Wi-Spy with an external antenna connector

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.

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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,


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Chuck Olson
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