antenna

The internal WiFi module on the Compute Module 4 (that's the bit under the metal shield in the picture above) routes its antenna signal via software. You can route the signal to either:

1. The built-in PCB triangle antenna (this is the default).
2. The external U.FL connector (which has an external antenna plugged into it in the picture above)

To switch the signal to the U.FL connector (for example, if you're installing your CM4 in a metal box where the PCB antenna would be useless), you need to edit the boot config file (sudo nano /boot/firmware/config.txt, and add the following at the bottom:

# Switch to external antenna.
dtparam=ant2

Then reboot the Pi.

[Required reading: Apple iPhone 4 Antennas... (by AntennaSys, Inc.)]

A lot of people have been mentioning how horribly terrible the iPhone 4's 3G antenna seems to be, due to an issue that has affected somewhere around half of iPhone 4 users (according to this MacRumors poll) so far.

Wanting to see this issue for myself, and knowing an iPhone 4 is on its way already, but will be delayed another week, I went to my local Apple Store, and tested five different iPhone 4's. All five exhibited the exact signal loss problem: If you grip the iPhone rather tightly with your wrist pressing against the lower-left corner of the iPhone, the 3G bars gradually diminish to zero, and the signal is lost.