The principle of operation of a Tapered Balun is presented on this page. This
is simple to understand - it gradually changes the shape of an unbalanced transmission line
to that of a balanced line.|
Examples of Tapered Baluns are shown in Figure 1.
Figure 1. Tapered Baluns - (a) coaxial cable and (b) Microstrip Transmission Line.
On the left in Figure 1 (a), a coaxial cable is slowly "peeled" and reshaped until it is a balanced transmission line. On the right in FIgure 1 (b), a microstrip transmission line (which is also inherently unbalanced), has the lower ground plane (dark grey) gradually resized until it is the same shape as the metal strip that lies above it (light grey line). At the end of the transition, the two lines are identical in shape (as shown in the side view figure), and represent a balanced transmission line.
For the tapered transition to work, it should vary slowly. The term "slowly" in electronics always mean "slow relative to a wavelength". Hence, for the desired frequency of operation, the transition should take place over at least a few wavelengths in length.
The nice property about tapered baluns (in addition to them being simple to understand), is that they are wideband. If the transition is slow at one frequency, then at higher frequencies (where the wavelength is smaller), the transition is also slow relative to a wavelength. Hence, these baluns operate over a wide frequency range. This is in contrast to the bazooka balun or the folded balun, which require components that are a quarter-wavelength long (at the desired frequency). This restriction makes them inherently narrow band.