A short loaded antenna cannot often be designed by making the total length (radiators + loading coil) equal to 1/4 wavelength. For your antenna (using EZNEC) the coil inductance to resonant the antenna to 7.15 MHz is 18 uH.

Given your 3" diameter form the coil can be made using insulated #18 wire and 13 turns close-spaced. This is based on a wire + insulation diameter or 0.06". Note that the total length of radiators plus wire is 16 ft.

Over perfect GND the feedpoint impedance would be 8 ohms. With coil loss and ground loss I would expect at least 25 ohms. That is for a decent ground that might consist of a dozen 15' radials on the ground. If you can have only one counterpoise wire it should be 32' 8", give or take. A 1:1 balun in the feedline will reduce feedline shield current (current on the outside of the shield). This can reduce the magnitude of the RF current induced into the apartment AC wiring. Without the balun the coax shield and the apartment AC wiring form a counterpoise. The path from the coax shield to the AC wiring is via the power supply DC terminals, through the power supply chassis, to the AC outlet GND terminal.

Here is a link to a coil calculator (I have not verified it):

http://electronbunker.ca/eb/InductanceCalc.html