A typical metal coffee can with diameter
approximately 10 cm has the dominant TE11 circular
waveguide mode cutoff wavelength [3] of approximately 17
cm corresponding to a cutoff frequency of 1.8 GHz, which
allows good performance for the laptop radar operation at
2.4 GHz. At 2.4 GHz, the free space wavelength is 12.5
cm, and to excite the TE11 mode a one-quarter wavelength
monopole thin wire probe with length 3.125 cm (as measured
from the tip of the probe to the inside metal surface of the
coffee can) is used installed within the coffee can by extending the center pin
of a SMA bulkhead receptacle jack. At 2.4 GHz, the coffee
can has a guide wavelength of 18.5 cm and, to provide a
good impedance match, the monopole wire probe is spaced
one quarter of the guide wavelength (4.6 cm) from the back
wall of the waveguide [4]. A plastic cover, which is typically
used to seal the coffee can after it is opened, also serves as a
microwave transparent radome material. Another example of
the use of metal cans for fabricating low cost, simple antennas
has been described [5].