Cigar antenna

The construction of this antenna is very similar to yagi array. Yagi is a type of endfire multielement array invented in 1920s by two Japanese university professors Hidetsugu Yagi and Shinoto Uda. Yagi array consists of a single driven element, and parasitic elements - reflector and directors.
Yagi type antennas are popular at lower frequencies. It is possible to build a yagi antenna for 2.4 GHz band, though some difficulties are involved. The main problem is to construct a dipole element and balun with close mechanical tolerances.

Figure 1 - Cigar antenna parts

An elegant solution which overcomes this problem is the cigar antenna. Cigar antenna is a cavity-fed yagi in which dipole and reflector elements are replaced by a probe-excited cavity which launches a wave along the slow-wave structure formed by the director array. Directors are metallic discs housed on threaded rod.

New version (updated when time permits)

First task is to construct a launcher. It is a probe-excited circular cavity. The cavity itself will have 50 Ohm feed-point resistance so it can be used as antenna without director array, for example as a feed for parabolic dish with low f/D ratio.

Cavity dimensions: inner diameter 88 mm, length 91 mm
Probe dimensions: diameter 3 mm, length 26.5 mm
Probe center to cavity closed end distance 44 mm

I simulated this antenna with method-of-moments based simulator IE3D. The simulation with IE3D was way faster and more accurate than with FDTD based simulator Fidelity.

IE3D simulation results:

bandwidth = 2.3 .. 2.9 GHz (dB[S(1,1)] < -15, SWR < 1.5)
gain @ 2.442 GHz = 7.5 dBi
3 dB beamwidth = 85 deg E-plane, 76 deg H-plane

Next task is to construct director element array to increase antenna gain. Distance between cavity open end and first parasitic element should be chosen so that antenna feed-point resistance is 50 Ohm @ 2.442 GHz frequency which is center frequency for IEEE 802.11b.

Old version (for reference)

I simulated this antenna with Zeland software's Fidelity package.
Waveguide dimensions - 84 mm inner diameter, 100 mm long. Coffee cans with such dimensions are common. Probe to back distance - 42 mm. Probe length - 26 mm. Probe diameter - 3 mm. Spacing between waveguide and disc array - 60 mm. Spacing between first and second disc - 20 mm. Spacing between rest of the discs - 35 mm. Diameter of director discs - 36 mm.
Described antenna with 16 director discs has 15dBi gain and SWR < 1.1 over IEEE 802.11b frequency range.

I'm trying to find best spacing and best disc diameter from available. As you can see from figure 2, elevation pattern has significant sidelobes.

Figure 2 - Elevation pattern (dBi)

Figure 3 - Azimuth pattern (dBi)

Figure 4 - SWR graph (50ohm)

Near field animation: [animated gif]. Antenna elements are not displayed, but you'll understand where they are located from field distribution.