On Mon, 08 Dec 2008 08:37:34 +1100, Clifford Heath
wrote:

Bill wrote:
looking at the photos it appears to be being used
with a DVM, depending on how you want to use it you may find an analogue
meter easier. It will show changes better than a DVM.

The author says he tried that and it didn't work as well:
"I tried connecting the antenna directly to a micro amp moving
coil meter, however there was very little meter deflection from
a Wireless LAN card."

Impedance too low, perhaps?

Clifford Heath.

Maybe, but more likely, not enough RF to do anything useful with an
un-amplified meter.

What's missing from everyone's posting is what they plan to do with
this field strength meter.

Let's play with the numbers. The average wi-fi access point belches
about +12dBm (32 mw) RF into an antenna with perhaps 2dBi gain. The
FSM has a gain of perhaps 0dB. At a distance of about 3 meters (10ft)
the path loss at 2.4GHz is about 50dB which will deliver about -30dBm
to the diode. On the most sensitive voltage range, the DVM has an
input impedance of at least 1Mohm and can be treated like an open
circuit load to the detector.

-30dBm into 50 ohms is 0.224 vrms. Transforming linearly from 50 ohm
reference to perhaps 150 ohms loop antenna impedance yields about
600mv rms. (Note: I know this is a bad guess but it's midnight and
I'm tired). Resonating the loop probably raises the impedance even
higher, thus resulting in even more rectified voltage.

The resultant DC voltage will be about the same at about 600mv DC.
However, for wi-fi use, the xmit duty cycle is quite low, sending just
beacons, when there is no traffic moving. The 1000pf cap will need to
be paralleled with a much larger cap in order to form a usable peak
detector.

So, with a high imput impedance DVM the antenna and detector is quite
efficient and can generate quite a bit of voltage. However, when the
DVM is replaced with a non-amplified meter, the relatively low
impedance of the meter shorts out the diode and signifigantly reduces
the Q of the resonant loop antenna, resulting in much lower detected
voltage.

I've built several transmitter hunt "sniffers" using almost exactly
the same circuit, except that I use a hot carrier Schottky diode for
the RF detector, and the cheapest Harbor Freight DVM (because it
always gets destroyed or lost during the hunt). For 2.4GHz, I use a
dish antenna, RF amplifier, step-up xformer (coax balun), diode
detector, and DVM.

I also made a really ugly hack that uses a satellite TV signal meter.
http://www.sadoun.com/Sat/Products/Accessories/Meters/SF95L-DSS-FTA-Satellite-Signal-meter.htm
These work from about 900 to 1800MHz with a minimum detectable meter
sensitivity of about -70dBm. They are comatose at 860 (cellular) and
2.4GHz but are great for 900 and 1900MHz (PCS) sniffing. The input
port is connected to a 900MHz or 1.2GHz antenna of any sorts. The
output goes to a 75 ohm load and a 12V battery (gel cell) through an
RF choke.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558