Power in a mobile phone call.


by amanno
Tags: mobile, phone, power
amanno
amanno is offline
#1
Jul18-13, 11:31 AM
P: 14
Hey guys,

Does anyone know about how much transmission power is outputted from a cell phone call? I have read it's around 100 mW (depending on distance from the tower), anyone ever verified this?

I want to try and use the signal created by a phone call (to detect when a phone call is made), so I have a recieving antenna and I am guessing I would need to amplify the RF signal before continueing correct?

I am assuming that a signal with 100mW of output power would be very weak (uW or nW) by the time it reaches the end of my recieving antenna?

Thanks
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Baluncore
Baluncore is offline
#2
Jul18-13, 09:04 PM
P: 1,268
Yes, a phone will adjust its output power between about 100mW and 1W depending on the range to the cell site.

There are going to be some complications. Your cell phone may exchange data with the local cell site even when you are not making calls. Your cell phone may transmit on up to 4 different bands.

Unless the phone is very close, to monitor the activity you will need to use a mixer to down-convert the bands being used to a baseband, then amplify those signals with a receiver chip that generates the usual logarithmic “received signal strength indicator” signal (RSSI). You can then detect when the RSSI exceeds a specified DC threshold that you have set. That will tell you when a nearby phone is transmitting. If you want to avoid autonomous chirps triggering your detector you will need to use a time gate to wait for a minimum time with a continuous signal detected before triggering the output.

To specify signal levels we need to know the maximum distance between the cell phone of interest and your detector's antenna. What is that range?
amanno
amanno is offline
#3
Jul19-13, 07:57 AM
P: 14
Very interesting idea, thank you.

I wasn't planning on the detectors antenna being too far from the phone itself, a couple of feet maybe (3ft). Unless you think it can go farther?

For my own knowledge: why do you need to down convert the bands (lets assume I am only going to use the 1900 MHz band)?

Baluncore
Baluncore is offline
#4
Jul19-13, 08:56 AM
P: 1,268

Power in a mobile phone call.


If the distance to the phone is significant then many other signals will exceed the phone signal in your detector's antenna. Amplification is obviously then not an option, without some form of pre-selection.

Down conversion is needed to use RSSI.

At three feet I would expect over a milliwatt so you should be able to use a diode detector like in an “RF sniffer” or a “bug detector”. Take a look at the more advanced examples here; http://dx.com/s/bug+detector


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