Antennas are sensitive devices. If you place a piece of metal near or put your hands around an antenna, you can see its s11 change rapidly, and its s11 below -10 dB band disappears. I've played with VNA and antennas and couldn't find a way to save antennas from hand-blocking.
How can mobile devices like smartphones and smartwatches not lose signal?
I know they probably have a diversity antenna, but it's quite easy to make all their antennas lose s11 <-10 dB band. I can just place my smartphone on a metal plate, and use both my hands to cover it. However, it can still make phone calls!
I have some guesses:
Textbooks did tell you that as an antenna designer, you should try to make s11<-10 dB. Textbooks didn't tell you antennas can also work at s11=-1 dB. It's just an efficiency loss. At s11=-3 dB, efficiency is 50%, at s11=-1 dB, efficiency is about 20%. An antenna does not lose signal immediately as s11 goes above -10 dB, you just lose some maximum communication distance.
Mobile device engineers can add some resistors on antennas to make those antennas not lose s11<-10 dB condition in most scenarios in daily use.
Today's mobile device baseband chips are very powerful. They have the function of VNA. In daily use, the chip frequently tests the antenna and calculates an optimal path to rematch. Then use its variable matching network to match the antenna to make the communication go again.
Which one is true about modern mobile devices?
