What the heck is this. Yes, it is some kind of capacitor. That is given by its value in pF's. Part of a mixer circuit. Is it something different? 3 legged capacitor? Capacitive coupling over the wire?

 

Feedthrough cap?

 

They are three-terminal feedthrough capacitors with an input, output, and ground terminal.
They minimize the capacitor's lead inductance, which otherwise would increase the capacitor impedance and thus it's ability to filter the signal, at high frequencies.

 

OK, that is a new one for me. Just how are the physically used in wiring a circuit? One has 4 leads and the other is a stud mount. Mounted into what? Screwed into the grounded chassis?

 

Yup, gotta keep lead length short in the VHF/UHF realm. I don't get above 70cm but somehow I've never seen these before in a wiring diagram. I do have some similar studs in a junk box of military parts a retiree gave me that I never recognized before. Now I know. Thanks!

 

Yup, gotta keep lead length short in the VHF/UHF realm. I don't get above 70cm but somehow I've never seen these before in a wiring diagram. I do have some similar studs in a junk box of military parts a retiree gave me that I never recognized before. Now I know. Thanks!

Really common for bias and power in high frequency gear.

 

Mounted into what? Screwed into the grounded chassis?

The are typically used for decoupling and isolation for the power, ground,. and other low frequency connections to a high-frequency circuit in a sealed metal box.
The capacitors are through-hole mounted in the box, which is connected to circuit common.

 

Part of what made me think of feedthrough caps is having taken apart one or two UHF tuners from TVs as a kid...

 

Your picture indicates the correct metal to panel feed through type .
Murata showing the lower insertion
Front.PM6