Electrical – Why use a PMOS transistor instead of a diode for ESD protection

Your question is a little ambiguous, but I'll take it you're asking how to protect the transistor if you only have access to ground and the low side of the relay coil, not the power supply the other side of the coil is connected to.

Here are two possibilities:

1. Use a zener diode. Connect the cathode to the collector and the anode to ground. This will conduct when the bottom side of the relay reaches the zener voltage. This voltage can be substantially higher than the relay supply voltage. It only needs to be below the maximum C-E voltage the transistor can handle.

   This method has the advantage of turning off the relay faster compared to when using a typical flyback diode as shown in your question. This happens when the zener voltage is substantially higher than the relay coil supply voltage.

   The zener max current must be rated for whatever the relay coil current it.

2. Use a snubber. That's a resistor and capacitor in series between the collector and ground. The relay coil current time the resistor must not exceed the transistor C-E voltage capability. The capacitor has to be sized so that the total charge dumped on it from the relay coil flyback current doesn't cause more voltage than the transistor can handle.

Your internal ESD devices will still take the discharge current themselves instead of letting it through to the sensitive MMIC input node. This assumes that there is some charge built up between the inner conductor and the shield of the coaxial cable. Some kinds of antennas have a naturally low impedance which pretty much precludes this kind of charge buildup.

But, as you say, if there is still a charge relative to earth, then it is not discharged until there is a ground path (like the USB plugged into the computer).