In a recent antenna study I was not very happy with the results of putting
an attenuator in the feed from my 100 mW router to the antenna, and checking
the signal 40 ft away with the Wi-Spy. The main problem was the difference
in weighting above -47 dB versus below -47 dB. I came to the conclusion that
simply telling the router to use "external antenna" was not enough to
disregard whatever incidental radiation remained from the router box.
Leakage through the diversity switch, and from the circuit board itself that
took a short-cut around the attenuator kinda screwed up the readings . Also,
having to adjust the attenuator at the source and read a reception level 40
feet away resulted in a lot of walking!
Will putting an antenna connector on the Wi-Spy be better? It will help.
What is really needed is to shield the dongle so that without an antenna,
the response in the presence of a normally saturating signal is
indistinguishable from the noise level. Then bring an antenna connector out
through the shield, and play all you want with an attenuator and a connected
receiving antenna. So I dug a nice raw aluminum box out of my collection and
put in feedthru capacitors for the +D, -D and Vbus lines so that nothing can
enter the box through the USB wires. With 100pf for the data feedthru
capacitors and 3000pf for the Vbus feedthru, just testing that showed the
Wi-Spy was completely shielded, showing only the small pips that
sporadically peaked to the -60dB line. Adding a Type N connector at the
other end of the box resulted in some retained peaks, but terminating it in
an N Terminator eliminated that small leakage. Then I snipped off a thin
piece of single-sided copperclad about 0.6mm thick, 2.5mm wide and 61mm long
and made a 2.4 GHz dipole by filing a gap in the center of the copper strip.
I hooked that dipole up to the N connector with a short piece of coax and
taped it down the middle of the Wi-Spy to couple into its antenna. How good
is this coupling? - - my best estimate is -13 dB (by attenuators) compared
to the Wi-Spy itself (or -8 dB on the Wi-Spy display). The good news is you
can take the Wi-Spy out of the box and use it as always when you get tired
of comparing receiving antennas or whatever experiments you have in mind.
One thing that may be important - - the quality of the shielding is related
to the quality of the box. The one I used is from Context Engineering Inc.
http://www.consite.com/context/cont_circuit.htm and is probably the 2506
size split body type with extrusions 4.3" long and plain end caps (middle
type in the exploded depiction). Man, I hope somebody tells them how crappy
their website is - - no drawing to relate the dimensions to, and that
idiotic roll-up/down button stuff - - exasperating! But the box has no holes
and goes together real tight to make a superb shield. Somebody said Fry's
carries them, so I guess that's where I got it, but I can't remember what I
paid - - it can't have been too much - - I'm real stingy when I'm looking at
electronic enclosure boxes.
When Metageek comes out with the external antenna connector, you can pop it
into this enclosure and eliminate the -13 dB coupling loss while enjoying
its excellent shielding. It then becomes a truly useful scientific
instrument for a very low price. Maybe Metageek will then calibrate the
display to agree with the performance of the circuitry.
Good luck,
Chuck
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