What happens if I feed a dipole without a balun?

The coax outer conductor becomes part of the antenna. Current flows back down the outside of the shield, radiating in an unpredictable pattern that distorts the antenna's designed radiation pattern, changes its input impedance, and couples RF into the feedline where it can cause common-mode interference. The cable becomes sensitive to its routing and length, making the antenna's performance unrepeatable. A choke balun (ferrite beads or coiled coax) blocks this common-mode current by presenting high impedance to it while leaving the differential signal unaffected.

What is the difference between a voltage balun and a current balun?

A voltage balun forces equal and opposite voltages at the balanced port, but does not guarantee equal currents if the load is slightly unbalanced. A current balun (also called a choke balun) forces equal and opposite currents regardless of load asymmetry, providing superior common-mode rejection. Current baluns are preferred for antenna applications because real antennas are never perfectly balanced due to nearby structures, mast effects, and asymmetric feeds.

How do I specify a balun for a double-balanced mixer?

Mixer baluns require tight amplitude balance (less than 0.5 dB) and phase balance (less than 3 degrees) over the full operating bandwidth. Poor balance degrades LO-to-RF isolation, increases even-order spurious products, and raises the mixer's noise figure. For broadband mixers (2 to 18 GHz), Marchand baluns on alumina or LTCC substrates achieve 0.3 dB amplitude and 2 degree phase balance. For narrowband applications, coupled-line baluns on PCB are sufficient and much cheaper.

RF Design

Balun

BALanced-to-UNbalanced

Connect a coaxial cable directly to a dipole antenna and the outer shield becomes a parasitic radiator: current flows back down the outside of the braid, distorting the pattern, shifting the impedance, and coupling RF into equipment on the feedline. A balun prevents this by converting the single-ended coax signal into a balanced differential drive where the two conductor currents are equal and opposite, leaving no net current to leak onto the shield. The same conversion appears in every double-balanced mixer, differential amplifier input, and push-pull PA output in RF engineering.

Category: RF Design

Key Specs: Amplitude & phase balance

Common Ratios: 1:1, 1:4, 1:9

Forcing Symmetry at the Boundary

Balun Topology Comparison

Topology	Bandwidth	Amplitude Balance	Phase Balance	Impedance Ratio	Best For
Marchand (coupled-line)	3:1 to 5:1	<0.5 dB	<3°	1:1	MMIC mixers, broadband receivers
Lattice (LC)	2:1 to 3:1	<0.3 dB	<2°	1:1	Narrowband, IC integration
Guanella (TL)	10:1+	<1.0 dB	<5°	1:4, 1:9	HF/VHF antennas, high power
Choke (ferrite)	5:1 to 10:1	N/A (1:1)	N/A	1:1	Antenna feedpoint, EMC
Microstrip rat-race	~20%	<0.2 dB	<1°	1:1	Planar mixers, narrowband
Stacked spiral (CMOS)	2:1 to 4:1	<0.5 dB	<5°	1:1 to 1:4	RFIC, mmWave, 5G front-end

Quantifying Balance Performance

Common-mode rejection ratio (CMRR):
CMRR = 20·log₁₀(|V_diff| / |V_cm|) dB

Effect of amplitude imbalance ΔA and phase imbalance Δφ:
CMRR ≈ −20·log₁₀(√(ΔA² + Δφ²))
where ΔA is in dB/20 (linear) and Δφ is in radians

At 0.5 dB amplitude and 5° phase imbalance: CMRR ≈ 25 dB. At 0.2 dB and 2°: CMRR ≈ 35 dB. For mixer LO-to-RF isolation, you need at least 20 dB CMRR.

Common Questions

Frequently Asked Questions

What happens without a balun on a dipole?

Current flows on the coax shield exterior, making the cable a parasitic radiator. The pattern distorts, impedance shifts unpredictably, and RF couples into equipment. A choke balun (ferrite or coiled coax) blocks the common-mode current while passing the differential signal.

Voltage balun vs. current balun?

Voltage baluns force equal voltages but tolerate current imbalance from asymmetric loads. Current (choke) baluns force equal currents regardless of load asymmetry, giving superior common-mode rejection. Current baluns are preferred for antennas since real antennas are never perfectly balanced.

How do I spec a mixer balun?

Require <0.5 dB amplitude and <3° phase balance over the full band. Poor balance degrades LO-RF isolation, raises even-order spurs, and increases NF. Marchand baluns on alumina achieve 0.3 dB / 2° for broadband mixers.

Related Terms

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