Why does an antenna need a 1:1 balun?

It seems like some kind of magic object, the 1:1 balun. Some say it can solve many antenna problems. It could turn a bad SWR into a good SWR. Increase antenne bandwith. Reduce interference in reception. Prevent RFI that causes interference to electrical devices at your neighbors and your own home. To even improving the radiation efficiency of the antenna. The 1:1 balun makes the hearts of many radio amateurs beat faster. But how magical is this piece of magic really and why should you use it at all?

Special function that comes in handy

Some amateurs swear by it, others think it’s just nonsense. Yet the 1:1 balun has a special function that comes in handy when feeding symmetrical antennas through an asymmetrical feed line. The 1:1 balun, in this capacity also called common mode choke or line isolator, ‘decouples’ the feed line from the antenna.

Symmetrical antennas

There are symmetrical (balanced) antennas and asymmetrical (unbalanced) antennas? They are easy to recognize in themselves. The dipole, with its two equal legs, is the prime example of a symmetrical antenna. The current distribution in both legs is equal and therefore symmetrical. This group of antennas also includes ‘variants’ of the dipole, such as the G5RV, ZS6BKW, Doublet, Cobweb and Hexbeam.

Cobweb antenna 20, 17, 15, 12, 10, 6 and 4 meter band

Asymmetrical antennas do not have equal power distribution. For example, the EndFed, inverted L, Windom (off center fed dipole) and groundplane.

Symmetrical and asymmetrical feedline

Symmetry can also be found in feedlines. An example of a symmetrical feed line is the open line, also called twin lead. This is two wires running parallel at equal distances. Along one wire the antenna is fed, while the other wire is the return path. This open line does not radiate, because the field of the ‘feed curren’ in one wire is cancelled out by the ‘return current’ in the other wire. You can feed a dipole with open line without any problem.

However, the most commonly used feed line is coaxial cable. The core of the coax feeds the antenna, with the shielding handling the return currents and shielding the core from outside influences. Coax is an asymmetrical feed line in which the currents are unevenly distributed. This creates problems when feeding a symmetrical antenna in which the currents are evenly distributed.

Unwanted currents; reduced performance and interference

That uneven current distribution in the coaxial cable can lead to currents on the shield of the coax, called common mode currents. That creates imbalance in the antenna, making the radiation anything but optimal.

The shield of the coax becomes an active part of the antenna and will also radiate RF. In addition to the coaxial shield, common mode curents run over everything that conducts current and is connected to that shield. So through the transceiver and the power supply, the electrical wiring in the house and everything connected to it. Yes, even the washing machine, refrigerator and oven can suddenly join in as an active part of the antenna!

The balun brings balance and restores symmetry

The 1:1 balun (derived from ‘balanced-unbalanced”) causes the uneven power distribution in the coaxial cable to ‘balance’ with the balanced dipole antenna. It converts the unbalanced signal from the coaxial cable to a balanced signal suitable for the dipole antenna. This restores symmetry.

You place the 1:1 balun between the feed point of the antenna and the coax.

The result

First of all, a more efficient and optimally radiating antenna. Because the coax cable is ‘decoupled’ from the antenna, it no longer conducts common mode currents. The chances of you interfering with equipment thus decrease significantly.

Better SWR?

A 1:1 balun does not improve SWR. The 1:1 ratio refers to the impedance that goes in and comes out. If the SWR of your antenna is around 1.5:1, it will still be around 1.5:1. Still, I get feedback from amateurs who did see more bandwidth on their antenna after installing 1:1 balun. A specific case: One had 500 kHz bandwidth with an SWR below 2:1 on the 10 meter band, after installing a 1:1 balun it had become 700 kHz. I have no explanation for this.

‘My antenna does not need a balun’

Now I see fellow amateurs proudly reporting on forums that their coaxial cable fed symmetrical antenna performs fine without a 1:1 balun, and it surely isn’t necessary to use a 1:1 balun.

Those I invite to watch this video from IZ2UUF. Davide shows the effect of a 1:1 balun when a dipole is fed with coaxial cable. Pay particular attention to the part where Davide uses an RF meter to ‘probe’ the equipment in his shack, including his morse paddle, for RF radiation.

The bonus; less interference signals

A 1:1 balun not only restores symmetry. It also eliminates common mode currents caused by nearby sources of interference. An interfering battery charger, power supply or solar panel inverter can cause common mode currents that, conducted through the coax shield, reach the receiver. The result, for example, is a high noise floor. But you have to place the balun close to the receiver, to achieve this.

If it does not help, it does not hurt.

Except for a negligible loss of 0.1-0.2 dB on HF, a 1:1 balun has no negative effect on your antenna system. It is rarely wrong to always feed your antenna through a 1:1 balun.

Want to know more?

Do you want to know more about the balun, unun, common mode choke or line isolator? Then check out this page with detailed explanations and my suggestions for homebrew.