Information below is from the old Brian Yee WEB page

Click on Title for PDF Copy

Slot Antenna Design

Stephen Bell, KB7TRZ

Introduction

This paper was presented at the 40th Annual West Coast VHF/UHF Conference held on May 5-7 (1995?) in Cerritos, CA. Some editing was done to make it suitable for this WEB page.

Abstract

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 omni-directional 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. (Note: A precise milling machine is almost a necessity…de W3RJW)

Getting Ready

The first step in construction of the antenna is to select a section of waveguide, which covers the desired frequency. 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. The program 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)
TABLE 1. Waveguide Dimensions
Usable Frequency
(GHz)
Typical
Waveguide
Long Dimension
"a" Inches
Short Dimension
"b" Inches
1.12-1.7 WR650
6.500
3.250
1.7-2.6 WR430
4.300
2.150
2.2-2.3 WR340
3.400
1.700
2.6-3.95 WR284
2.840
1.340
3.3-4.9 WR229
2.290
1.145
3.95-5.85 WR187
1.870
.0870
4.9-7.05 WR159
1.590
.0759
7-11 WR102
1.020
0.510
8.2-12.4 WR90
0.900
0.400
10-15 WR75
0.750
0.375
18-26.5 WR42
0.420
0.170

Running the Computer Aided Design File

Once the waveguide has been selected, just enter its cross-sectional dimensions and the desired operating frequency into 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 MS Windows.

The program file references the slot antenna dimensions shown in Figure 1. Note that all calculated dimensions (from the MathCad program) 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.

Machining the Slots

Drill Press - "Just" drill many overlapping holes, then file out the slots to the desired dimensions.

Milling Machine – The ideal method: fast and accurate, less prone to mistakes.

Note: 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.

Feeding the Antenna

The simplest feed for the slot antenna is to use a coaxial to waveguide transition. These transitions are often available on the surplus market. (Note: Another method is build the antenna long on the bottom and use a N chassis mount jack and probe antenna to replicate a transition as part of the antenna …de W3RJW)
FIGURE 1. Front View of Waveguide Slot Antenna
MathCad Software

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.

Ham Band Slot Antennas

Here are some worked examples of waveguide slot antennas using the MathCad program by Stephen Bell. These examples are based on a 16-slot design (8 slots per face). All dimensions are in inches.
Band
(MHz)
Wave-Guide
"a"
Dim.
"b"
Dim.
Offset
from
Center
 Slot Length
Slot Vert. Spacing
Slot width
 Head Space
1296 WR650 6.50 3.25 0.49 4.46 6.39 0.64 6.39
2304 WR340 3.40 1.70 0.21 2.56 3.90 0.39 3.90
3456 WR284 2.84 1.34 0.31 1.71 2.57 0.26 2.57
3456 WR229 2.29 1.15 0.15 1.71 2.57 0.26 2.57
5760 WR187 1.87 0.87 0.25 1.02 1.23 0.12 1.23
5760 WR159 1.59 0.76 0.15 1.03 1.34 0.13 1.34
10368 WR102 1.02 0.51 0.14 0.57 0.69 0.07 0.69
10368 WR90 0.90 0.40 0.08 0.57 0.74 0.07 0.74
10368 WR75 0.75 0.38 0.05 0.57 0.88 0.09 0.88
24192 WR42 0.42 0.17 0.05 0.24 0.30 0.03 0.30
Below are several more antennas using the MathCad program. These are 32 slot antennas (16 Slots per Face ... de W3RJW
Band
(MHz)
Wave-
Guide
"a"
Dim.
"b"
Dim.
Offset
from
Center
Slot Length
Slot Vert. Spacing
Slot width
Head Space
5760 WR159 1.59 0.76 0.103 1.025 1.341 0.134 1.341
10368 WR90 0.90 0.40 0.059 0.570 0.736 0.074 0.736
Below is an 8 Slot design for 2304 MHz (4 Slots per Face) ... de W3RJW
Band
(MHz)
Wave-
Guide
"a"
Dim.
"b"
Dim.
Offset
from
Center
Slot Length
Slot Vert. Spacing
Slot width
Head Space
2304 WR340 3.40 1.70 0.296 2.563 3.901 0.390 3.901
* Brian Yee's home page no longer exists as far as I can tell. Likewise, the links to the programs are also gone. Here is a link to a basic program that purports to perform the same calculations as the MathCad program.

http://www.ham-radio.com/sbms/sd/slotant.htm

Here is a Visual Basic program that calculates the Slot Antenna parameters. This program was sent to me by Peter, VK3KCG. Click here to download.

de W3RJW