I want to create a switch for an approximately 990MHz signal path. All I have is a piezo sensor that creates a weak voltage pulse of 0.5v every 2 seconds. the length of the pulse is around 1ms. Want to create a switch from that voltage but not sure how to proceed?

 

You can't do much with 500 mv so you might was to put an amplifier between the piezo element and the switch. https://forum.allaboutcircuits.com/...-signals-from-the-piezoelectric-disks.198802/

You will also need to decide on what kind of switch to use. What kind of signal is it that you want to switch?

 

For the amplifier, given that you want a logic level pulse out, and that you need a fair amount of gain, I suggest using a CMOS inverter as a linear amplifier. That is very simple although i I don't recall the feedback resistor value. But that will give you a logic level pulse when you feed it to a second inverter.
For the 990 MHz switch, I suggest using a diode and forward biasing it to swith the UHF signal,depending on the power level.