Balun
| Balun | |
|---|---|
| Stands for | Balanced to unbalanced |
| What it does | Joins a balanced antenna or line to unbalanced coax, and may also transform impedance |
| Main types | Current (choke) baluns and voltage baluns |
| Common ratios | 1:1 and 4:1 |
| In use | |
| Usual location | At the antenna feed point |
| Built from | Ferrite cores wound with wire or coax, or coiled coax |
| Related device | Unun (unbalanced to unbalanced), such as the 9:1 and 49:1 types for end-fed wires |
A balun is a device that connects a balanced antenna or feed line to an unbalanced one. The name is a contraction of balanced to unbalanced. Its most common job in amateur radio is joining coax to the center of a dipole, but many baluns also transform impedance, so that, for example, a 200-ohm antenna can be fed with 50-ohm cable. A typical balun is a ferrite core wound with a few turns of wire or coax, sealed in a small weatherproof box at the antenna's feed point.
Why a balun is needed
A dipole is balanced: its two halves are mirror images, and at the feed point they carry equal currents flowing in opposite directions. Coax is unbalanced: one conductor sits inside the other, so the two are not alike. Radio-frequency current travels only on the surfaces of conductors, an effect called the skin effect, so the shield of a coax cable behaves as two separate conductors, its inside surface and its outside surface.
When coax is connected straight to a dipole, the current on the inside of the shield divides at the feed point. Most goes into the antenna wire, but some flows back down the outside of the shield. That stray current is called common-mode current, and it turns the feed line into part of the antenna. The results can include a distorted radiation pattern, RF in the shack, an SWR reading that changes when the coax is moved, and extra noise on receive as the feed line picks up interference from household electronics. Stopping that current is the main reason for putting a balun at a dipole's feed point.
Current and voltage baluns
There are two basic designs, often named after the engineers who first described them.
- A current balun, or Guanella balun, forces the currents at its two output terminals to be equal and opposite. A 1:1 current balun is, in effect, a common-mode choke: it presents a high impedance to current trying to flow on the outside of the coax while passing the normal signal through untouched.
- A voltage balun, or Ruthroff balun, forces its two output terminals to equal and opposite voltages with respect to ground. That produces equal currents only if the antenna is perfectly symmetrical, which a real antenna, with one end nearer a tree or a roof than the other, seldom is.
For feeding antennas, current baluns are generally preferred, because they suppress common-mode current even on an imperfect installation. Voltage baluns are still found in some older equipment and inside some antenna tuners.
Ratios and uses
A balun's ratio is its impedance ratio. A 1:1 balun changes only the balance; a 4:1 balun transforms 200 ohms to 50. An unun is the unbalanced-to-unbalanced equivalent, used where neither side is balanced, most often between coax and an end-fed wire.
| Ratio | Type | Typical use |
|---|---|---|
| 1:1 | Balun | Coax-fed dipoles, inverted Vs and Yagi driven elements |
| 4:1 | Balun | Off-center-fed dipoles; ladder line at the output of an antenna tuner |
| 9:1 | Unun | Random-length end-fed wires, always used with a tuner |
| 49:1 or 64:1 | Unun | End-fed half-wave antennas |
The 9:1 unun doesn't produce a match by itself. It brings the widely varying impedance of a non-resonant wire into a range a tuner can handle. The 49:1 and 64:1 types are designed for the very high impedance, typically a few thousand ohms, found at the end of a half-wave wire.
Choosing the wrong ratio creates a mismatch of its own. A 4:1 balun on a dipole whose impedance is already near 50 ohms presents about 12.5 ohms to the coax, an SWR of 4:1.
Choosing and installing
- Match the ratio to the antenna. Use 1:1 unless the antenna's feed-point impedance calls for something else.
- Allow for power and duty cycle. Ferrite warms up under high power, high SWR and digital modes such as FT8, which hold full power for the whole of each transmission. A hot balun is a lossy one and may fail, so choose one rated well above the power you run.
- Mount it at the feed point and run the coax away from the antenna at a right angle where possible. Many commercial dipole center insulators have a 1:1 balun built in.
- Keep water out. Point the connector downward and seal it as described under RF connectors.
- Add a second choke if needed. On a long feed line, another choke where the coax enters the building deals with current picked up along the way.