Chris, Assuming that you've got a suspended ceiling with steel grid, just get a mag mount antenna and stick it upside-down to the grid. Attach the feedline to the grid for a couple of feet so it's perpendicular to the radiator for 1/4 wavelength or so. And turn down the TX power. That close to the repeater, you could even do without the PA, I suspect. So if your radio shop has a mobile with a blown PA you might be able to give it a home.
I used to operate indoors at home by sticking a mag mount to the top of the fridge; a metal file cabinet would serve equally well as a groundplane. But you'd have to be careful not to transmit when anyone was near the file cabinet. You could also make a ground plane with an SO239 female connector and five 1/4 wavelength scraps of AWG14 solid copper house wire (one for the radiator and four as radials). Put a loop in the top of the radiator and suspend the antenna from the ceiling grid (or a hook) with monofilament line. Take a piece of coaxial cable, strip the jacket for something over 1/4 wavelength, spread the shield braid and fold it back over itself, and over the jacket. You've got a coaxial dipole. You need to strip more than 1/4 wavelength because the braid shortens as you spread it. Trim the braid and the exposed center conductor to 1/4 wavelength each, apply tape or shrink tubing, and hang as above. The end of any antenna will be a high impedance (hence high voltage) point, so use several inches of monofilament to suspend it. If your office has a window, you may have enough room to make a J-pole out of burglar-alarm or copper foil attached to the glass. Just some ideas. Bob N3HAT
