This paper was presented at the 40th Annual West Coast VHF/UHF conference held on May 5-7 in Cerritos, California. Some editing was done to make it suitable for this web page.
Slot antennas can be used for fixed stations, satellite ground stations and beacons. With proper mounting, a slot antenna can also be used for "microwave mobile". With a 16-slot total, the antenna can have 10-12 dbi gain.
Slot antennas can be built from surplus waveguide sections, which will give an ominidirectional pattern (and horizontal polarization). This paper offers a computer-aided method to calculate the proper dimensions for the slots and their locations.
Because the antenna is of one-piece construction, it is rugged and can be built cheaply, requiring only access to a reasonably precise drill press or milling machine.
The first step in construction of the antenna is to select a section of waveguide which covers the desired frequency. Approximate waveguide dimensions for several microwave bands are listed in table 1. If your waveguide doesn't match the dimensions exactly, go ahead and run the program. It will tell you if the waveguide is too small (if you attempt to operate a waveguide below its cutoff frequency, it will have unacceptable SWR and losses).
| Usable Frequency (Ghz) | Typical Waveguide | Long Dimension "a", inches | Short Dimension "b", inches | 1.12 - 1.7 | WR650 | 6.5 | 3.25 | 1.7 - 2.6 | WR430 | 4.3 | 2.15 | 2.2 - 3.3 | WR340 | 3.4 | 1.7 | 2.6 - 3.95 | WR284 | 2.84 | 1.34 | 3.3 - 4.9 | WR229 | 2.29 | 1.145 | 3.95 - 5.85 | WR187 | 1.872 | .0872 | 4.9 - 7.05 | WR159 | 1.59 | .0759 | 7 - 11 | WR102 | 1.02 | 0.51 | 8.2 - 12.4 | WR90 | 0.9 | 0.4 | 10 - 15 | WR75 | 0.75 | 0.375 | 18 - 26.5 | WR42 | 0.42 | 0.170 |
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Once the waveguide has been selected, just enter its cross-sectional dimensions and the desired operating frequency into the Mathcad program file slot_ant.mcd or slot_a2.mcd. In order to edit the file and make the calculations, you must have a copy of Mathcad, running under Microsoft Windows.
The program file references the slot antenna dimensions shown in Figure
1. Note that all calculated dimensions are given in both inches and millimeters.
For the best pattern, a pair of ground planes (wings) can be added to either
side of the antenna. The wings should be flush with the top of the waveguide
and extend at least a half-wavelength below the bottom slot. The wings need
to protrude out from the sides by several wavelengths, if possible.
Drill Press - Just drill many overlapping holes, then file out the slots to the desired dimensions.
Milling Machine - The ideal method: fast and accurate, and less prone to an accident of drilling or filing.
Note that the front -face and rear-face slots are exactly across the waveguide
from each other. You should be able to "see through" any front/back pair
of slots if the antenna is constructed correctly.
The simpliest feed for the slot antenna is to use a coaxial-to-waveguide transition. These transitions are also available (sometimes) on the surplus market, having BNC or Type N connectors.
There are two versions of the Mathcad slot antenna program. It was discovered
that the original program running on Mathcad 3.1 had some problems when
users tried to run it on later versions, namely version 6.0, etc. For users
of the older Mathcad, use slot_ant.mcd, otherwise use slot_a2.mcd. Many thanks to Steve Muther, WF6R, for getting the program running in
Mathcad 6.0.
Some worked examples
About Mathsoft's Mathcad
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