wood from a WG corner, toward the center of the WG, the insertion loss will increase. This technique can be used as an attenuation Fine Tuner. Again, with proper taper, the VSWR will not change appreciably, as the absorber is moved toward the WG center.

(2) A full-sized WG low loss bullet could be constructed from Balsa wood. This low density material will have a much slower attenuation constant in dB per inch.

PAD CONSISTENCY -- The Dissipation Factor (or Loss Tangent) that is caused by the wooden bullet is strongly affected by the moisture content of the wood. Therefore, I recommend painting the wooden bullets with a weather-proofing paint so as to maintain their moisture content (loss consistency). Depending on the Dissipation Factor of the paint that is chosen, I suspect there will be a slight loss increase after the bullet is painted. Don't judge the final insertion loss until the paint has dried. There probably are some low Dissipation Factor paints, such as lacquer, which will have very little impact on the bullet's additional loss.

MORE PAD & ATTENUATOR DATA -- Concerning WG components, here's THE BOOK: George Southworth, "Principles and Applications of Waveguide Transmission", D. Van Nostrand Co., 1950; 689 pages (an oldy but goody). Sometimes this book is available from used book web sites; it's a good buy. It contains some of the best PICTURES of how rectangular and circular WG really works with lots of performance curves (you won't need the math to understand the pictures (pages 166 & 169), it's almost an animation) -- amazing stuff for 1950.

On pages 269 to 276 you'll find pictures of linear, binomial, gaussian, and exponential WG impedance stepping functions for broadband impedance matching, 14 designs for dummy loads (pages 368 to 371), about 25 attenuator designs.

Page 121 (A & B) has pictures are 21 of the circular WG modes (with the relative sizes of pipe shown, same frequency) made with an "RF absorbing camera".

The book shows some great transition devices, hybrids, mode killing devices & devices for launching higher modes (pages 354 to 362), round WG components (pages 269, 327 & 328), circular guide fin line (page 133), a great section explaining choke flanges (page 201), a circular pipe polarization rotating device that's "home brewable" (page 207), the shapes of circular and rectangular WG (of constant periphery) that give minimum loss (page 193)(the popular ones are not optimum), "skeleton WG" (page 175), about 15 kinds of WG irises (page 246 & 255), circular WG filters (page 307), the Qualcomm duplexing filter explained (page 309).

(Southworth continued) rotary vane phase shifter (page 333), rotary vane attenuators defined (page 375), a way of designing a variable conductance dissipative film (page 377), 33 pages of horn data (only portions have appeared in other WG or antenna books), 8 kinds of "backfire" feeds including the Cutler (pages 448 to 454), eight types of WG slot antennas (pages 425 & 430), five kinds of corner reflectors, waveguide lens antennas, some TWT and magnetron info, etc. The picture on page 186 shows me how I could make S-band WG out of rain gutter down spout tubing. Let a Microwaver stand in a good hardware store with that book in hand and I think he'll get some great and crafty microwave ideas.

ARTICLE REPEAT (IN PART) -- Below I have repeated two sections of the 1999 article. The numbers refer to the section numbers of the original article; there were 14 sections. If there is sufficient interest I could "re-publish" the 1999 article. Those who need a copy can request it from me.

Two sections of the 1999 K2RIW article entitled, "Circular WG Frequencies, More Accuracy, More Experiment Data" ------------------------------------- 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.

The completed circular WG dummy load consists of a ~ 7" piece of 3/4" pipe with the tapered broom stick handle (absorber) in it plus a copper pipe coupler at the open end. Some of the broom stick absorber can stick out the pipe far end, if you prefer. It is easy to place this load on any other piece of circular WG, while running component tests. These pipe couplers really are "sexless" connectors. For experienced rectangular WG users, it will feel strange to make connections in 2 seconds and not worry about screwing down the flanges to get a good VSWR. ...................

9. PADS -- We never did this, but it would be easy to design circular WG fixed attenuators by decreasing the length of broom stick absorber and tapering both ends to have a good impedance match from either direction. In this case I would recommend painting the absorber to keep the moisture content (absorption) constant.

If it is found that the loss is too great for a convenient length of tapered wood absorber, consider making the absorber out of six "splines" by using thin sheets of wood, or out of balsa wood. These low density materials (with tapered ends) will allow a lower insertion loss to be constructed from a longer length of wood absorber. Also, the slower loss characteristic will cause a lower VSWR for a particular taper rate.

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