ask for the information of diodes

1) it should be chip d.
2) if the power is 23dBm and higher u need shunt to gnd

give more info , any way there are alot of diode with cp<1 pf

best performances with "beam lead" diodes

Hi.

What about schottky diodes?

Thanks.

schottky diodes mostly use in power supply,but it can not up to 20GHz,

Not really, Schotty diode can be used up to 20 GHz.

Consider Tunnel or varactor diode...

i want to buy some diode cores

used for preselect switch in front_end of receiver

Most types of diodes may be used as switch, Schottky, PIN etc.
But the switching performancese are more related to the artwork rather than the type of semiconductor.
Beam lead diodes mounted on "hard", thin film substrate ustrip, placed on a "star" artwork and shielded by a deep, milled metal jig, give the best performances on frequency domain and reach the highest frequency. Under this mounting conditions you may get an hi end compromise between switching time and frequency range.

In any case, to get the highest isolation and the lower loss, mechanical switch is the best.

FET switch is the fastest.