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Use a laptop as a mobile wireless power meter Network
This hint is for people trying to optimise their wireless signal reception in strange places. I recently had to get a wireless network operating over 100 meters away from the base station, and in a metal shed -- not the best environment for microwave communication!

This hint basically does the same job as AP Grapher, but can be run from the command line or over SSH -- so I was able to have someone else carry my MacBook Pro around the shed while I stayed near the wireless router and fiddled with its antennas.

I ssh'd into my MacBook Pro from a computer at the router, and ran the following command in Terminal:
while x=1; do /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | grep CtlRSSI; sleep 0.5; done
This periodically displays the received signal strength indicator (RSSI), basically the power of the microwave radio signal from your router.

The value is in dBm, which is a logarithmic scale so that an increase of 10 units means a factor of 10 increase in power; an increase of 20 means a factor of 100 increase in power; etc. Watching this reading, I was able to position the router and antenna for maximum signal strength at various places in the shed.
The airport -I command alone displays other useful information about your airport connection, like the network name, channel, authorization mode, and the noise on the microwave signal.
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Use a laptop as a mobile wireless power meter
Authored by: googoo on Apr 08, '10 09:47:04AM

This is very useful. Thanks!

-Mark



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Use a laptop as a mobile wireless power meter
Authored by: dankna on Apr 08, '10 01:00:12PM

They're not microwaves. They're radio waves. Microwaves would cook you!

But very useful tip, thanks!



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Use a laptop as a mobile wireless power meter
Authored by: ALT147 on Apr 08, '10 05:56:31PM
Actually, they are microwaves, and, interestingly, they are at almost the same frequency (2.45 GHz) as those in your microwave oven. I'm not sure why that band was chosen.

But, in any case, the power present in your oven is in the hundreds of watts to kilowatts, whereas the power level sitting a metre away from a strong wireless router is less than -20 dBm, which is 0.00001 W. The power from the router I'm connected to now is -84 dBm, which is less than 0.00000000001 W. So we're probably OK.

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Use a laptop as a mobile wireless power meter
Authored by: barko192 on Apr 08, '10 07:43:30PM

Just because EMR is in the microwave band, doesn't mean it will warm food. The 2.4 GHz and 5 GHz frequencies that wireless routers operate on are actually in the microwave band of 300 MHz to 300 GHz, the microwave band is a subset of the radio band of 3 kHz to 300 GHz.

Only microwaves of a specific frequency warm food, because the energy per microwave photon corresponds to a ro-vibronic energy transition in the water molecule.

For more info check out: http://en.wikipedia.org/wiki/Microwave_oven



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Use a laptop as a mobile wireless power meter
Authored by: ALT147 on Apr 11, '10 03:51:26PM
Only microwaves of a specific frequency warm food, because the energy per microwave photon corresponds to a ro-vibronic energy transition in the water molecule.
In fact, microwaves of anywhere from 1 GHz to a few hundred GHz will warm food. See this page for far too much detail. I just thought it odd that the frequency of a microwave oven coincided with channels 7, 8 and 9 of the 802.11 standard. Even though the power levels may be way too low to heat anything (or anyone), surely you'd choose a band far away from any commonplace, powerful source like a microwave oven for the sake of interference?

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Interesting! Thanks
Authored by: tice on Apr 08, '10 02:08:46PM

Only: How do you stop that task? q doesn't work in that case. Sure quit does it, but what is the correct way?



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Interesting! Thanks
Authored by: zhell on Apr 08, '10 03:12:20PM

Press Ctrl-C

This shortcut in general quits the running application in Terminal and gives you back the shell prompt. To accomplish this, it sends a signal called SIGINT, for "interrupt", to the application.

Programs may choose to ignore some signals though. In such a case they can still be terminated by other signals:
1. possibly SIGTERM ("terminate") is sufficient; otherwise
2. SIGKILL always works.

Sending arbitrary signals to a program is done with the "kill" command. See "man kill".

Note that sending a SIGTERM or SIGKILL to the wrong process, e.g. your Pages application, will irrevocably terminate it immediately without asking if you want to save your work.



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Use a laptop as a mobile wireless power meter
Authored by: joteff on Apr 10, '10 12:28:17AM

Just out of curiosity: what router did you use to bridge100 metres? And what is considered to be a sufficient signal strength?



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Use a laptop as a mobile wireless power meter
Authored by: ALT147 on Apr 11, '10 04:04:51PM
I was using a Belkin F5D8236-4 N wireless router, but not by choice, and I wouldn't recommend it. You can't even set an administrator password on it.

The question of signal strength is interesting. I'd advise you to check it out for yourself using the airport -I command in various places (note that the reported RSSI doesn't respond instantly to a change in signal strength, and the airport signal icon in the menubar is even more delayed). The signal I got at the other end of the shed was about -60 to -70 dBm, which is a signal-to-noise ratio of 20-30 dB, or 100 to 1000.

The signal on the network I'm using right now is at -83 dBm, a signal-to-noise ratio of only 5 dB = 3 (there's about -88 dBm of noise), but I still have full reception according to the airport icon in the menubar. If I go further away from the router, the SNR drops to 0 dB (as much signal as noise), but the airport icon still reports 2 bars.

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Use a laptop as a mobile wireless power meter
Authored by: hgregorian on Apr 10, '10 07:55:42AM
Great hint, thank you!

I modified the command a bit to simply show the current RSSI dBm value as opposed to a running list:
clear; while x=1; do /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | grep CtlRSSI | sed -e 's/^.*://g' | xargs -I SIGNAL printf "\rRSSI dBm: SIGNAL"; sleep 0.5; done


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Use a laptop as a mobile wireless power meter
Authored by: pachy on Apr 12, '10 05:58:25AM

The optimum length of an antenna is a multiple of the wavelength (range) that it is receiving. That's why your AM radio's antenna isn't 3 miles long.



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