How does a bilog antenna achieve such wide bandwidth?

The bilog achieves 100:1 bandwidth (30 MHz to 3 GHz) by combining two inherently broadband antenna types. The biconical section uses two conical or triangular metallic elements fed at their apexes, creating a frequency-independent impedance characteristic over roughly a 10:1 bandwidth. Below 300 MHz, the biconical elements are electrically small to moderate in size and provide omnidirectional coverage. The log-periodic dipole array (LPDA) section uses a series of dipole elements of progressively decreasing length, spaced logarithmically along a boom. At any frequency, only the dipole elements near resonance (length approximately lambda/2) are active, creating a traveling-wave excitation that sweeps along the array with frequency. The LPDA achieves 10:1 or greater bandwidth with consistent gain (5-7 dBi) and impedance. A passive crossover network at approximately 200-300 MHz transitions from the biconical to the LPDA section. This network must maintain impedance matching (VSWR < 2:1) and smooth antenna factor variation through the transition. Well-designed bilogs achieve antenna factor variation of less than 1 dB through the crossover region.

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

Antenna factor (AF) converts the voltage measured at the antenna's output terminal to the electric field strength at the antenna's location: E (dBuV/m) = V (dBuV) + AF (dB/m). For emissions measurements, AF is critical because the regulatory limit is specified in field strength (dBuV/m), not received voltage. AF varies with frequency and must be calibrated for each individual antenna. Calibration methods per ANSI C63.5 and SAE ARP-958: (1) Standard Site Method (SSM): the antenna is measured on a validated Open Area Test Site (OATS) or semi-anechoic chamber against a reference antenna with known AF. Accuracy: +/-1.5 dB. (2) Three-Antenna Method: three antennas are measured in pairs, and the individual AFs are computed from the three insertion loss measurements without needing a reference standard. Accuracy: +/-1.0 dB. (3) Reference Antenna Method: comparison against a calibrated reference dipole at each frequency. Typical bilog AF values: 30 MHz: 20-25 dB/m, 100 MHz: 10-15 dB/m, 1 GHz: 22-28 dB/m, 3 GHz: 28-35 dB/m. The AF includes the effects of antenna gain, impedance mismatch, and cable loss (if included in the calibration). Recalibration is typically required every 1-3 years per ISO 17025 accreditation requirements.

When should you use a bilog versus separate biconical and LPDA antennas?

The bilog trades performance for convenience. Advantages of bilog: single antenna covers 30 MHz to 3 GHz without swapping, saving 5-15 minutes per test setup. No antenna swap means no cable reconnection errors, which eliminates a common source of measurement uncertainty (0.5-2 dB). Compact: one antenna, one tripod, one cable. Ideal for pre-compliance testing and screening measurements where speed matters more than ultimate accuracy. Advantages of separate antennas: the standalone biconical (30-300 MHz) typically has better-characterized omnidirectional pattern and lower VSWR than the biconical section of a bilog. The standalone LPDA (200 MHz to 6+ GHz) can extend to higher frequencies than a bilog and has higher gain (7-8 dBi vs. 5-6 dBi for bilog LPDA section). For accredited compliance testing per CISPR 16-1-4, some test labs prefer separate antennas because each can be independently calibrated with lower uncertainty. The separate LPDA also provides better pattern directivity for immunity testing. Cost comparison: a quality bilog costs 3000-8000 USD, while a biconical (1000-2000) plus LPDA (1500-3000) totals 2500-5000 USD but requires two calibrations. For labs performing high-volume testing: bilog saves time. For accredited compliance labs: separate antennas may provide lower measurement uncertainty.

Antenna / EMC

Bilog Antenna

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

Antenna factor (AF) converts the voltage measured at the antenna's output terminal to the electric field strength at the antenna's location: E (dBuV/m) = V (dBuV) + AF (dB/m). For emissions measurements, AF is critical because the regulatory limit is specified in field strength (dBuV/m), not received voltage. AF varies with frequency and must be calibrated for each individual antenna. Calibration methods per ANSI C63.5 and SAE ARP-958: (1) Standard Site Method (SSM): the antenna is measured on a validated Open Area Test Site (OATS) or semi-anechoic chamber against a reference antenna with known AF. Accuracy: +/-1.5 dB. (2) Three-Antenna Method: three antennas are measured in pairs, and the individual AFs are computed from the three insertion loss measurements without needing a reference standard. Accuracy: +/-1.0 dB. (3) Reference Antenna Method: comparison against a calibrated reference dipole at each frequency. Typical bilog AF values: 30 MHz: 20-25 dB/m, 100 MHz: 10-15 dB/m, 1 GHz: 22-28 dB/m, 3 GHz: 28-35 dB/m. The AF includes the effects of antenna gain, impedance mismatch, and cable loss (if included in the calibration). Recalibration is typically required every 1-3 years per ISO 17025 accreditation requirements.

Q: When should you use a bilog versus separate biconical and LPDA antennas?

The bilog trades performance for convenience. Advantages of bilog: single antenna covers 30 MHz to 3 GHz without swapping, saving 5-15 minutes per test setup. No antenna swap means no cable reconnection errors, which eliminates a common source of measurement uncertainty (0.5-2 dB). Compact: one antenna, one tripod, one cable. Ideal for pre-compliance testing and screening measurements where speed matters more than ultimate accuracy. Advantages of separate antennas: the standalone biconical (30-300 MHz) typically has better-characterized omnidirectional pattern and lower VSWR than the biconical section of a bilog. The standalone LPDA (200 MHz to 6+ GHz) can extend to higher frequencies than a bilog and has higher gain (7-8 dBi vs. 5-6 dBi for bilog LPDA section). For accredited compliance testing per CISPR 16-1-4, some test labs prefer separate antennas because each can be independently calibrated with lower uncertainty. The separate LPDA also provides better pattern directivity for immunity testing. Cost comparison: a quality bilog costs 3000-8000 USD, while a biconical (1000-2000) plus LPDA (1500-3000) totals 2500-5000 USD but requires two calibrations. For labs performing high-volume testing: bilog saves time. For accredited compliance labs: separate antennas may provide lower measurement uncertainty.

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Hybrid broadband antenna combining a biconical dipole (30–300 MHz) with a log-periodic dipole array (200 MHz–3 GHz) in a single structure. Passive crossover blends sections at 200–300 MHz. Provides continuous 30 MHz to 3 GHz coverage for CISPR 16, MIL-STD-461, and FCC Part 15 emissions and IEC 61000-4-3 immunity testing without antenna swaps.

Range: 30 MHz – 3 GHz

Gain: 5–7 dBi (LPDA section)

VSWR: <2:1 across band

Understanding Bilog Antennas

The bilog antenna solves a practical problem in EMC testing: radiated emissions measurements span 30 MHz to several GHz, but no single antenna element type covers that entire range with adequate performance. The bilog integrates a biconical section (inherently broadband due to its frequency-independent impedance geometry) with a log-periodic dipole array (broadband through its progressively-scaled resonant element architecture), connected through a passive crossover network.

This hybrid approach provides continuous coverage with a single antenna, eliminating the need for antenna swaps during a test sweep. The antenna factor (AF) calibration data, unique to each antenna, converts measured voltage to regulatory field-strength units (dBμV/m).

Antenna Factor Conversion

Field Strength Measurement:
E (dBμV/m) = V (dBμV) + AF (dB/m)

Typical AF by Frequency:
30 MHz: 20–25 dB/m
100 MHz: 10–15 dB/m
1 GHz: 22–28 dB/m
3 GHz: 28–35 dB/m

Calibration Accuracy:
SSM: ±1.5 dB | Three-antenna: ±1.0 dB
Recal interval: 1–3 years (ISO 17025)

Bilog vs. Separate Antennas

Factor	Bilog (Integrated)	Separate Bicon + LPDA
Coverage	30 MHz – 3 GHz	30 MHz – 6+ GHz
Setup time	Single antenna, fast	Swap required at ~300 MHz
Uncertainty	Crossover region +0.5 dB	Lower (independent cal)
LPDA gain	5–6 dBi	7–8 dBi (standalone)
Cost	$3k–$8k (1 cal)	$2.5k–$5k (2 cals)
Best for	Pre-compliance, screening	Accredited compliance labs

EMC Standards Coverage

Standard	Test Type	Freq Range	Bilog Suitable?
CISPR 16 / EN 55032	Radiated emissions	30 MHz – 1 GHz	Yes
FCC Part 15	Radiated emissions	30 MHz – 40 GHz	To 3 GHz
MIL-STD-461 RE102	Radiated emissions	10 kHz – 18 GHz	30 MHz–3 GHz
IEC 61000-4-3	Radiated immunity	80 MHz – 6 GHz	To 3 GHz

Common Questions

Frequently Asked Questions

How does it cover 100:1 bandwidth?

Biconical: frequency-independent impedance (30–300 MHz). LPDA: progressively-scaled resonant elements (200 MHz–3 GHz). Passive crossover blends at 200–300 MHz with <1 dB AF variation.

What is antenna factor?

AF converts terminal voltage to field strength: E(dBμV/m) = V(dBμV) + AF(dB/m). Calibrated per ANSI C63.5 or SAE ARP-958. Accuracy ±1.0–1.5 dB. Recalibrated every 1–3 years.

Bilog vs. separate antennas?

Bilog: faster (no swap), one cable, pre-compliance ideal. Separate: lower uncertainty, higher LPDA gain (7–8 vs. 5–6 dBi), extends to 6+ GHz. Accredited labs may prefer separate for lower measurement uncertainty budget.

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