Capacitor for Rectenna

Hi,
I just got done designing my patch microstrip antenna which resonates at 2.4GHz. My goal is to make a rectenna but I am not sure what size my capacitor should be. Is there a formula or rule of thumb?

Thanks

I assume you mean the capacitor in the bias line? The capacitor just needs to keep RF from floating around on the DC lines, and reflect back an open circuit. Anything bigger than probably 220 pF would do. You will need a bigger capacitor SOMEWHERE in the system to make a low ESR supply, but it does not have to be right at the antenna elements.

Thanks..
How did u arrive to that number?
Also I'm going to Digikey.com to see what kind of capacitor I should order but there are many types such as ceramic, film, thin film. which one do u think is best for those kind of frequencies...

Simple. You want a RF capacitor to look pretty much like a dead short circuit between its two terminals. The impedance of a 220 pf capacitor would be:

Z = |1/2Π(2.4X10^9)(220X10^-12)| = 0.3 ohms

Looks like a short circuit to me.

Somewhere further away from the microwaves, I would liberally sprinkle 10,000 uF or larger caps, maybe even some of those super capacitors, to handle transient loads.

Rich