You say balcony, but I'm unsure if that means no ceiling/roof above. If there is no roof, and the sky is the limit, for an antenna to not require a counterpoise, you need it to be a 1/2 wavelength electrically so your feedpoint is at a voltage loop and your current loop is at the antenna's mid-point.
So for a 20M antenna, you are talking about a 10M (~32' ) tall radiator and of course a 49:1 BalUn to do the impedance transformation from the coax to the antenna feedpoint. Unfortunately, that size antenna, whatever you make it out of, will be noticeable unless you restrict yourself to after dark operation. You can also do a coil loaded 1/2 wave antenna that could shorten the antenna, but you'd have to tell me what parts of any band your interested in, and what your maximum height you think you could get away with, and I'd do some calculations for you; unfortunately, coil-loaded antennas are narrow banded, so you have to pick what part of a band you want, and if you go outside that part, you will need a tuner.
If you're wondering what band to start out on, 20M is a great band for reliable skip. An antenna for 10M would be less noticeable, but activity is hit or miss.
If you can't do a vertical 1/2 wave, you can look at this design for 10M, and 20M if you dare, you can build this balcony antenna out of PVC and coax, and not need a BalUn since the feedpoint impedance is already ~50 Ohms. Basically, everything left of the coax choke hangs outside your balcony or on your deck if it's not enclosed by cement and wire mesh lath, and the coax choke, along with some creativity, keeps the antenna vertical and stable on your balcony (that part of the design is up to you). The design uses 1/2" PVC outside the balcony for weight savings, and 3/4" PVC for strength and counterbalancing, inside the balcony. If you're ambitious, you can use threaded fittings here and there so you can take it apart and keep your better half happy when you're not using it.
The formula (2952/frequency in MHz) for stripping your coax cover in inches and separating your braid from your center conductor, will get you close to your desired frequency. To tune the antenna, you must have the coax choke in place, otherwise your coax will carry common-mode current as it acts as a second counterpoise. You will want to set your radio to its lowest power output, and go through the band you designed the antenna for and find the frequency where your VSWR is lowest; if that frequency where the VSWR is lowest, is above your desired "center" frequency, then trim a couple of inches off the center conductor radiator, and the braid counterpoise, and repeat until the VSWR is lowest at your "center" frequency. If your VSWR is lowest below your desired frequency, woops, trash that formula, and pull out the coax and strip back more of the coax cover, and separate more shield and center conductor (this is the reason why you want a foot or so of the center conductor radiator, and shield counterpoise, hanging beyond the 1/2" pipe).
Repeat test until satisfactory.
Notes on design: Normally a 90 degree counterpoise would give you a 37 Ohm feed-point impedance, but not when you only have 1 counterpoise, it will be closer to 46Ω, and if the PVC droops under its own weight, it may actually get to 50Ω; and for what it's worth, your signal will be favoring the counterpoise, away from the building. And orientation of the coax center conductor can point to the sky or the ground or anywhere in-between to get the best VSWR or best receive of another station.
