Cell phone signal detector part 1

I’ve been building circuits to detect a cell phone signal.
I use Verizon service for my smartphone that runs on the 3G network. 3G by the way, stands for 3rd generation, 1st gen being analog, and the 2nd being PCS.
Verizon uses the 800 and 1900MHz frequency band for the 3G network.

The basic concept of a cell phone signal detection circuit is to design your antenna to “catch” the right frequency. The signal is then amplified through a standard opAmp like the LM324. On the output of the opAmp, I use a Radio Shack variety BJT transistor in these circuits to amplify the voltage enough to light an LED.

It is challenging to detect the signal strength without interference from noise. I have a Droid One and it is receiving signal in the -80dBm range, which corresponds to 10 pW. On my phone, I go to Settings, then Status – the received signal strength in dBm is located there. There’s unfortunately no place to read strength of transmitted signal.
Link to dBm to Watts chart.

This image is from Wikipedia’s Dipole antenna page


Sounds simple, right?
Except that the cell phone doesn’t always give off a signal. Mine (and presumably others in the Verizon 3G network) gives an easily detectable signal when I first make a call or am about to receive a call. When the 3G icon on my phone flickers, so does my LED.
But unfortunately, when I’m talking on the phone or texting or using the internet, no easily detectable signal.

Link to the second post in this series
Link to the third post in this series