On my mini quadcopter I have a 2.4 GHz quarter wavelength monopole antenna that is close to defective (due to crashes and therelike) and that I need to replace. This antenna is used to recieve control input and send back telemetry data, so it is sending and receiving at the same time.
The ground plane of the quadcopter does not seem to be too good, so I was thinking about replacing the monopole antenna with a dipole antenna, e.g. from an old WIFI router.
Is it possible to use a dipole antenna in a monopole design? Can I just connect the second half of the dipole antenna to ground?
If I build the dipole antenna myself from two pieces of wire, do both halves of the antenna need to be aligned directly in one line or can I place the halves apart from each other to where they are convenient on the board?
Is there anything else I need to know about that topic?
Best Answer
A dipole antenna requires a balanced feed for it's impedance and emission shape to remain good. A monopole is fed with an unbalanced signal i.e. signal and (local) ground) - it's basically half a dipole.
People use what is called BALUN to convert an unbalanced signal to a balanced signal and, by the sound of it this is all you need. It can be very simple - a couple of turns of the coax feed to the antenna wound around a ferrite toroid might just do the job or a little RF transformer (can be easily hand wound) will also suffice.
There are many variations on a the theme of dipoles and there are such devices called folded dipoles that bend the two limbs around in a circle but, it's always best to keep both limbs as symmetrical as possible and far away (as much as is feasible) from other circuitry.