Cantenna Musings
Thinking aloud about antenna design and construction. All of this information comes from sites on the Internet and shouldn't be trusted.
A ultra cheap antenna designed based on [http://www.turnpoint.net/wireless/cantennahowto.html Nalleys Beef Stew Cans] got me to thinking about ways to do something similar. In particular that site has a calculator to help you determine the right can diameter for Wifi antennas. This design is a waveguide antenna. You can find some [http://www.saunalahti.fi/~elepal/antenna2.html waveguide antenna theory here].
Of interest is the calculator on the Nalleys cantenna page. Input your can diameter and it displays the frequency ranges captured (filtered?) and spacing for the connector feeds and such. Plugging the formulas into a spreadsheet showed that useful can diameters are 2 7/8" to 3 2/3" (or 73mm to 93mm). A table of sample sizes shows:
Diam - MM |
73 |
76 |
79 |
83 |
86 |
88 |
92 |
93 |
Fractional Inches |
2 7/8 |
3 |
3 1/8 |
3 1/4 |
3 3/8 |
3 1/2 |
3 5/8 |
3 2/3 |
Decimal Inches |
2.875 |
3 |
3.125 |
3.25 |
3.375 |
3.475 |
3.625 |
3.66 |
T11 cut off |
2406.00 |
2305.75 |
2213.52 |
2128.39 |
2049.56 |
1990.58 |
1908.21 |
1889.96 |
T01 cut off |
3142.56 |
3011.62 |
2891.15 |
2779.95 |
2676.99 |
2599.96 |
2492.37 |
2468.54 |
Guide WaveLen |
30.46 |
14.96 |
11.58 |
9.94 |
8.95 |
8.40 |
7.79 |
7.67 |
1/4 Guide |
7.62 |
3.74 |
2.89 |
2.49 |
2.24 |
2.10 |
1.95 |
1.92 |
3/4 Guide |
22.85 |
11.22 |
8.68 |
7.46 |
6.71 |
6.30 |
5.84 |
5.75 |
The numbers are:
- The first three rows are can diameter expressed in millimeters, fraction and decimal inches
- Low and high end of the frequency filter (?) range, these should be outside of the 802.11b range of 2416 to 2462.
- Guide wave length
- 1/4 wave - distance from base to N connector center
- 3/4 wave - minimum length of overall can
From the table you can see that the fatter the can the shorter its length should be. See [http://www.saunalahti.fi/~elepal/antenna2.html the theory page] for why.
Current questions I have:
- Does it need to be a metal can? Can I line a cardboard tube with aluminum foil?
- What happens if you put a Yagi-style rod in front of a waveguide can?
- What are the downsides to Waveguide antennas vs. Yagi vs. dipole vs dish?
Why didn't I get an N connector from MichaelCodanti?
What this space for more thoughts as I dig deeper into this issue and people who know better read the page and modify it with more/better information. --MichaelRasmussen