A vertical clearance of 2.5-3 metres (8.2-9.2 feet)
An horizontal clearance of 3 or more metres (9 feet or more)
A hook on the ceiling at the center of the horizontal clearance, as close as possible to the free-space
Possibly a metallic fence
A few words about it
The Cedevita-20 is a hybrid monoband antenna, the result of merging the radiator of a shortened vertical with one element of a dipole. It fits on most balconies and must be hung from the ceiling. It might work from indoor as well, hanging from the lamp ceiling hook in larger rooms but I have not tried it.
It appears as a vertical antenna with a center-loaded radiator (center loading a radiator increases efficiency with respect to base loading) and at least one quarter wavelength counterpoise.
This simple setup is actually a dipole, so the use of a 1:1 balun is advised in order to reduce RF radiation from the coax, RTX, power supply and AC mains... so to avoid annoyance to electronical devices in the neighbouring apartments.
Additional counterpoises of the same length can be added and laid in other directions if the location allows.
Cedevita-20 is a cheap and simple way to be on the air when erecting a proper antenna is not possible. Don't expect superior performance from it!
Construction is straightforward.
Identify how much vertical space you want to allocate to the radiator, possibly 20-30 cm (8-12 inches) less than the overall height.
Decide at which height you want the loading coil to be. Then download G4FGQ's VERTLOAD.EXE (a DOS executable, works in all MS Windows and under Linux dosemu). This program will give you an idea of how many turns you need.
You can either cut three pieces of wire (top, bottom and coil) or do everything with just one length, as you prefer. Drill a hole on the top and bottom of the cylindrical support, and two holes on the side. Follow the drawing to visualize what I can't express with words.
Wind more turns than theoretically needed: this will lower the resonance of the antenna, and make tuning as simple as cutting wire.
Cut at least one ¼ lambda counterpoise and keep it handy.
Connect the counterpoise(s) to the cold end of the balun (coax) and the loaded radiator to the hot end. Check SWR around the chosen HF band.
If the best SWR is lower in frequency than the desired band you need to remove turns from the coil. Otherwise you have to add turns. At 14 MHz one turn moves resonance of about 100 kHz, but YMMV.
How about other bands?
There is no reason why the Cedevita-20 cannot be turned into a Cedevita-30, -40, -17 or whatever. The coil will have more or fewer turns if you build it for a lower or a higher band respectively.
I have built a Cedevita-30 and brought it to resonance. I was surprised to discover how much RF was being radiated from the coax and everything else! I could feel CW key paddles warm up when touched! (Yep, they're made of metal.) I haven't tried the Cedevita-30 with the 1:1 balun.
I have successfully operated a 3-band Cedevita with a single tapped coil for 40, 30 and 20m HAM bands. 17 and 15m taps would fit the coil support as well, but I noticed that coil taps create reciprocal capacitance, making tuning harder especially on higher bands (closer taps). I would rather go for monoband pluggable coils.
A bonus band
If planned accurately, the Cedevita-20 may also resonate on 6m. Simply make sure that the radiator length between the feedpoint and the loading coil is ¼ lambda at 50 MHz, that is about 1.4m (15 feet). The loading coil will have a high impedance at 50MHz, so that is works like a trap. Do not forget to add at least one resonating 6m counterpoise!
The name of this antenna comes from the support I used to wind the coil: it originally cointained a popular Croatian drink called Cedevita. It is a plastic cylinder about 10cm high and 6cm diameter that can be drilled very easily and provides satisfying mechanical resistance.
I believe any thick-plastic bottle may work, such as milk bottles.