Antenna Factor & Gain Calculator
Perform fast bidirectional conversion between Antenna Gain (dBi) and Antenna Factor (AF) for 50Ω systems. Also includes an auxiliary utility to calculate the actual electric field strength from EMI receiver voltage readings.
Formula: AF (dB/m) = 20 * log10(f_MHz) - G_dBi - 29.78
*This conversion formula assumes the antenna input is perfectly matched to a 50Ω RF system, excluding reflection loss.
Auxiliary Utility: Field Strength (E) Calculation
Measurement Inputs
Calculated Field Strength (E)
Core Concepts: Antenna Gain vs. Antenna Factor
Antenna Gain (dBi)
The language of Telecom engineers. It measures the antenna's ability to focus radiated power in a specific direction. A higher value means a more focused "beam" or higher directional sensitivity.
Antenna Factor (AF, dB/m)
The language of EMC engineers. It measures the "conversion efficiency" of an antenna acting as a sensor. It's the logarithmic ratio of the incident Electric Field (E) to the output Voltage (V). A lower AF value indicates higher receiving efficiency.
Deep Dive: Why Frequency Matters in the Formula
Even if an antenna's Gain remains constant, its physical ability to capture spatial energy (effective aperture) drastically shrinks as frequency increases. This happens because higher frequencies have shorter wavelengths, resulting in a smaller physical capture area. Therefore, a 20*log10(f) compensation term must be introduced, which forms the physical core of the logarithmic conversion formula.
Engineering Trivia: Why isn't Biconical Antenna Gain flat?
A typical 30-300MHz biconical antenna has very low gain at 30MHz (down to -15 dBi) but rises to +2 dBi at 200MHz. This is because, constrained by the physical dimensions of an anechoic chamber, the antenna operates in a highly inefficient "electrically short" state at low frequencies.
However, this creates a brilliant engineering coincidence! Since AF ∝ 20*log(f) - Gain, as frequency increases, the 20*log(f) term grows, but the Gain also grows. By subtracting the two, the enormous variance is effectively canceled out, resulting in a much flatter, shallow U-shaped Antenna Factor (AF) curve across the band. This flatness is highly desirable as it stabilizes the dynamic range requirements of EMI receivers.
Understanding Field Strength Auxiliary Calculation
In radiated emissions testing, EMI receivers and spectrum analyzers measure the RF voltage present at their input terminals. However, EMC standards (like FCC Part 15 or CISPR 32) specify limits in terms of the actual Electric Field Strength in the air.
- Receiver Voltage (V): The raw reading from your instrument in dBµV.
- Antenna Factor (AF): Provided by the antenna manufacturer. It characterizes how effectively the antenna converts the incident electric field into a voltage at its connector. It is frequency-dependent.
- Cable Loss: High-frequency RF signals attenuate as they travel through coaxial cables. This loss must be added back to the measurement.
- Preamp Gain: If an external Low Noise Amplifier (LNA) is used to boost the signal before it reaches the receiver, its gain must be subtracted from the final calculation.
Calculator quality evidence
Purpose of this calculator
Use the antenna-factor panel to sanity-check the theoretical relation between antenna gain, frequency, and AF before replacing it with the antenna's accredited calibration data.
Formula and variables
For the displayed 50 Ω theoretical estimate: AF(dB/m) ≈ 20·log10(fMHz) − G(dBi) − 29.78. Field strength is then E(dBµV/m) = V(dBµV) + AF + cable loss − preamp gain.
Physical assumptions and scope
The gain-to-AF estimate assumes a matched 50 Ω system, free-space far-field interpretation, and idealized gain. Cable loss and preamp gain are explicit corrections in the field-strength panel; antenna calibration, mismatch, balun, and site factors are not inferred.
Valid input scope
Use positive frequency and numeric gain or AF values within the antenna's characterized band. For compliance work, use the calibration table and measurement geometry instead of treating the theoretical curve as a certificate.
Independently checked test vectors
- 100 MHz, 0 dBi → 10.22 dB/m AF(forward)
- 200 MHz, 2 dBi → 14.24 dB/m AF(forward)
Conversion-specific misuse to avoid
Do not mix dBi gain with dB/m antenna factor, or add cable loss twice. A calculated AF is a theory check; accredited calibration data takes precedence for a test report.
Source and content review
- Keysight: Antenna FactorsSupports the relation between measured voltage, antenna factor, and electric field, with calibration taking precedence over theory.
Content review date：