Wi-Spy useful for antenna study
 

Termed by some as the most inexpensive 2.4GHz spectrum analyzer you can buy,
the Metageek Wi-Spy USB dongle http://www.metageek.net/index.php has a lot
going for it. I recently attempted to use it to judge the performance of
various Wi-Fi antennas, with the intent to improve the connection to my
local community wireless network access points.

For a source, I used an old D-Link 802.11b Router that I found at a flea
market, set to beacon at 3 millisecond intervals with an identification
string and a long error correction preamble to provide strong spectrum
peaks. At first I located the test antenna and an in-line variable
attenuator at the Router output, but leakage through its diversity switch
and incidental radiation from its unshielded circuitry took a short cut
around the attenuator and produced misleading results.

I then decided a better way to do this testing was to shield the Wi-Spy
device in a small box and couple the test antenna to it through my in-line
attenuator. I filtered the USB leads with feed-thru capacitors, using 100pf
for the +D and -D and 3000pf for the Vbus lines, fastened a dipole antenna
to the Wi-Spy case and connected it by a short coax lead to a Type N
connector at the other end of the box. I feel the quality of the shielding
is related to the quality of the box, and I found a very nice one from
Context Engineering Inc. http://www.consite.com/context/cont_circuit.htm
It's the 2506 size split body type with extrusions 4.3" long, plain end caps
(middle type in the exploded depiction), and is available at Fry's, in the
S. F. Bay Area for $10.99 + tax.

Comparing shielded Wi-Spy display levels with a 1/4-wave whip against the
USB device directly receiving the test signal showed about an 8 dB coupling
loss on the display. But most important is the effectiveness of the shield
so that without an antenna, the response in the presence of this normally
saturating signal is now indistinguishable from the noise level. The Wi-Spy
manufacturer promises to have an improved model with an external antenna
connector available this summer, and that will eliminate the coupling loss,
but for now, the ability to employ the Wi-Spy either open or shielded
answers a variety of needs.

Chuck, W6PKP