The length of your antenna
To fix our ideas, take the case of a wire antenna, one of the cheapest aerial offering excellent performances and used by thousands amateurs worldwide. By the length of your wire antenna we understand the complete physical length of your wire from one end to another, knowing that most longwires are constituted of a simple copper or steel wire, sometimes linked together to reach the expected length.
The diameter of the wire antenna would influence the bandwidth of the antenna and the radiation resistance, which represents the loss of energy due to heating of the antenna wire.
If u want to design dipole ant on theory half wave length of operating freq, give max gain but in fact use 0.8-0.95 of half wave because at half wvae it's generate multi beam.
Diameter of wire effect to loss of radiated power(Skin effect and surface wave=> see Emag theory) you should not use solid tube if you warry about gain.
Text book for beginner => Antenna Theory:Ballanis