[HomeBrew] Variable capacitor

[hwhall at compuserve.com]

 Rather than agonizing about losses in a rotary joint, just bypass it old-school style. Connect a short, flexible pigtail between  the rotor plate(s) and the frame that carries the rotor plate(s). A half or one turn loop of pigtail will allow easy rotation (unless you were planning on spinning the rotors around and around!). Unless the antenna is intended for VHF or UHF, a short pigtail will be virtually invisible as an inductance, or even a resistance. 



A butterfly style  or split stator cap seems inefficient at yielding capacitance if you have to build one, because it is essentially two caps in series, so you get less bang for your construction effort. 

Another idea that might be worth thinking about: if the CD disks don't work out (too little metal or too difficult to get to the metal layer) take a look at some old 5 & 1/4 inch hard disk drive platters. Dead drives ought to be cheap and the plates are very smooth. I believe you can disassemble the platter stacks non-destructively.

Wayne
WB4OGM