Aside from range extension, especially valuable,
given the tiny 1mW XBee transmitter power, such antennas may also help reduce
unwanted RF noise picked up from the myriad of devices (WiFi, cordless and
Bluetooth phones, microwave ovens, video senders etc) now flooding that same
2.4GHz spectrum slice.
Rev.Ed’s XBee units come with a small factory-fitted wire
radiator, although this looks shorter than the theoretical quarter-wave whip
(31.5mm) of a 125mm 2.4GHz wave.
Outdoor range checks gave a good 50-100m between bare units but
were hugely influenced by the usual microwave line-of-sight (LOS) limitations.
Even shielding at 100m with your body will completely cut
signals, as will vegetation in full leaf (it’s actually the water in both which
knocks the signal about).
You can make a variety of parabolic (and
near parabolic) reflectors for ZigBee and even WiFi using simple
(and cheap!) materials. More details are on Stan's website:
Indoors, through timber and plaster walls, gave ~20m range.
Remember that brick walls, especially external ones, absorb moisture so are
always likely to be more of a problem.
Wireless data links likely to be subject to such obstacles may
show more "punch" with cheaper (but slower) UHF 433MHz modules (see
SILICON CHIP, December 2005).
Since each 6dB antenna gain doubles range, a simple 6dB antenna
"point to point" at each end should yield 12dB (or four times) the bare range.