The purpose of a Balun is to allow the connection of a coaxial feed line ( unbalanced feedline) to a balanced antenna , such as a dipole.
Picture at left is a Circularly polarised turnstile antenna. Although this is for 75ohm coax, similar strusture are possible for 50ohm as well. Significant in this particular design is the fact that the 75ohm phasing cable section serves the function of the Balun as well.
What we are trying to achieve with a Balun, or its electrical equivalent, is to prevent common mode currents from flowing down the coax feedline. These currents are induced when a dipole is simply connected directly to a coax feedline, one element to the inner conductor, and one to the outer. The currents flowing in the dipole elements result in the electromagnetic wave being generated and radiated by the dipole elements. However, the current flowing in the element connected to the coax outer sleeve has to return to the generator ( the transmitter) and so does this by flowing on the outer sleeve of the coax cable. This current induces radiation from the coax outer sleeve, thereby distorting and destructively interfering with the dipole radiation pattern. This current can also return all the way back to the transmitter, and induced energy flows in adjacent cables and looms - In a small UAV with wiring close to everything, this sometimes manifests as servos twitching in sync with the video or datalink transmissions, etc, among other phenomena.
The balun on the left could be used to feed a folded dipole. Such a dipole has a feedpoint impedance of around 300ohms, and this balun exhibits an impedance step-up ratio pf 4:1. SO a 75ohm feedline would result in a good match to the 300ohm dipole, with a unbalanced to balanced trasformation in the deal.
However, there exist a number of alternative 'baluns' that can be used where an impedance transformation is not desired, a sort of 1:1 Balun.
Note that these are not really baluns in the true sense, but actualy perform the same duty by acting as a choke for the RF currents flowing on the coax outer sleeve.
The left image is often referred to as the 'Bazooka' Balun. It is a 1/4 wavelength of tubing, snugly fitted over the coax sleeve insulation, with the bottom end of the tube soldered all the way around to the coax sleeve braid. The top end of the tube is open and insulated from the rest of the antenna. This works by the 1/4 wave section forming a short circuit to the flowing currents at the base, and a high impedance at the top, choking of said currents.
The following are variations of the theme:
The Bazooka balun is preferred and is more efficient.
In order to not distort the antenna radiation patterns ans not cause EMI with other on-board electronics, it is always desirebale to use a Balun type feed for balanced antenna such as dipoles, Turnstiles, etc. The examples shown can be used with 1:1 and 4:1 impedance match for all dipole types.