How does the biconical antenna achieve wideband impedance matching?

The biconical antenna achieves broadband behavior because its geometry creates a uniform transmission line structure. Schelkunoff's analysis (1943) treats the biconical antenna as a spherical transmission line: the two cones form the conductors, and the space between them carries the TEM mode. For an infinite biconical antenna, the characteristic impedance is purely real and frequency-independent: Z₀ = (η/π)·ln(cot(θ/2)) = 120·ln(cot(θ/2)) ohms, where η = 377Ω (free-space impedance) and θ is the half-cone angle measured from the axis. This is analogous to a coaxial line's Z₀ = (60/√ε_r)·ln(b/a) being frequency-independent. The infinite cone has no end reflections, so the input impedance equals the characteristic impedance at all frequencies, providing infinite bandwidth. For a finite biconical antenna of total length L: the cone truncation creates an open-circuit reflection at the cone end. This reflection returns to the feed point after a round-trip delay of 2L/c, creating impedance oscillations. The input impedance oscillates around Z₀ with period Δf = c/(2L). For L = 50 cm (typical for 30-300 MHz): Δf = 300 MHz. The oscillation amplitude depends on the half-cone angle: wider cones have lower Q reflections and smoother impedance. At θ = 45-60° half-angle, the impedance variation is ±30% of Z₀, yielding VSWR < 2:1 over a 10:1 bandwidth with a simple matching balun. This is the fundamental reason biconical antennas can cover 30-300 MHz (10:1 BW) in a single aperture.

What is antenna factor and how is it calibrated for biconical antennas?

Antenna factor (AF) converts the voltage measured at the antenna terminals to the incident electric field strength: E (V/m) = V_terminal (V) × AF (1/m), or in dB: E (dBμV/m) = V (dBμV) + AF (dB/m). For an ideal half-wave dipole: AF = (λ·η)/(4π·R_rad·Z_load)... simplified, AF is related to gain: AF (dB/m) = 20·log₁₀(f_MHz) - G(dBi) - 29.78 dB. Biconical antenna AF is typically: 20 MHz: AF ≈ 15 dB/m. 100 MHz: AF ≈ 25 dB/m. 300 MHz: AF ≈ 32 dB/m. The AF increases with frequency because the antenna's effective area decreases (A_eff = λ²·G/(4π), and G is roughly constant while λ² shrinks). Calibration methods per ANSI C63.5 and IEEE 149: 1) Standard Site Method (SSM): compare DUT biconical against three calibrated antennas on an open-area test site (OATS) or semi-anechoic chamber. Three-antenna method eliminates the need for a known standard. Uncertainty: ±1-2 dB. 2) Reference Antenna Method: compare against a calculable standard dipole. The half-wave dipole gain is known from theory: G = 1.64 (2.15 dBi). 3) GTEM cell method: known field inside a GTEM cell calibrates the antenna. Faster but potentially less accurate (±2-3 dB). Calibration data is provided as an AF-vs-frequency table, typically at 1 MHz intervals. This data is essential: using an uncalibrated antenna can introduce 5-10 dB measurement error, potentially causing a passing product to appear non-compliant or vice versa.

How do biconical antennas fit into EMC test configurations?

EMC compliance testing requires measuring electromagnetic emissions from equipment under test (EUT) across specific frequency ranges defined by standards: CISPR 16 / EN 55032 (IT equipment): 30-1000 MHz radiated emissions. FCC Part 15: 30 MHz to 40 GHz (depending on device class). MIL-STD-461G RE102: 2 MHz to 18 GHz. The biconical antenna covers the lower portion of these ranges: Typical configuration: Biconical antenna: 30-200 MHz (or 30-300 MHz). Log-periodic dipole array (LPDA): 200-1000 MHz (or 300-1000 MHz). Horn antenna: 1-18+ GHz. Test setup (CISPR 16-2-3): antenna distance 3, 5, or 10 m from EUT. Antenna height scanned from 1-4 m to find maximum emission. Both horizontal and vertical polarization measured (biconical rotated 90°). Turntable rotates EUT 360° in azimuth. Result compared to emission limit (e.g., EN 55032 Class B: 30 dBμV/m at 10 m for 30-230 MHz). The biconical antenna's omnidirectional azimuth pattern (±2 dB) ensures that emissions from any direction are captured with consistent sensitivity. Its calibrated AF converts measured terminal voltage directly to field strength. Measurement chain: E (dBμV/m) = V_receiver (dBμV) + AF (dB/m) + Cable_loss (dB). Example: receiver reads 35 dBμV, AF = 25 dB/m, cable loss = 2 dB at 100 MHz. E = 35 + 25 + 2 = 62 dBμV/m. Limit at 100 MHz: 30 dBμV/m at 10 m. Result: FAIL by 32 dB.

Antennas

Biconical Antenna

Q: What is antenna factor and how is it calibrated for biconical antennas?

Antenna factor (AF) converts the voltage measured at the antenna terminals to the incident electric field strength: E (V/m) = V_terminal (V) × AF (1/m), or in dB: E (dBμV/m) = V (dBμV) + AF (dB/m). For an ideal half-wave dipole: AF = (λ·η)/(4π·R_rad·Z_load)... simplified, AF is related to gain: AF (dB/m) = 20·log₁₀(f_MHz) - G(dBi) - 29.78 dB. Biconical antenna AF is typically: 20 MHz: AF ≈ 15 dB/m. 100 MHz: AF ≈ 25 dB/m. 300 MHz: AF ≈ 32 dB/m. The AF increases with frequency because the antenna's effective area decreases (A_eff = λ²·G/(4π), and G is roughly constant while λ² shrinks). Calibration methods per ANSI C63.5 and IEEE 149: 1) Standard Site Method (SSM): compare DUT biconical against three calibrated antennas on an open-area test site (OATS) or semi-anechoic chamber. Three-antenna method eliminates the need for a known standard. Uncertainty: ±1-2 dB. 2) Reference Antenna Method: compare against a calculable standard dipole. The half-wave dipole gain is known from theory: G = 1.64 (2.15 dBi). 3) GTEM cell method: known field inside a GTEM cell calibrates the antenna. Faster but potentially less accurate (±2-3 dB). Calibration data is provided as an AF-vs-frequency table, typically at 1 MHz intervals. This data is essential: using an uncalibrated antenna can introduce 5-10 dB measurement error, potentially causing a passing product to appear non-compliant or vice versa.

Q: How do biconical antennas fit into EMC test configurations?

EMC compliance testing requires measuring electromagnetic emissions from equipment under test (EUT) across specific frequency ranges defined by standards: CISPR 16 / EN 55032 (IT equipment): 30-1000 MHz radiated emissions. FCC Part 15: 30 MHz to 40 GHz (depending on device class). MIL-STD-461G RE102: 2 MHz to 18 GHz. The biconical antenna covers the lower portion of these ranges: Typical configuration: Biconical antenna: 30-200 MHz (or 30-300 MHz). Log-periodic dipole array (LPDA): 200-1000 MHz (or 300-1000 MHz). Horn antenna: 1-18+ GHz. Test setup (CISPR 16-2-3): antenna distance 3, 5, or 10 m from EUT. Antenna height scanned from 1-4 m to find maximum emission. Both horizontal and vertical polarization measured (biconical rotated 90°). Turntable rotates EUT 360° in azimuth. Result compared to emission limit (e.g., EN 55032 Class B: 30 dBμV/m at 10 m for 30-230 MHz). The biconical antenna's omnidirectional azimuth pattern (±2 dB) ensures that emissions from any direction are captured with consistent sensitivity. Its calibrated AF converts measured terminal voltage directly to field strength. Measurement chain: E (dBμV/m) = V_receiver (dBμV) + AF (dB/m) + Cable_loss (dB). Example: receiver reads 35 dBμV, AF = 25 dB/m, cable loss = 2 dB at 100 MHz. E = 35 + 25 + 2 = 62 dBμV/m. Limit at 100 MHz: 30 dBμV/m at 10 m. Result: FAIL by 32 dB.

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Wideband antenna formed by two conical conductors fed at their common apex. Impedance: Z₀ = 120·ln(cot(θ/2)) Ω (infinite cone). Typical: 20 to 300 MHz, 10:1 bandwidth, VSWR < 2:1, omnidirectional in azimuth. AF: 10 to 35 dB/m (frequency-dependent). Primary use: EMC compliance testing per CISPR 16, MIL-STD-461G, and FCC Part 15.

Range: 20–300 MHz

BW: 10:1

AF: 10–35 dB/m

Understanding Biconical Antennas

The biconical antenna is one of the few antenna geometries whose performance can be derived analytically. Schelkunoff's 1943 spherical-wave transmission line analysis showed that two infinite cones form a uniform transmission line with frequency-independent characteristic impedance. This means the input impedance is constant across all frequencies, providing theoretically infinite bandwidth. Practical finite-length biconical antennas truncate the cones, introducing end reflections that cause impedance oscillations, but the fundamental broadband character is preserved.

This broadband impedance behavior makes the biconical antenna the standard measurement instrument for EMC testing from 20 to 300 MHz. Every EMC test laboratory worldwide uses biconical antennas with calibrated antenna factors to convert measured terminal voltages to incident field strengths with traceable accuracy. The antenna's omnidirectional azimuth pattern ensures consistent sensitivity regardless of the equipment orientation on the turntable.

Impedance & Antenna Factor

Characteristic Impedance (infinite cone):
Z₀ = 120·ln(cot(θ_half/2)) Ω
θ = 30°: Z₀ = 158Ω
θ = 47°: Z₀ ≈ 100Ω
θ = 60°: Z₀ = 66Ω

Antenna Factor:
AF (dB/m) = 20·log(f_MHz) − G(dBi) − 29.78
E (dBμV/m) = V (dBμV) + AF + Cable Loss

Impedance Oscillation Period (finite cone):
Δf = c/(2L)
L = 50 cm: Δf = 300 MHz

EMC Antenna Coverage by Frequency

Antenna Type	Freq Range	Gain	AF Range	Standard
Biconical	20–300 MHz	0–4 dBi	10–35 dB/m	CISPR 16-1-4
LPDA	200–1,000 MHz	5–7 dBi	15–30 dB/m	CISPR 16-1-4
Horn	1–18 GHz	10–20 dBi	20–45 dB/m	CISPR 16-1-4
Ridged horn	1–40 GHz	8–15 dBi	25–50 dB/m	MIL-STD-461G
Active bicon.	30 MHz–6 GHz	−10–5 dBi	5–40 dB/m	Custom

Common Questions

Frequently Asked Questions

Why is it wideband?

Infinite cone = uniform spherical transmission line with Z₀ = 120·ln(cot(θ/2)). Frequency-independent. Finite truncation adds oscillations with period Δf = c/2L. Wide cone angles (θ > 45°) smooth oscillations for VSWR < 2:1 over 10:1 BW.

Antenna factor calibration?

SSM: three-antenna method on OATS (±1 to 2 dB). Reference dipole: compare to calculable G = 2.15 dBi. GTEM cell: fast but ±2 to 3 dB. AF table at 1 MHz intervals. Uncalibrated: 5 to 10 dB error.

EMC test setup?

3/5/10 m from EUT. Height scan 1 to 4 m. H and V polarization. Turntable 360°. E = V + AF + cable loss. Paired with LPDA (200+ MHz) and horn (1+ GHz) for full 30 MHz to 18 GHz coverage.

Related Terms

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EMC Testing

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