What Use Is An F-call?

What use is an F-call? In a previous discussion I talked about decibels. The take home from that was that a decibel represents a ratio between two things. The gain of an antenna over the gain of a standard reference antenna, or the power loss between the start of a coax cable and it's end. I also mentioned that there are several other things with dB in them. Today I'd like to introduce the dBm, or Decibel milliwatt. It's a unit used to compare and contrast different levels of output. Unlike the Decibel, which is a ratio, the dBm is an absolute unit. It is referenced to a Watt. In audio and telephony, it's relative to a 600 ohm impedance, but in our RF patch, it's relative to a 50 ohm impedance. So, how do you use it, what does it mean and why is it useful? Let's look at some large and small numbers. If you look at an FM broadcast radio station, it typically uses 100 kilowatt, a 1 with 5 zeros. If you look at the received signal power of a GPS satellite, you might get 0.2 femtowatt, or 0.000 and 12 more 0