The Infinite Balun
In the case of a coaxial cable feeding an antenna, a balun is used to choke (eliminate or reduce) the currents flowing on the outside shield of the coaxial cable.
The idea of an Infinite Balun is to use the currents flowing on the outside of the coaxial cable as part of the antenna. In this case, we won't need to choke the current at all; we just need to align the cable with where we want the current to flow on our antenna.
As an example, consider the spiral antenna. The spiral antenna has two equal arms, which needs fed in the center (center conductor of the coaxial cable to one arm, and the ground or shield of the cable to the second arm of the spiral). For spiral antennas to work properly, we need balanced currents on either arm of the spiral.
Suppose we try to run the cable up to the spiral antenna from below, as in Figure 1:
Figure 1. A Spiral Antenna Fed With a Coaxial Cable from Below.
In Figure 1, the spiral has two arms the blue and the green. The center conductor of the coaxial cable (red wire) is attached to the blue arm. The outer conductor of the coaxial cable (black) is attached to the green arm of the spiral antenna.
For the antenna of Figure 1 to work properly, the current on the outside of the coaxial cable (black region) must be choked. This could be done with a bazooka balun; however, this balun is farely narrow bandwidth. Since spiral antennas are extraordinarily wideband, a better balun needs used.
The solution is to use the coaxial cable's outer conductor as one arm of the spiral antenna. In this sense, the current can flow on the outside of the coaxial cable, so that the unbalanced operation becomes desirable (current flows on both arms). Remember: current exiting a coaxial cable views the outside of the coaxial cable as a comletely independent surface from the inside surface (where the current is flowing in the transmission line mode). Hence, when the coaxial cable "opens up" (i.e. begins to feed an antenna), the current has no problem flowing on the outside of the coaxial cable.
The infinite balun uses the coaxial cable as one arm of the spiral, as shown in Figure 2:
Figure 2. The Infinite Balun Used With The Spiral Antenna.
In Figure 2, the black coaxial cable leaves the radio (transmitter and receiver). The coaxial cable wraps around and forms one arm of the spiral antenna. The center conductor of the coax (shown in red) attaches to the other arm of the spiral antenna (shown in Blue).
Why does this work? Imagine current flowing down the transmission line, a forward travelling wave on the inner conductor and the opposite on the inside surface of the outer shield of the coaxial cable. Once the coaxial cable ends (the center conductor is exposed and connected to the blue arm), the current on the center conductor of the coaxial cable must travel to the blue arm of the spiral antenna.
Now, when the current flowing on the inside of the outer shield of the coaxial cable is exposed (where the shield is broken at the feed point), the current has nowhere to flow except the outside of the coaxial cable. Hence, the current must travel down the outside of the coaxial cable, which happens to double as the black arm of the spiral antenna.
This balun works wonderfully. It has no bandwidth problems and is a very clever design. In addition, since the spiral antenna is a good radiator, the currents die off quickly as the spiral winds away from the center. Hence, terminating one arm of the spiral (the coaxial cable) to the radio does not affect the performance.
This balun can be used whenever a separated ground region is available to merge the coaxial cable outside shield with one region of the antenna. Care must be taken when exiting the coaxial cable from the structure, so that the lead of the coaxial cable to the radio (transmitter/receiver) does not detune or negatively affect the antenna.