902MHz Slot Antenna
I came across an amazing wesbite, talking about all sorts of interesting antenna designs and their theories, antenna-theory.com. One of the more unique designs talked about is the Cavity-Backed Slot Antenna. The site goes through the design of one for 2.55GHz. I decided to adapt the design to 902MHz.
By taking their design and calculated size to wavelength, I got a general feel for how big it would need to be. It happened to end up being just about the size of a 19in rack! The tightest dimension was height, with the monopole active element being 3.27in and a 2U being 3.5in. Because of the construction style I used for my first attempt, I went with a 3U (5.25in) height so I had plenty of room.
Using a standard 19in rack as a starting point, the cavity is 17.75x13x5.25 inches. The slot is 1/2 wavelength long and ~1cm tall. The slot size is the the main size to change and tune to optimize performance.
The slot is spaced 1/4 wavelength in front of the back wall. This allows reflections to add with the wave to increase gain.
For my first attempt I wanted to try it cheaply and quickly. This meant any machining or custom water cutting was out of the picture. Empty rackmount cases are available, but around around $100 - which seemed like too much for a design I wasn't even sure about. I ended up going with foam core and HVAC Foil duct tape. I created the basic shape out of the foam core and hot glue, then covered the inside with the foil tape. I tried to overlap as much as possible to improve performance. I created a flap on the top panel to allow me to adjust the feed point.
Initial testing wasn't great. There was a dip in SWR centered around 845MHz, but it only went down to around 2.3. To me, this indicates that the active element is too long. The spacing from the back wall to the active element is spaced for 902MHz which might cause the bad performance.