radiates efficiently when it has an outer circumference of at least one wavelength this means it needs a maximum diameter of about one third wavelength the upper frequency limit for efficient spiral radiation is set by the size of the feed point attachments and the lower frequency limit is set by the outer diameter of the spiral structure within these limits the spiral radiates efficiently in a frequency independent manner the input impedance and the radiation patterns will vary little over this frequency range with any spiral structure an upper frequency of 10 gigahertz presents no problem the spiral antenna is constructed by etching the pattern on a printed circuit board a planar printed circuit spiral antenna radiates perpendicularly to the plane of the spiral the spiral itself is located at the end of a cylindrical metal cavity the cavity back to provide isolation from neighboring elements and electronics typically an absorber is used on the top side of the spiral inside the cavity to make sure the element responds only downward separate transmit and receive antennas are used to simplify the electronics provide spatial separation and reduce very shallow reflections suitable processing hardware includes a powerpc 750 processor running at 350 mhz on a pmc mezzanine form factor the powerpc 750 is ideally suited for portable applications due to the small die size and low power consumption of less than 4 watts and its low cost the described systems and techniques promise to provide low cost ease of use and higher performance better detection and lower false alarm rate a low cost.