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I was wondering if I can measure my phone's transmitted power with an SDR by tunning in the transmission bandwidth but then, I'm not sure how to know whether this power is coming out of my phone or it's the base station received power at my phone?

Also, I tried using this app but again, I'm not sure if the dBm correspond with the Rx or Tx power. I contacted the developpers but they didn't answer. Maybe anyone knows another app or a way to do this?

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    \$\begingroup\$ You're not meaningfully going to be able to do this with random consumer gear, especially as you do not have a way to make a wired connection to the phone's transmitter or have a calibrated field strength antenna. What you can do is look up the specifications for the designed maximum power for that model. However the network will try to manage the phone to transmit with as little power as possible for success, so often the actual power will be below the maximum. \$\endgroup\$ – Chris Stratton Sep 3 '18 at 10:36
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How can I measure my smartphone transmitted power?

You can't.

At least not in a very meaningful way. I mean, using an SDR receiver you can get an indication of the frequency your phone is using and when it is transmitting but that's about it.

It is in no way an accurate way to measure the real output power of the phone as the SDR's input and phone's antenna output aren't coupled properly (you would need to use matching networks and an RF cable to prevent power from "escaping"). Also the phone will transmit in short "bursts" of power which is difficult to detect for an SDR.

To measure the real output power of a phone you need a stack of equipment worth much more than a couple of hundred thousand $. And then you can only measure if you manage to get the phone in some kind of testmode (like used during manufacturing). If not you will need some equipment to emulate a base station, that's another couple of hundred thousand $.

Anyway, how is the output power important? Your phone needs to fulfill certain requirements to pass qualification tests. That sets the maximum output power and that is what it will be. Usually it's a couple of Watt.

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  • \$\begingroup\$ I also read about the emulation of a base station but I don't understand how the base station could measure the phone's power. Would it be in a data packet or would it be the actual received power at the base station? Thank you!! \$\endgroup\$ – itd Sep 3 '18 at 10:47
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    \$\begingroup\$ The base station does not measure the actual value, it only measures the signal it receives. The base station can only tell the phone to increase or decrease power. So if the reception is bad the base station will tell the phone to increase its power if it can. Likewise if the phone is nearby the base station will tell it to lower the power so that the base station can better receive phones that are further away and only have a weak signal. You can compare it to talking to someone standing next to you and someone far away. \$\endgroup\$ – Bimpelrekkie Sep 3 '18 at 10:55
  • \$\begingroup\$ It is interesting to note that the highest power handling capability of commonly available PIN based RF antenna switches is +33dBm (2W). \$\endgroup\$ – Peter Smith Sep 3 '18 at 13:07
  • \$\begingroup\$ The basestation has an RSSI relative-signal-strength-indicator output, from a logarithmic (multi-stage compression circuit) amplifier. Examine datasheet on SA604 for example. \$\endgroup\$ – analogsystemsrf Sep 3 '18 at 22:07

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