See my embedded answers with ***
My only question is when you start turning the all-thread to move the coil up and down, what prevents the coil from spinning? Does the tightness of fit with the fingerstock overcome any torsion that is put on the coil from the motor or is there some kind of a slot and pin that keeps it lined up? I didn't see a slot in the plans so I am assuming that it is the first thing. It also looks like I can get just about everything that I need except for the finger stock from home depot.
*** I have the actual W6AAQ SCREWDRIVER antenna. The base tube is made of heavy gauge aluminum irrigation pipe 2" diameter. Since I never have actually tore mine down not sure if the fingerstock is the device which keeps the coilform from turning but don't see anything on the outer face of hte coil form to keep it from rotating but it doesn't. The screw shaft inside has a clutch mechanism that clicks when you hit the end of travel.
--Can the thing be used with 2 meter?
*** Not sure - never tried but it will do 6M ok but not the best 6M antenna.
--Will I be able to tune the thing with a MFJ-259?
*** Yes, I did that to test mine but if you want to tune it all the time with a 259 you will need to rig up a coax switch to go between the antenna and rig/259.
--What resonates? Is it just the whip or is it the whip and the coil, or the whole thing;whip lower tube and coil?
*** The length of the lower base tube (2" dia), the coil and the whip. The coil that is inside the base tube is essentially not there as far as RF is concerned. Simply put it is a shortened center loaded whip antenna.
--If I mount it on my steel mirror bracket, will I need to insulate it or ground the outside tube?
*** I highly doubt you will have a mirror left very long. The stress on the mirror from the windload on the antenna of this length (especially 20M and lower) will rip it off unless it is one MONDO mirror. As stated by W6AAQ, the base of the antenna is at ground PROVIDING you setup the lower matching coil section correctly. The lower end of base tube is actually turned on a lathe on my model and matching coil is part of the base tube assembly. My tube assembly is at ground with center of coax attached above the matching coil section. If you use the older method of using a toroid for matching, the base will need to be insulated from ground and the feed point will be at the lowest point of the base tube.
--Would anyone have a copy of QST October 1982 and can e-mail me the plans for the tuning unit therein?
*** If you check my website (click on my call for my profile) I have a schematic of the small control box I use. It is based on W6AAQ's design with a little expansion. Not much you can expand on for 12V and a switch
*** My DK3 is a great antenna and works like gangbusters. Usually if I can hear'em, I can work'em. I may not be the loudest signal on the band but I do get very good reports and several times, the other stations are surprised to hear I am mobile. I use the Yaesu FT-100D with it and set the tuning manually using the SWR bar meter on the FT-100D.
Hope this info isn't too late. I just came across this forum posting today.