Balun Symbol
Definition: The Balun symbol represents a transformer-based or transmission-line device that converts between a balanced signal (two conductors carrying equal and opposite signals relative to ground) and an unbalanced signal (one signal conductor referenced to ground), drawn in RF and telecommunications schematics as a rectangular block with an Unbalanced single-ended pin on one side and Bal+ and Bal− differential pins on the other, used in antenna feed systems, RF circuits, and audio distribution per ANSI/IEEE 315-1975 and IEC 60617 conventions.
Also known as: BAL-UN transformer, balancing transformer, balanced-to-unbalanced transformer, RF balun, antenna balun, transmission-line transformer.
What the Balun symbol means
The Balun symbol in a circuit diagram marks the transition point between a single-ended (unbalanced) transmission line — such as a 50 Ω coaxial cable — and a balanced two-wire system — such as a dipole antenna feedline or differential signal pair. The Unbalanced pin represents the coaxial or single-ended side referenced to ground, while Bal+ and Bal− represent the two equal-amplitude, phase-opposite conductors of the balanced side.
Baluns appear in RF schematics wherever an unbalanced RF source or receiver must connect to a balanced antenna or load. Without a balun, common-mode current flows on the outside of the coaxial shield, causing RF interference, pattern distortion on a dipole antenna, and feed-line radiation. The balun suppresses this common-mode current and ensures pure differential-mode signal flow on the balanced side. Baluns are also used in audio, video, and data circuits to reject common-mode noise on balanced lines.
How to identify the Balun symbol
The Balun symbol is a rectangle labelled 'BALUN' or 'BAL' with the impedance ratio noted as an annotation (e.g., '1:1', '4:1', '9:1'). The Unbalanced pin exits the left side of the block (single coaxial/unbalanced connection). Two pins exit the right side: Bal+ (top right) and Bal− (bottom right) representing the balanced differential output pair. Some schematic representations show a coil or transformer symbol inside the block. The impedance ratio annotation (e.g., 50 Ω : 200 Ω = 1:4 or 1:1) is critical information in the balun schematic symbol.
Function in a circuit
A balun performs two functions simultaneously: impedance transformation (if the ratio is not 1:1) and conversion between balanced and unbalanced modes. A 1:1 current balun forces equal currents to flow in both balanced conductors without changing impedance. A 4:1 voltage balun transforms a 50 Ω unbalanced source to a 200 Ω balanced load (or 300 Ω feed line), matching a half-wave dipole (72 Ω) to a 50 Ω coaxial feed with a 1.5:1 impedance transformation. Baluns are implemented as wound toroid transformers, coaxial choke baluns, or transmission-line transformers depending on frequency, power level, and bandwidth requirements.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 covers transformer and RF component symbols; a balun is represented as a two-winding transformer symbol with the balanced side explicitly marked, or as a rectangular function block with BAL annotation. IEC 61196 covers RF cable assemblies that include balun connectors. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 Section 11 covers transformer symbols; baluns are shown as transformer blocks with balanced-output labelling or as rectangular function blocks with BAL/UNBAL labelling. The IEEE standards for antenna systems (IEEE 149, IEEE 145) describe balun use in antenna measurement setups. |
| Key difference | IEC and ANSI/IEEE both represent baluns as transformer blocks or rectangular function blocks. IEC uses the two-winding transformer symbol with a dot convention for polarity; ANSI may show a simplified block with BAL/UNBAL labels. No fundamental glyph difference exists; both convey the balanced vs. unbalanced distinction through pin labelling and annotation. |
Terminals / pins
| Pin | Name |
|---|---|
| unbal | Unbalanced |
| bal_pos | Bal+ |
| bal_neg | Bal- |
Typical values
Impedance ratios: 1:1, 4:1, 9:1, 16:1; frequency range: HF baluns 1–30 MHz, VHF/UHF 50–1,000 MHz, wideband baluns 1–500 MHz; power handling: QRP 5 W, amateur radio 100–1,500 W, broadcast kW range; unbalanced impedance: 50 Ω (coaxial); balanced impedance: 50 Ω, 200 Ω, 300 Ω, 450 Ω (ladder line); insertion loss: <0.5 dB (well-designed ferrite core).
Where the Balun symbol is used
- Dipole and Yagi antenna feed connections converting the balanced dipole to unbalanced 50 Ω coaxial cable
- Amateur radio HF and VHF antenna systems including the G5RV doublet, off-centre fed dipoles, and loop antennas
- Broadcast FM and TV antenna systems matching balanced folded dipoles to 50 Ω or 75 Ω transmission lines
- Audio and professional A/V systems where balanced XLR lines connect to unbalanced RCA or phono inputs
- EMC and conducted emissions test setups where baluns decouple common-mode current from EUT feed cables
- ADSL and DSL modem line interfaces converting balanced telephone pairs to unbalanced circuit boards
- RFID and NFC reader antenna circuits matching a balanced loop antenna to an unbalanced 50 Ω reader IC
Example
In an HF dipole antenna schematic, a 1:1 current balun symbol appears at the feed point of a half-wave dipole. The Unbalanced pin connects to the centre conductor of a 50 Ω RG-8 coaxial cable feed line running to the transceiver. Bal+ and Bal− connect to the two dipole arms. The balun prevents RF from flowing back down the coaxial shield as common-mode current, eliminating TVI (television interference) from the antenna system.
Key facts
- A balun converts between a balanced differential signal (Bal+, Bal−) and an unbalanced single-ended signal (Unbalanced/coaxial), also performing impedance transformation if the ratio is not 1:1.
- The three pins on the schematic symbol are: Unbalanced (single-ended, left), Bal+ (differential positive, top-right), and Bal− (differential negative, bottom-right).
- Common impedance ratios are 1:1 (choke balun, 50 Ω unbalanced to 50 Ω balanced), 4:1 (50 Ω to 200 Ω), and 9:1 (50 Ω to 450 Ω), matching different antenna feed impedances.
- A current balun (choke type) forces equal currents in both balanced conductors; a voltage balun forces equal voltages. Current baluns are generally preferred for antenna feed applications because they perform correctly under any load impedance.
- Without a balun at a dipole feed point, RF current flows on the outside of the coaxial shield, causing the feed line to radiate, distorting the antenna pattern, and creating EMI on other equipment.
- Baluns are implemented as ferrite toroid wound transformers, coaxial choke baluns (coiled or sleeved coax), or transmission-line transformers depending on power level, frequency, and bandwidth requirements.
- Insertion loss of a well-designed ferrite-core balun is less than 0.5 dB across its operating band; poorly designed baluns exhibit high losses at high VSWR loads.
- The balun schematic designator is typically T (transformer) or BL (balun), e.g., T1 or BL1, per ANSI/IEEE 315-1975.
Frequently asked questions
What does the balun symbol look like in a circuit diagram?
The balun symbol is a rectangle labelled 'BALUN' or 'BAL' with an impedance ratio annotation (e.g., '1:1', '4:1'). The Unbalanced (coaxial/single-ended) pin exits the left side, and two differential pins (Bal+ on top-right and Bal− on bottom-right) exit the right side. Some representations show a coil or transformer symbol inside the block.
What does the balun symbol mean in a schematic?
The balun symbol marks the transition between an unbalanced (single-ended, coaxial) signal path and a balanced (differential, two-wire) signal path. It indicates that common-mode currents are suppressed and the impedance is transformed (if the ratio is not 1:1) between the two signal domains.
What is the difference between a 1:1 balun and a 4:1 balun?
A 1:1 balun converts between balanced and unbalanced modes without changing impedance — a 50 Ω unbalanced source connects to a 50 Ω balanced load. A 4:1 balun transforms a 50 Ω unbalanced source to a 200 Ω balanced load, matching a centre-fed dipole (approximately 200 Ω at resonance when fed with open-wire line) to a 50 Ω coaxial cable.
What is the difference between a balun and an unun?
A balun converts between balanced and unbalanced modes (BALanced-to-UNbalanced). An unun converts impedance between two unbalanced (single-ended) circuits without any balanced-to-unbalanced conversion — for example, a 9:1 unun matches a 50 Ω coaxial feeder to a 450 Ω end-fed antenna wire, both of which are unbalanced. In a schematic, both appear as transformer blocks but the unun's output pins both reference ground, unlike the balun's differential (floating) output.
What standard defines the balun symbol?
The balun symbol follows ANSI/IEEE 315-1975 transformer symbol conventions (Section 11) and IEC 60617-06 for transformer/RF component symbols. IEEE 149-1979 (Standard Test Procedures for Antennas) and IEEE 145-1993 (Antenna Definitions) describe balun usage in antenna systems. No single standard defines an exclusive balun symbol; the rectangular block with BAL/UNBAL labelling is a de-facto convention in RF schematics.
Why is a balun needed at a dipole antenna feed point?
A dipole antenna is a balanced device — its two arms carry equal and opposite RF currents. A coaxial cable is an unbalanced device — only the centre conductor carries the signal current; the shield provides the return. Without a balun, the outside of the coaxial shield forms a third conductor and carries RF current back down the feed line, causing the shield to radiate, distorting the antenna radiation pattern, and inducing interference on nearby equipment.
What are the three pins on the balun schematic symbol?
The three pins are: Unbalanced (single pin, left side of the block) — connects to the coaxial or single-ended 50 Ω source/load; Bal+ (top-right) — the positive/first conductor of the balanced differential pair; and Bal− (bottom-right) — the negative/second conductor of the balanced differential pair. Bal+ and Bal− carry equal-amplitude, phase-opposite signals relative to each other.
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