Some antennas can also be BPF filters.
It may have multiple resonsant bands though. Otherwise add SAW filter.
The sensitivity of a circuit like this is not very good. Certainly, nothing from the schematic makes the 3GHz reception look intentional. The opamp is setup as a transimpedance amplifier, which makes it suspectible to interference with Rf=6.8M, but at the same time AC input current is shunted to GND via the capacitor. However, because its such a shitty capacitor at RF (aluminium cap), you can probably look at it as a fairly high-value inductor for 3GHz..
It fixes the DC point, but how it behaves at 3GHz is a bit of an unknown.
As I eluded in my previous (emotional
) response.. I don't think these circuits are trustworthy at all. Sure they may light up if you hold them next to some device. But is that WiFi? GSM traffic? Proprietary 434/868MHz? Or even just 50Hz mains? From your neighbours? Who knows!
The poor sensitivity is perhaps advantegous for the 'neighbours' part. But other than that its anyone's guess what this circuit is receiving.
By the way, shielding does not work for self-resonance. This circuit purely operates in interference principles, and the antenna is already the most sensitive part (its directly on the net of the transimpedance amplifier). So I wouldn't expect it to be a gamechanger.