What is an Unun: RF Antenna Unun

RF antenna Ununs are unbalanced to unbalanced RF transformers typically used with antennas like end fed wires where impedance transformation is required to match the coaxial feeder.

RF Antenna Baluns & Ununs Includes:
Antenna Balun basics Current balun Unun basics

Although the name "Unun" sounds a little unusual, it refers to a component that has many similarities to the more familiar balun.

A balun provides a balanced to unbalanced transformation for feeder systems. By the same logic, an unun provides an unbalanced to unbalanced transformation.

The main purpose of an antenna unun is to provide an impedance transformation, and it is often used with end fed wire antennas, and in particular with end fed half wavelength wires. They can also be used with vertical antennas which are also unbalanced.

Antenna unun applications

An antenna unun is effectively a form of RF transformer that is used to interface two different impedance levels both of which have an unbalanced line.

Coaxial cable is an unbalanced form of feeder, i.e. it has a screen and a centre and the screen is normally earthed. A balanced feeder, by contrast has two lines, neither of which is earthed - the two lines carry equal and opposite signals, so the fields cancel out and no power is radiated.

Also antennas can be balanced or unbalanced. The dipole is an ideal example of a balanced form of antenna because neither of the two sections at the feed point is earthed.

Conversely there are many antennas like end fed wires or vertical antennas where the antenna uses an earth as part of the antenna, and this requires an unbalanced form of feeder like coaxial cable.

If an impedance transformation is required at the feed point of the antenna, then this can be accomplished using an unun.

Often quarter wave vertical antennas have a feed impedance of around 20Ω. This is too low if it is to be fed directly using 50&Omega coaxial feeder cable. To overcome this issue, an autotransformer is often included in the base assembly. This is effectively an unun which matches the 20Ω of the vertical antenna with the 50Ω of the coaxial feeder.

The ununs used for matching quarter wave or similar vertical antennas are normally contained within the base of commercial manufactured vertical antennas ad therefore they are not often seen as separate items.

However there is an increasing level of use of end fed wire antennas. In past years, these were often brought directly into the radio room and interfaced via an antenna tuning unit.

Basic concept of the end fed half wave EFHW antenna

To obtain the best performance and also keep the levels of RF energy away from occupied areas, the best way is to install the end fed wire in the open and away from inhabited areas and then to feed the antenna using coaxial cable. As the feed impedance of an end fed wire, and in particular the increasingly popular end fed half wave antenna, a means of matching the feeder to the antenna is required at its feed point. This matching can be obtained using an unun.

Basic autotransformer unun circuit — Basic unun concept

Difference between balun and unun

As baluns an ununs are both used in antenna systems and they both provide functions associated with matching the antenna to the feeder, there is often a lot of confusion between the two. Many people ask what the difference is between a balun and an unun.

IN essence a balun enables a transition between an unbalanced and a balanced system, whereas an unun works with unbalanced systems at both the input and output.

Naturally there are circuit differences as well. The unun has an earth side connection for both the input and output, whereas the balun only has an earth side connection for the unbalanced side.

Comparison between the Unun and Balun showing how the unun has one connection connected to earth for RF and the balun earth is connected to the centre tap. - balun vs unun — Comparison between the Unun and Balun

It is worth noting that in the case of the balun, the outer of the coaxial cable which is at earth potential is connected to the centre point, leaving the ends of the autotransformer balanced and not earthed to RF.

Unun basics

The theory behind the unun is basically the same as any transformer. That said, with high frequencies being used than for power line transformation or audio, etc, factors such as the loss in any formers, interwinding capacitance, etc need to be considered more closely.

As with any transformer the impedance transformation is equal to the square of the turns ratio.

\frac{Z_{in}}{Z_{out}} = \frac{N_{in}^{2}}{N_{out}^{2}}

This means that for an unun with the output circuit having twice as many turns as the input, the impedance will be transformed by a factor of four.

Practical ununs

Ununs are often wound as an autotransformer. The windings are also often wound together so that there are two, three, four windings together.

This means that where there are two windings together, the impedance transformation ratio will be 2², i.e. 1:4 giving a match to 200Ω with 50&Omega coax.

Some ununs will have three windings, so the the impedance ratio is 2³, or 1:9 giving a match to 450Ω when used with 50Ω coax feeder.

The 9:1 ununs are often seen for use with end fed half wave antennas. These end fed antennas may have an impedance that is higher than the 450Ω that would give a match to &50&Omega coax.

Accordingly some ununs have a higher turns ratio giving a higher impedance. Accordingly some ununs are seen with turns ratios of 49 or even 64:1. Notice that the impedance ratios are always a square of turns ratio on the former: 4, 9, 16, 25, 36, 49 . . . and so forth.

Antenna tuner used with an unun, antenna tuner & end fed half wave antenna

Whatever the unun used, it is always advisable to use an antenna tuning unit at the transmitter. There is likely to be a relatively high level of SWR on the line, and although this will cause a small signal loss, the main issue will be to protect the transmitter from seeing a high level of SWR.

If the transmitter does see a high level of SWR, then the voltage and current peaks could cause damage to the transmitter power amplifier. If the power amplifier has protection circuitry, then it could result in the transmitter reducing its power output.

With end fed wires now being an ideal option for multiband HF antennas, along with the need for matching vertical quarter wave and similar antennas, the unun is being widely used for these and many other applications.