What’s a Balun? Why a Balun? How Do I Make a Balun?
Most antennas are “balanced” radiators — with two identical sides — while coaxial cable feedline is “unbalanced.” How do we make them work well together? With a “balun” in-between. W1IS and KCiDSQ cover the basics.
Whats a Balun. “Balun” (BALanced to UNbalanced) is a catch-all term for a variety of devices we use on our antenna-feed-line systems. This article will take a brief look at what they’re all about, why they’re important and how you can build your own.
There are two classes of balun, choke and impedance transformation. Here’s a brief look at each:
A choke coil’s reactance isolates one part of the antenna-feedline system from another, as shown in Figure 1. With bifilar wires or coax wound on a common core, a choke coil provides a high impedance at the design frequencies of the antenna system that limits the flow of common mode current through it.
A choke is often called a current balun when the output is connected to an antenna or balanced feedline. Another application: With coax connectors on each end, it can isolate two sections of coax or a rig from an antenna tuner and is called an UNUN (UNbalanced-to-UNbalanced).
Impedance transformation baluns match the impedance of one part of the antenna-feedline system to a different impedance on the other side. There are two kinds of impedance transformation baluns: transformer and autotransformer.
A transformer-type balun uses separate coils wound on the same core as in power transformers and some “wall warts.” Since the coils only connect through the flux in the core, each side is isolated from extraneous currents, called common mode currents, on the other side, as seen in Figure 2.
The voltage on the output side is proportional to the ratio of number of turns on each coil. If the number of turns on each side are the same, the ratio is 1:1 and the voltage and impedance on each side are the same. If the turns ratio is 2:1, the voltage on the output is twice the input voltage and the impedance on the output side is the square of the voltage ratio or four times the input impedance.
Baluns are specified by impedance ratio so this would be a 4:1 balun. Because the core of a transformer balun can easily saturate, they are mostly used in low-power applications, such as a receiving antenna.
An autotransformer has the coils connected in series, as in a Variac®, Figure 3. As you may have guessed, if the number of turns on each coil is the same, the voltage on each coil must be the same.
Most baluns are wound with the same number of turns so the output of the balun in Figure 3 mustbe twice the input and it is a 4:1 balun. Unlike a choke balun or a transformer balun, autotransformers by themselves provide little or no isolation. Though some manufacturers call them current baluns, they are more accurately voltage baluns.
For new hams and even some old hands, baluns seem to be mysterious devices that either should or must be used on antennas. The first encounter with RF burns or noise coming from the rig or other device in the shack or house when transmitting may be the principal motivator that leads to a balun.
But there are other reasons to appreciate the value of a balun. For example, a dipole can be made to work with two pieces of wire cut to one-quarter wave each for the band of interest and fed directly with coax.
The unwary operator then believes that the radiation pattern will mostly be broadside to the antenna. This won’t be the case, though, because — as a result of skin effect — coax actually has three conductors, the center, the inside of the shield and the outside of the shield. Since the outside of the coax is connected to one leg of the dipole, it can radiate from current flowing back down the outside of the cable as well as from the quarter-wave wires.
To ensure the antenna is radiating where you expect it, the outside of the coax must be isolated from the inside of the coax as well as the wires of the antenna. You can make the simplest isolator by rolling up some of the coax, making some of the outside of the cable into an inductor that will impede the flow of RF down the outside.
There are several articles describing how to physically do this. Unfortunately, the coils may be heavy, clumsy and have a limited bandwidth, so they may not be as effective as you need or want. For example, it takes a lot of coax to make a choke that works well on 160 meters. However, that coil of coax is a rudimentary choke. When used at the interface between the feedline and the wire elements, it serves to interface an unbalanced medium, coax, to the balanced antenna.
However, it has great use elsewhere in the antenna system. For example, when the antenna is fed with balanced feeders, such as ladder line or open wires, it isolates the balanced side of the feed system from the unbalanced coax that you connect to your rig. The 1:1 balun, or choke or UNUN, described in this article will provide excellent isolation from 1.8 to 54 MHz.
A different kind of balun is used as an impedance matching device from 50-ohm coax to an off-center-fed (OCF) antenna whose impedance at the feed point varies but is often around 200 ohms, calling fora4:1 impedance transformation.
Some impedance-matching baluns provide good isolation and others little or no isolation. Baluns can be built with a wide range of matches, but 1:1, 4:1, and 9:1 cover the needs of most hams. Next to the 1:1 balun, the 4:1 balun described in this article is the next most useful for most antennas and feed systems from 1.8-54 MHz.
A hybrid balun consists of a 4:1 voltage balun connected to a 1:1 current balun / choke / UNUN that connects to the coax. This combination is particularly good for OCF antennas as the voltage balun provides the impedance match while the 1:1 balun properly isolates the feedline from the antenna. It is also useful for connecting balanced feed lines, such as open wire or ladder line, to coax. Isolation is key to both keeping RF off the feedline and preventing the feedline from affecting the tuning and performance of the antenna.
This article describes how to make and use a 1:1 current balun, a 4:1 balun and the combination, a hybrid balun. For a full treatment of baluns see “Understanding, Building and Using Baluns and Ununs,” by Jerry Sevick, W2FMI (SK)³. This book is available from CQ.
The 1:1 Balun
A 1:1 balun isolates the outside conductor of the feedline from the antenna so the feedline doesn’t radiate as part of the antenna, making your antenna truly balanced and keeping RF out of the shack.
An UNUN is often used at the junction where coax enters the building to keep RF induced onto the outside of the coax from entering the building. At the feed point of an antenna, such as a dipole, beam, etc., a balun transforms the unbalanced feed from the coax to a balanced feed at the antenna. The following shows you step-by-step how to make a 1:1 balun / choke / UNUN.
Teflon®, or PTFE, coax is used for its size, low loss, powerhandling ability, and high breakdown voltage. The outside of the coax wrapped around the toroid becomes a relatively large inductor between the radiating wires and the coax thereby electrically isolates the antenna from the outside of the coax.
It can be built as the “center insulator” with eyebolts to attach the antenna wires or, with two SO-239s, to provide isolation at any place along the feedline on the way to the rig.
The balun described here provides isolation from the lowest frequency, depending on the number of turns through the toroid, up to 6 meters. Table 1 lists the parts you’ll need to build one, and PhotosA-D will help you visualize the steps.
1. The number of turns needed depends on the lowest frequency you wish to operate.
2. Use 12 turns for 3.5 MHz, and 16 turns for 1.8 MHz (the photos show a 16-turn balun).
3. Loosely attach two cable ties 180° apart on a 140-43 toroid.
4. Put the end of the coax through the lower cable tie.
5. Note: every time the coax goes through the toroid is one turn, even if it doesn’t go all the way around. Photo B shows two turns.
6. Wrap the coax around the toroid either 4 or 8 more turns. The sixth or eighth turn goes diagonally across the toroid completing the sixth or eighth turn through the toroid, Photo C.
7. Wind the coax as shown in Photo D.
8. The last turn on the left side goes out the lower side of the toroid and is fastened by tightening the cable tie.
9. There should be plenty of coax to route inside the box to either a second SO-239, the eyebolts that connect to the antenna, or to the 4:1 balun in the hybrid balun configuration that follows.
10. A 1:1 balun is typically packaged two ways; for connection to antenna wires (Photo E) or in line to block RF (Photo F).
Materials for 1:1 balun/choke/UNUN
1 – (For up to 600 watts) 140-43 toroid ( FT-140-43, or Mouser 623-5943002701)
1 – (For up to 1,500 watts) 240-43 toroid ( FT-240-43, or Mouser 623-5943003801) 600 watts
4 feet RG-316 for 600 watts ( or ) 1,500 watts
6 feet RG-303 for 1,500 watts
2 – 4 inches of tie wraps
1 – (up to 600 watts) Awclub waterproof dustproof enclosure 3.3” x 3.2” x 2.2”
1 – (up to1,500 watts) waterproof dustproof enclosure 4.53” x 3.53” x 3.15” (WA-28 Polycase.com) or Carlon A-273,
4” x 4” x 2” weatherproof box available at local hardware and electrical stores.
3 (To connect to antenna wires) – 10-24 stainless steel routing eyebolts ( #9489T54)
10 – 10-24 Stainless steel hex nuts (local HW store)
10 – #10 Stainless steel flat washers (local HW store)
10 – #10 Stainless steel lockwashers (local HW store)
5 – #10 Ring terminals (local HW store)
4 – 6-32 x 1/2-inch Stainless steel machine screws (local HW store)
4 – #6 lockwashers (local HW store)
4 – 6-32 Stainless steel hex nuts (local HW store)
2 – (Only need 1 to connect to antenna wires) SO-239 coax socket
The 4:1 Balun
A 4:1 balun is actually a type of autotransformer similar to that used in a Variac® that steps up voltage, in this case by a factor of two. Because the voltage goes up by two and impedance goes up by the square of the voltage, the impedance transformation is 4:1.
Most voltage baluns only provide an impedance transformation and little or no isolation or common mode rejection. The 4:1 balun described below is a single-core Guanella balun used in many commercial off-center-fed (OCF) antennas.
Though single-core Guanella baluns are often called current baluns, they are actually voltage baluns and, used alone, their limited isolation can be a problem. The usual solution is the two-core Guanella balun that has better common mode rejection. However, the single-core Guanella can provide excellent performance and can keep the voltage on each side of the antenna identical if its input is properly isolated from the coax and the rest of the antenna system.
We found that a robust current balun is absolutely necessary for an OCF operating at its lowest design frequency when placed between the Guanella balun and the feedline. This exploits the limited rejection of the single-core Guanella balun and the robust isolation of a 1:1 balun. We have found this most effective for our OCF antennas.
In addition to OCF antennas, 4:1 baluns are used in some antenna tuners for connecting balanced line, open wire, or ladder line to the tuner. More commonly, they are used to couple balanced line to coax at house entrances where coax is fed through the wall or a window to the rig. The balun described below, using four coils connected in series, is illustrated in Figure 4.
The input voltage, V1, splits in half across the two coils, leaving V1/2 on each coil. Since all the coils have the same number of turns of wire and are on the same core, the voltage on each coil must be the same, V1/2. Therefore, the output voltage is the sum of the voltage on each coil, 2V1. And, since the impedance is the square of the voltage, the impedance of the output is 4 times the impedance of the input. In our case, 50 ohms in and 200 ohms out.
Building the 4:1 Balun
The limited isolation of the 4:1 balun makes it imperative to use it with a 1:1 balun / choke. You’ll find the parts list in Table2. Photos G-J illustrate the steps.
Once again, the number of turns you use depends on the lowest frequency you wish to use. Six turns on each side for 80 meters and up and 8 turns on each side for 160 meters and up.
1. Loosely attach two cable ties about 180° apart on the toroid.
2. For up to 600 watts, Cut 18 inches of red and 18 inches of green wire. For up to 1,500 watts, cut 24 inches of each wire.
3. Tape them together every 3 or 4 inches with a short piece of electrical tape or a piece of shrink tubing to keep them together as you wind the toroid.
4. Set the colors of the wire as in the photos with the green wires on the right side for the right-side windings and on the left side for the left side windings.
5. Every time the wire goes through the toroid is one turn even if it doesn’t go all the way around. Photo G shows two turns.
6. Leave a few inches of wire outside the toroid and secure the tie wrap to hold the wire as you wind.
7. Add four or six more passes through the toroid and secure the wires with a cable tie with the wires emerging from the bottom of the toroid, Photo H.
8. Repeat the process for the other half of the toroid, starting the winding at the bottom of the toroid, Photo I. Keep the green wire on the left side as in the photo. Secure the wires on the top of the toroid after the wires go through the toroid six or eight times, Photo J.
Wiring and Mounting a Hybrid Balun
Use the hardware specified for the 1:1 balun above, and Photos K and L for guidance.
1. Strip both ends of the coax on the 1:1 current balun / choke by running a razor blade or sharp hobby knife around the outside about 1 inch from the end and pull off the insulation, Photo K.
2. Tin the whole shield and strip the top 3/4 inches of the cable with a wire stripper.
3. Then run the blade around the center conductor insulation about 1 /4-inch from the end and pull off the insulation.
4. Tin the center conductor and around the exposed shield. PTFE/Teflon® insulation doesn’t melt at soldering iron temperatures. For your installation, you may need to solder a short wire to the shield.
5. Strip the about 3/8 inches from the four short wires at the input (50-ohm) end of the balun.
6. Wrap and solder the red wires together. Wrap and solder the green wires together. This is where you will attach one side of the 1:1 balun.
7. On the other (output) side of the balun, cut the red and green wires closest together so they are about 1 inch long. Strip and solder them together. Leave the other red and green wires as they will be connected to the output connections on the balun case.
8. Wire as in the Figure 5 and laid out in Photo L after you determine how and where you will mount the coax input and the two output connectors.
Materials for 4:1 Balun
1 – (For up to 600 watts) 140-43 toroid (, FT-140-43, or Mouser 623-5943002701)
1 – (For up to 1,500 watts) 240-43 toroid (, FT-240-43, or Mouser 623-5943003801)
For 300 or 600 watts:
3 feet #18 Red PTFE/Teflon® wire ( #18PTFESTRRED25)
3 feet #18 Green PTRE/Teflon® wire ( #18PTFESTRGRE25)
For 1,500 watts:
4 feet #16 Red PTFE/Teflon® wire ( #16PTFESTRRED25)
4 feet #16 Green PTRE/Teflon® wire ( #16PTFESTRGRE25)
4 – 4-inch tie wraps
There are many ways to package the hybrid balun, Photos M and N. The specified boxes have bosses on the bottom. Remove the bosses using a Dremel-type tool with a grinding wheel to give you more space to mount the toroids using double-sided foam mounting pads.
Toroids can be configured many ways, as in the photos. It is critical that the toroids be separated to keep the coupling between them to a minimum, either rotated 90° or separated by at least 1 inch. Tightly coupled coils can affect the performance of the antenna.
The antenna terminals can be brought out through the ring mount as in Photos Mand N. Cutting the ring terminal with diagonal cutters as shown on the right side of Photo M makes it is easier to attach the antenna wire to the hybrid balun.
We hope this article will help you better understand the importance and function of baluns, and will help you get the most out of your antenna system by building the type or types that best meet your needs.
Reprinted with permission from the January 2021 issue of CQ Amateur Radio magazine.