How Is Antenna Gain Measured? A Clear Technical Explanation

　　How Is Antenna Gain Measured? A Clear Technical Explanation

　　Antenna gain is not a measure of “signal strength” (it doesn’t create more power) but of signal focusing ability—how well an antenna directs radio waves toward a target area instead of radiating them evenly in all directions. To quantify this, gain is measured as a relative value (not absolute) against standard reference antennas, with strict testing methods to ensure consistency across products. Below’s a step - by - step breakdown of how it works.

　　1. First, Understand the Core: Gain Is a “Relative” Metric

　　Unlike power (measured in watts) or voltage (in volts), gain has no absolute value—it compares an antenna’s performance to a reference antenna (a theoretical or physical model with known radiation characteristics). This is critical because:

　　An antenna with “10dBi gain” doesn’t mean it produces 10x more power; it means it focuses 10x more signal in a specific direction than the reference.

　　Without a reference, gain numbers are meaningless (e.g., “10dB” alone tells you nothing—you need to know the reference to interpret it).

　　2. Two Key Reference Standards for Measuring Gain

　　The industry uses two universal references to calculate antenna gain, each tied to a specific unit (dBi or dBd) that you’ll see on spec sheets:

　　A. Isotropic Radiator (Unit: dBi)

　　What it is: A theoretical, “perfect” antenna that radiates signal equally in all 3D directions (like a light bulb emitting light uniformly). It doesn’t exist in real life—but it’s the global standard for gain measurement because it provides a consistent baseline.

　　How it’s used: Most consumer and industrial antennas (including the patch antennas in your previous guide) use dBi (decibels relative to isotropic) as the unit. For example:

　　A 3dBi omnidirectional patch antenna directs 2x more signal (10^(3/10) ≈ 2) in its best direction than the isotropic reference.

　　A 10dBi directional patch antenna (for outdoor cameras) focuses ~10x more signal (10^(10/10) = 10) in its narrow beam than the isotropic model.

　　B. Half - Wave Dipole Antenna (Unit: dBd)

　　What it is: A real, simple antenna (two metal rods, each ¼ wavelength long) that radiates signal more strongly in horizontal directions (like a flashlight’s side - to - side beam) and weakly above/below. It’s a common reference for ham radio or legacy communication antennas.

　　How it’s used: dBd (decibels relative to dipole) compares gain to this physical antenna. Importantly, dBd values are “higher” than dBi for the same antenna—because the dipole itself has 2.15dBi of gain over the isotropic reference.

　　Conversion formula: Gain (dBi) = Gain (dBd) + 2.15

　　Example: A 5dBd ham radio antenna = 5 + 2.15 = 7.15dBi.

　　Note: Patch antennas (your focus earlier) rarely use dBd—stick to dBi for Wi-Fi, GPS, or IoT use cases.

　　3. Practical Testing Methods: How Manufacturers Measure Gain

　　To get accurate gain numbers, manufacturers use controlled environments and standardized equipment. The most common method is anechoic chamber testing:

　　Step 1: Set Up the Test Environment

　　An anechoic chamber is a room lined with foam pyramids that absorb radio waves (no reflections, no external interference—mimicking “free space”).

　　Place the antenna under test (e.g., a 6dBi patch antenna) at the center, and a reference antenna (calibrated to match the isotropic or dipole model) at a fixed distance (typically 1–10 meters, depending on frequency).

　　Step 2: Transmit and Compare Signals

　　Connect both antennas to a calibrated radio transmitter/receiver (signal generator + power meter).

　　First, transmit a signal with the reference antenna, and measure the power received at the test point (this is the “baseline” power).

　　Then, replace the reference with the antenna under test, transmit the same power signal, and measure the received power.

　　Step 3: Calculate Gain

　　Use the decibel formula for relative power:

　　Gain (dB) = 10 × log₁₀ (Received Power from Test Antenna / Received Power from Reference Antenna)

　　For example: If the test antenna’s received power is 4x higher than the isotropic reference, gain = 10 × log₁₀(4) ≈ 6dBi (matching the patch antenna spec in your earlier guide).

　　Key Controls for Accuracy

　　Frequency matching: Test at the antenna’s target frequency (e.g., 2.4GHz for Wi-Fi patch antennas)—gain changes with frequency.

　　Beamwidth mapping: For directional antennas (like 8–15dBi outdoor patch models), measure gain across all angles (360° horizontal, 180° vertical) to create a “gain pattern” (tied to the radiation pattern spec you learned before).

　　Environmental stability: Keep temperature, humidity, and cable length consistent—small changes can skew results.

　　4. How to Interpret Measured Gain Values (Avoid Misconceptions)

　　Now that you know how gain is measured, here’s how to use the numbers wisely (building on your patch antenna knowledge):

　　Higher gain ≠ better for all cases: A 15dBi directional patch antenna (measured against isotropic) has a narrow beamwidth (~60° horizontal), so it’s great for long - range outdoor cameras but terrible for covering a small office (it creates dead zones).

　　Match gain to your scenario:

　　Indoor Wi-Fi routers: 3–6dBi (omnidirectional, measured in dBi) → wide coverage for 100–200㎡.

　　Outdoor security cameras: 8–10dBi (directional, dBi) → focused signal for 50–100m range.

　　Ignore “unrealistic” claims: An antenna claiming “30dBi gain” for 2.4GHz Wi-Fi is impossible—physical limits (size, frequency) cap consumer patch antenna gain at ~15dBi. Such numbers likely skip the reference (e.g., no “dBi”/“dBd”) or use a non - standard baseline.

　　Final Summary

　　Antenna gain is measured as a relative value against two references:

　　dBi: Compares to a theoretical isotropic radiator (most common for patch antennas).

　　dBd: Compares to a real half - wave dipole (rare for Wi-Fi/IoT).

　　Manufacturers use anechoic chambers to test and calculate gain by comparing the antenna’s signal to the reference. When using the numbers, focus on matching the gain (and its unit) to your use case—not chasing the highest number. This ensures you get the coverage or range you need, just like selecting the right 3–6dBi or 8–10dBi patch antenna for your router or camera.