Baluns and Chokes

When you connect a coaxial cable to the center of a dipole antenna, you are connecting an unbalanced feedline (where one conductor — the shield — is at ground potential) to a balanced antenna (where both elements carry equal and opposite currents relative to each other). Without a balun at this junction, common-mode current flows on the outside of the coaxial cable's outer conductor, causing the feedline to radiate, disturbing the antenna's radiation pattern, and potentially bringing RF energy into the shack where it can interfere with your equipment and cause RF burns. A balun solves this problem. Every coaxial-fed dipole installation benefits from one.

Three common balun types for amateur radio. The choke balun suppresses common-mode current without changing the 1:1 impedance ratio. The W2DU bead balun is the most compact approach. The 4:1 balun transforms the balanced feedpoint impedance to match the coaxial cable.

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Why You Need a Balun at the Dipole Feedpoint

A dipole antenna is a balanced structure. Both halves carry equal RF currents flowing in opposite directions — one half carries current flowing outward from the feedpoint, the other carries current flowing back toward the feedpoint. At any instant, the currents in the two elements are equal in magnitude and opposite in direction (relative to the feedpoint).

A coaxial cable is an unbalanced structure. The outer conductor (shield) is connected to the ground system at the transmitter end and is physically connected to the earth via the station ground. The inner conductor carries the signal. When you connect coax directly to a dipole feedpoint without a balun, you force one element of the dipole to be connected to the grounded shield.

This creates a problem: the coax shield is part of the ground system, and current wants to flow on its outer surface as well as its inner surface. Inside the coax, equal and opposite currents flow on the inner conductor and the inner surface of the shield — this is the normal transmission-line current. But outside the shield, additional current flows from the feedpoint down the outer surface of the coax toward the shack. This outer-surface current is called common-mode current because it is in addition to — and in the same direction on — both sides of the feedline.

Common-Mode Current Explained

Think of the feedline as having two separate paths for current: the differential mode (inside the coax, equal and opposite currents on the center conductor and shield inner surface) and the common mode (on the outside of the shield). The differential mode carries the actual RF signal. The common mode is unwanted and causes several problems:

• Feedline radiation: The common-mode current turns the feedline into an antenna. It radiates RF in directions that distort the dipole's intended radiation pattern. Instead of the classic figure-eight pattern of a horizontal dipole, you get a modified pattern that depends on the feedline routing.
• RF in the shack: The common-mode current travels all the way down the feedline to the shack. RF voltages appear on the transceiver's chassis, the microphone, and other equipment connected to the radio. This causes RF feedback in voice transmission (the microphone picks up its own RF), interferes with computer equipment, and can cause painful RF burns when touching equipment while transmitting.
• Interference coupling: Common-mode current on the feedline can couple interference from nearby power lines, motors, or other equipment into the antenna system, degrading the receive noise floor.
• SWR instability: Because the feedline is radiating, changes in its routing or nearby objects change the antenna's effective feed impedance. SWR may appear to change when you move the feedline — a classic sign of common-mode current problems.

Common-mode current flows on the outside of the coaxial shield when an unbalanced feedline is connected to a balanced antenna. A ferrite choke at the feedpoint presents high impedance to this common-mode current without affecting the differential signal inside the coax.

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Current Balun vs. Voltage Balun

There are two fundamentally different types of balun, and they solve the balanced/unbalanced problem in different ways. The distinction matters greatly for antenna applications.

Voltage balun

A voltage balun forces the voltages at its balanced output terminals to be equal in magnitude and opposite in phase relative to ground. It achieves this by its transformer-like structure: the voltages are constrained by the number of turns on the windings. A voltage balun works correctly only when the antenna impedances are equal on both sides — that is, when the antenna is electrically symmetric.

In a real antenna installation over imperfect ground, the two antenna elements rarely present exactly equal impedances. When the antenna is asymmetric, the voltage balun tries to force equal voltages on both terminals, and significant current flows in its windings to achieve this. This current heats the windings and the ferrite core — a voltage balun can fail or distort at high power with asymmetric loads.

Current balun

A current balun forces the currents at its output terminals to be equal and opposite — by presenting high impedance to common-mode current. It does not directly constrain the voltages. Instead, it adds a large common-mode choke inductance in series with the path for common-mode current, making it difficult for common-mode current to flow. Differential current flows freely (it creates equal and opposite magnetic fields in the choke that cancel out, so the choke presents no significant impedance to differential mode).

The current balun is the preferred type for antenna work because it:

• Works correctly even when the antenna is asymmetric (due to ground effects, nearby objects, etc.)
• Does not develop high currents in its windings under unequal load conditions
• Is not affected by SWR on the antenna side
• Can be built very simply from a short length of coaxial cable wound on a ferrite core

The Coaxial Choke Balun

The simplest current balun is made by winding several turns of the coaxial feedline itself on a ferrite toroid core just before the antenna feedpoint. The differential signal inside the coax is unaffected — the equal and opposite currents on the inner conductor and shield inner surface create canceling magnetic fields in the core, so the core has no effect on the differential signal. But any common-mode current on the outside of the shield creates a net magnetic field in the core, and the inductance of this common-mode path is high (due to the ferrite's high permeability), presenting high impedance to common-mode current.

A coaxial choke balun is also called a ferrite sleeve balun, coaxial choke, or simply a RF choke. It is the most common type of balun in amateur radio use.

Construction of a 1:1 coaxial choke balun:

1. Select a ferrite toroid of the appropriate material and size (see ferrite selection section below). FT-240 toroids (outer diameter 2.4 inches = 61 mm) are commonly used for HF choke baluns.
2. Wind 8–12 turns of the coaxial cable (RG-303 or smaller for compact winding, or the actual feedline if the toroid is large enough) through the toroid hole. Each pass through the center counts as one turn.
3. The more turns, the higher the common-mode impedance — but more turns also increase the inter-winding capacitance, which limits effectiveness at higher frequencies. 8–12 turns is a good compromise for a broadband HF choke.
4. Weatherproof the completed choke by enclosing it in a weatherproof enclosure or potting with silicone.

The common-mode impedance of a choke balun depends on the number of turns and the permeability of the ferrite material. A well-built choke balun using FT-240-43 ferrite (Mix 43) with 10–12 turns of coax can achieve over 3000 ohms of common-mode impedance across the 1–30 MHz HF range. This is enough to effectively suppress common-mode current on any practical installation.

Ferrite Bead Balun (W2DU Type)

An alternative construction uses a string of small ferrite beads threaded onto the coaxial cable at the feedpoint. Each bead presents a small amount of common-mode impedance, and the total of 50–100 beads provides the required high impedance. This design was popularized by Walt Maxwell W2DU and is commercially available from several manufacturers.

The W2DU balun is compact, lightweight, and highly effective. It uses type 73 ferrite beads (equivalent to Mix 73, which is a nickel-zinc ferrite with high permeability useful from 1–30 MHz). The commercial versions use 50 beads of this type threaded on a short length of coax. A 1:1 current balun version is used at dipole feedpoints; a 4:1 version uses a transmission line winding to achieve both balun action and impedance transformation.

4:1 Baluns for Ladder Line Connections

A 4:1 balun transforms a 200-ohm balanced impedance to 50-ohm unbalanced coax — or equivalently, a 50-ohm unbalanced input to a 200-ohm balanced output. This is useful when connecting:

• A folded dipole (natural feedpoint impedance ~300 ohms) through a 6:1 balun to 50-ohm coax
• A 450-ohm ladder line system to a 50-ohm transmatch in the shack (with a 9:1 or other ratio)
• A center-fed 2-element Yagi with 200-ohm feedpoint to 50-ohm coax

A 4:1 current balun is built as a transmission-line transformer: two short lengths of coaxial cable (or two-wire twisted line) are wound on a ferrite core and connected in a specific circuit topology that produces 4:1 impedance transformation while maintaining balanced-to-unbalanced conversion. The construction is somewhat more complex than a 1:1 choke balun; commercial units from reputable manufacturers (MFJ, Palomar, Balun Designs, Rugged Circuits) are available and recommended for power levels above 100 watts.

Choosing the Right Ferrite

The ferrite material (often called the "mix" number) determines the frequency range over which the core provides effective choking impedance. Choosing the wrong mix means the balun may work poorly at your operating frequency.

For most amateur HF dipole installations operating on 160 through 10 meters (1.8–30 MHz), Mix 43 is the first choice. It provides high choking impedance across the full HF range and is available in FT-240-43 toroids (2.4-inch diameter) that are large enough to wind 8–12 turns of RG-8X or RG-213 through.

For 2 meters (144 MHz) and 70 cm (432 MHz), Mix 61 provides much better choking effectiveness. Mix 43 becomes ineffective above about 50 MHz because its permeability falls off rapidly at higher frequencies.

For a broadband choke that works from 160 meters through 6 meters, some builders use a stack of two or three toroids of different mixes (Mix 43 + Mix 61, for example) to obtain good choking impedance across the complete range.

Ununs for End-Fed Antennas

An unun (Unbalanced-Unbalanced) is a transformer that converts between two different unbalanced impedances without requiring balanced ports. End-fed wire antennas are popular because they are easy to install — one end is at the feedpoint, the other end hangs in the air. But an end-fed wire antenna (not a true resonant end-fed half-wave, which is a special case) typically presents a very high impedance — several thousand ohms — at its feed end, and must be transformed to 50 ohms with an unun.

The most common unun ratios for end-fed antennas are 9:1 (transforms 450 ohms to 50 ohms) and 49:1 (transforms 2450 ohms to 50 ohms). These ratios are achieved with wound transmission-line transformers on ferrite cores. The 9:1 unun is appropriate for multi-band end-fed antennas (like the EFHW-type that use a half-wave resonant length on the lowest band), and the 49:1 unun is used for non-resonant random wire antennas where the feedpoint impedance is very high.

Installation Guidelines

Where to install the balun and how to weatherproof it:

• Mount the balun at the antenna feedpoint, not in the shack. A choke balun in the shack does not stop common-mode current on the feedline between the shack and the antenna — it only stops current below the choke. The common-mode current source is at the antenna feedpoint where the unbalanced coax meets the balanced antenna. The choke must go there.
• Weatherproof all connections at the feedpoint. Use self-amalgamating tape to seal the SO-239 or N-type connector junction. Cover the ferrite core if it is outdoors — ferrite absorbs water over time and its properties can change.
• Connect the balun case or enclosure to the antenna support structure if the support is metallic. Do not allow the balun body to dangle in the air on the coax connector alone — the mechanical load will eventually break the connector.
• Use the correct power rating. Commercial ferrite baluns are rated for specific power levels. The combination of high SWR and high power can generate significant heat in the ferrite, leading to thermal runaway and failure. Derate by 50% for operation with SWR above 2:1.