Why is the bicone antenna broadband?

An infinite biconical antenna has a characteristic impedance that depends only on the cone half-angle, not on frequency: Z = (120/pi) * ln(cot(theta/2)), where theta is the half-angle. This frequency-independent impedance provides a constant match across all frequencies. A finite bicone truncates the cones, which creates a low-frequency cutoff determined by the cone length (approximately lambda/4 at the lowest frequency). Above this cutoff, the impedance remains nearly constant, giving bandwidths of 10:1 or more. This is fundamentally different from resonant antennas like dipoles, which are impedance-matched only near their resonant frequency.

What is the difference between a bicone and a discone antenna?

A discone replaces the upper cone with a flat disc (ground plane), creating a monopole version of the bicone. The discone has only vertical polarization (vs. the bicone's horizontal polarization in the equatorial plane), unidirectional pattern in elevation (radiates only above the ground plane), and approximately half the gain of a bicone. Discones are more practical for base station and monitoring applications because they mount easily on a mast with the disc on top. Bicones are preferred for EMC testing (CISPR/ANSI standards) because of their more predictable and calibrated antenna factor.

How are bicone antennas used in EMC testing?

EMC compliance testing per CISPR 16, MIL-STD-461, and FCC Part 15 requires measuring radiated emissions from 30 MHz to 1 GHz. A calibrated biconical antenna covers 30-300 MHz (the lower portion), while a log-periodic dipole array covers 300 MHz to 1 GHz. The bicone antenna's calibrated antenna factor (AF) converts the received voltage to field strength: E(dBuV/m) = V(dBuV) + AF(dB/m). The antenna is placed at a specified distance (3m, 10m, or 30m) from the equipment under test in an anechoic chamber or open area test site (OATS). Measurement accuracy depends on the bicone's calibration uncertainty, typically +/- 1.5 to 2 dB.

Broadband Antennas

Bicone Antenna

/by-kohn an-ten-uh/ (biconical)

A Bicone Antenna is a broadband omnidirectional antenna consisting of two conical conductors placed apex-to-apex. Its impedance depends only on cone angle, not frequency, providing 10:1+ bandwidth. The standard antenna for EMC radiated emissions testing from 30-300 MHz per CISPR 16 and MIL-STD-461, and widely used in SIGINT and spectrum monitoring.

Category: Broadband Antennas

Bandwidth: 10:1+ (frequency-independent Z)

Application: EMC testing, SIGINT

Understanding Bicone Antennas

The bicone is one of the earliest examples of a frequency-independent antenna. Its broadband nature comes from the fact that it has no resonant dimension: an infinite bicone has impedance determined only by geometry (cone angle), not wavelength. Truncating the cones to a finite length creates a low-frequency cutoff (approximately lambda/4), but above that the impedance stays flat. This makes bicones essential for applications that must cover wide frequency ranges with a single antenna.

Bicone Antenna Design

Bicone Antenna:
A Bicone Antenna is a broadband omnidirectional antenna consisting of two conical conductors placed apex-to-apex. Its impedance depends only on cone angle, not frequency, providing...

Key specifications:
-300 MHz | 16 a | 1 m | 75 MHz

Gain: G = η_ap×4πA/λ²

Broadband Antenna Type Comparison

Antenna	Bandwidth	Gain	Polarization	Application
Bicone	10:1+	0-2 dBi	Horizontal (E-plane)	EMC 30-300 MHz
Discone	10:1+	0-2 dBi	Vertical	Monitoring, scanner
Log-periodic dipole	10:1+	5-8 dBi	Linear	EMC 300 MHz-1 GHz
Bow-tie	3:1	2-5 dBi	Linear	UWB, GPR
Spiral	10:1+	2-5 dBi	Circular	EW, direction finding

Key Equations

Friis transmission:
P_r = P_tG_tG_r(λ/4πd)²

Antenna gain:
G = η_ap × 4πA_eff/λ²

Beamwidth (3 dB):
θ ≈ 70λ/D degrees

Common Questions

Frequently Asked Questions

Why is it broadband?

Impedance depends only on cone angle, not frequency: Z = (120/pi)*ln(cot(theta/2)). Infinite bicone: frequency-independent. Finite: cutoff at f = c/(4*L_cone). Above cutoff, impedance stays flat. No resonant dimension = broadband.

Bicone vs. discone?

Discone replaces upper cone with flat disc (ground plane). Vertical polarization only. Easier mounting. Half the gain. Bicones preferred for EMC testing (calibrated antenna factor). Discones preferred for base station monitoring.

EMC testing use?

CISPR 16 / MIL-STD-461 / FCC Part 15 radiated emissions: bicone covers 30-300 MHz, log-periodic covers 300 MHz-1 GHz. Calibrated antenna factor converts voltage to field strength. Placed at 3/10/30m in anechoic chamber or OATS. Calibration uncertainty: +/- 1.5-2 dB.

Related Terms

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