If you want to design a reliable communication system, you really need to understand how an antenna sends and receives energy. The radiation pattern shows how an antenna spreads its power out in space. Gain, on the other hand, tells you how well it focuses energy in a certain direction compared to some ideal reference.
Radiation pattern and gain together give you a clear picture of what to expect from an antenna out in the real world.
Engineers and technicians use these measurements to pick the right antennas for the job, improve coverage, and cut down on interference. By looking at the main lobe, sidelobes, beamwidth, and gain values, they can predict performance before installing anything.
Accurate measurement helps ensure the antenna actually meets its design goals, whether it’s for satellites, radar, or wireless networks.
Modern test setups—like compact antenna ranges and anechoic chambers—let us evaluate antennas precisely in controlled environments. These places make it possible to measure patterns and gain without interference from other objects, so you get data you can actually trust for both development and verification.
Fundamentals of Antenna Radiation Patterns
An antenna’s radiation pattern shows how it sends or receives energy in different directions. These patterns help you figure out coverage, signal strength, and how well the antenna focuses energy where you actually want it. They’re closely tied to gain, directivity, and the beam’s shape.
Understanding Radiation Patterns
A radiation pattern is basically a graph or mathematical description of how an antenna radiates energy into space. You can show it in two or three dimensions.
Usually, people normalize the pattern so the maximum value is 0 dB. That makes it easier to compare different shapes. The main lobe points to the strongest direction, and side lobes or back lobes show where unwanted radiation leaks out.
Radiation patterns can be omnidirectional—same strength in all horizontal directions—or directional, focusing energy in just one direction. Directional antennas usually have higher gain since they push energy into a smaller area.
Engineers match these patterns to applications, like using wide coverage for broadcasting or narrow beams for point-to-point links.
Azimuth and Elevation Patterns
People usually describe radiation patterns in two main planes:
| Plane | Description | Common Use |
|---|---|---|
| Azimuth | Horizontal cut of the pattern, viewed from above | Shows coverage around the antenna |
| Elevation | Vertical cut of the pattern, viewed from the side | Shows vertical beam shape and tilt |
An azimuth pattern helps you see how well an antenna covers different compass directions. This matters a lot for base stations or broadcast antennas.
An elevation pattern shows how energy spreads above and below the antenna’s horizon. That’s important for controlling coverage distance and avoiding interference with nearby systems.
If you analyze both patterns together, you get a much better idea of the antenna’s full three-dimensional radiation characteristics.
Directivity and Beamwidth
Directivity tells you how concentrated the radiation is in a particular direction compared to an isotropic radiator. Higher directivity means the antenna focuses more energy into a specific spot, boosting signal strength that way.
Beamwidth is just the angular width of the main lobe, usually measured where the signal drops by 3 dB from the peak.
A narrow beamwidth increases directivity but shrinks the coverage area. A wide beamwidth covers more area but lowers directivity.
Choosing the right mix of directivity and beamwidth really depends on the job, available bandwidth, and interference around you. You want to use transmitted power efficiently and get the best coverage you can.
Antenna Gain and Efficiency Concepts
An antenna’s performance comes down to how well it directs energy and how much of the input power it actually radiates. These things determine how well it can send or receive signals in any direction.
Definition of Antenna Gain
Antenna gain describes how much power an antenna can focus in a certain direction compared to a reference antenna. Usually, the reference is an isotropic radiator (which radiates equally in all directions) or a half-wave dipole.
We express gain in dBi (isotropic) or dBd (dipole). For example:
| Gain Type | Reference Antenna | Example Value |
|---|---|---|
| dBi | Isotropic | 6 dBi |
| dBd | Half-wave dipole | 3 dBd |
Gain isn’t the same as amplification. It just means the antenna can concentrate radiated energy in certain directions, making the signal stronger there. High-gain antennas have narrower main beams, while low-gain ones spread energy out more evenly.
Antenna Efficiency
Antenna efficiency is the ratio of radiated power to the total input power. Losses happen because of conductor resistance, dielectric materials, and impedance mismatch.
Here’s the formula:
Efficiency (%) = (Radiated Power / Input Power) × 100
So if you feed an antenna 10 W and it radiates 8 W, it’s 80% efficient.
There are two main types:
- Radiation efficiency – covers ohmic and dielectric losses.
- Total efficiency – includes both radiation efficiency and mismatch losses from impedance differences.
High efficiency matters a lot for battery-powered and low-power systems. Any wasted energy just means less range and worse performance.
Relationship Between Gain and Directivity
Directivity tells you how well an antenna focuses energy in one direction compared to an isotropic radiator, without worrying about losses. Gain takes both directivity and efficiency into account:
Gain = Directivity × Efficiency
If an antenna’s directivity is 10 dBi and its efficiency is 0.8 (80%), its gain comes out to:
10 dBi + 10·log10(0.8) ≈ 9 dBi
So, even with high directivity, poor efficiency can drag down the gain. That’s why you really need to look at both numbers when you’re judging performance.
Key Parameters Affecting Antenna Performance
Several measurable factors affect how well an antenna transmits or receives signals. These include the electric field’s orientation, how efficiently power transfers, and the range of frequencies where the antenna works well. Each parameter can impact signal quality, coverage, and system reliability.
Polarization
Polarization describes the electric field’s orientation in the radiated wave. You’ll usually see linear (vertical or horizontal) and circular polarization.
Matching the transmitter and receiver’s polarization is crucial. If you don’t, you can lose a lot of signal—sometimes more than 20 dB.
Circular polarization can help if the antennas move around or rotate, but it might have slightly less gain than a perfectly aligned linear system.
For satellite communication, circular polarization is often the go-to because it handles signal path rotation. For ground-based links, linear polarization is more common since the antennas are easier to build and more efficient when aligned.
VSWR and Impedance
Voltage Standing Wave Ratio (VSWR) measures how well the antenna’s impedance matches the transmission line, usually 50 ohms. A perfect match is a VSWR of 1:1.
High VSWR means reflected power, which lowers efficiency and can even damage transmitters if the power’s high enough. Most systems can live with a VSWR below 2:1.
You can fix impedance mismatch by tuning the antenna, using matching networks, or changing the feed point location.
| VSWR | Return Loss (dB) | Power Reflected (%) |
|---|---|---|
| 1.0 | ∞ | 0 |
| 1.5 | 14.0 | 4 |
| 2.0 | 9.5 | 11 |
Keeping impedance matching right ensures maximum power transfer and steady performance across the frequency range.
Bandwidth Considerations
Bandwidth is just the range of frequencies where an antenna meets its targets for gain, VSWR, and pattern.
Narrowband antennas work best over a small frequency range, so they’re good for single-frequency systems. Wideband designs can handle multiple services or frequency-agile radios, but they’re usually more complicated.
You can describe bandwidth as absolute bandwidth (in MHz) or fractional bandwidth (percentage of the center frequency).
Things like temperature, nearby objects, and even how you mount the antenna can nudge the usable bandwidth a bit. Designers usually make sure the antenna’s bandwidth covers the whole operating range, just to be safe.
Antenna Measurement Techniques and Setups
Getting accurate antenna measurements depends on controlling the test environment, knowing what each part does, and making sure the signal conditions fit the test. Key factors include how you position the antennas, the measurement region, and how you create a stable, uniform electromagnetic field.
Antenna Under Test and Source Antenna
The Antenna Under Test (AUT) is the one you’re checking for pattern, gain, and efficiency. You mount it on a positioner so you can rotate it in azimuth and elevation.
The source antenna sends out the reference signal. Usually, this is a calibrated horn or some other standard antenna with known performance. You keep the source fixed so it always lights up the AUT the same way.
You align both antennas so their main beams face each other. The distance between them depends on whether you’re in the near-field or far-field region. In controlled environments like anechoic chambers, absorbers on the walls cut down reflections, making it feel like free space.
Near-Field and Far-Field Regions
The near-field region is close to the antenna, usually within less than 3λ (three wavelengths). Here, the electromagnetic field hasn’t settled into a stable pattern yet. Measurements in this zone need some math to convert them into far-field results.
The far-field region is where the wavefront flattens out and the radiation pattern is fully formed. You can estimate the far-field distance with:
[
R_{ff} \approx \frac{2D^2}{\lambda}
]
where D is the largest antenna dimension and λ is the wavelength. Far-field measurements are easier for direct pattern recording since you don’t need to do any conversions.
Picking the right region is crucial for accuracy. Large antennas often need near-field testing because far-field distances would be huge and just not practical.
Plane Wave Generation
For good pattern and gain measurements, you need a plane wave—a wavefront that’s uniform and has constant phase across the test area. In outdoor far-field setups, you get this naturally if you’re far enough away.
In compact ranges, a reflector or lens system turns the spherical wave from the source antenna into a plane wave. The feed system—usually a horn or array—lights up the reflector with a clean beam.
The plane wave’s quality depends on how well the reflector’s designed, how you align things, and how well you calibrate the feed. Any wavefront distortion can mess up your measurements of gain, beamwidth, or sidelobes.
Antenna Test Environments and Ranges
You need controlled environments to measure antennas accurately, since you want to minimize interference and reflections. Test facilities give you predictable conditions, so you can measure things like pattern, gain, and efficiency with confidence.
Anechoic Chambers
An anechoic chamber is a shielded room lined with radio-frequency (RF) absorbing material. These absorbers kill reflections from the walls, floor, and ceiling, making it feel like free space indoors.
The chamber blocks outside RF signals, so you don’t get interference from nearby transmitters. This isolation is especially important for measuring weak signals or small tweaks in antenna performance.
Key features include:
- Absorber panels: Usually pyramidal foam coated with conductive material.
- Turntable or positioner: Lets you rotate the antenna under test (AUT) in azimuth and elevation.
- Measurement equipment: Vector network analyzers, receivers, and signal generators.
Anechoic chambers are great for repeatable measurements because you don’t have to worry about weather, ground reflections, or multipath effects.
Compact Range Reflectors
A compact range uses a specially shaped metallic reflector to create a plane-wave test zone in a small space. The reflector turns spherical waves from a feed antenna into parallel rays, so you get far-field conditions indoors.
The compact range reflector is usually offset-fed to cut down on blockage and scattering. Its surface accuracy really matters—any imperfections can distort the wavefront.
Advantages include:
- Needs less space than a traditional far-field range.
- Lets you test high frequencies without huge distances.
- Supports large antennas you couldn’t fit in a chamber of equal size.
People often use compact ranges for satellite, radar, and high-gain antenna testing, especially when they need really precise beam measurements.
Indoor and Outdoor Test Ranges
Indoor ranges include anechoic chambers and compact ranges. These spaces offer controlled conditions and keep out environmental changes. They work best for precision measurements and research.
Outdoor ranges use open-air setups. You’ll often see large separation distances to reach far-field conditions. Engineers manage ground reflections by using elevated towers or slant ranges.
Comparison Table:
| Feature | Indoor Ranges | Outdoor Ranges |
|---|---|---|
| Weather Impact | None | Yes |
| Space Requirements | Moderate to High | High |
| RF Isolation | Excellent | Limited |
| Typical Use | Precision lab measurements | Large antennas, long distances |
Some projects need outdoor ranges for testing very large antennas. In those cases, you have to pick the site carefully to avoid interference and unwanted reflections.
Procedures for Radiation Pattern and Gain Measurement
Accurate antenna testing takes a careful setup, controlled conditions, and precise measurement techniques. You need to prepare the test system, capture reliable data, and watch out for anything that could distort the results.
Calibration and Positioning
Calibration keeps all measurement equipment working within known accuracy limits. This covers signal generators, network analyzers, and reference antennas. Testers often use a calibrated reference antenna with known gain to compare against the Antenna Under Test (AUT).
You need to clear the test environment of unwanted reflections. Indoor tests usually happen in an anechoic chamber lined with RF-absorbing material. Outdoor ranges require setting up in the far-field region.
Align the AUT with the test system’s coordinate axes. A rotating positioner lets you measure in azimuth and elevation. Getting the position right really matters, since misalignment can change the measured beam direction and gain values.
Data Acquisition and Analysis
During measurement, you rotate the AUT in small angular steps to record its radiation pattern. Data gets collected in both E-plane and H-plane cuts, or sometimes as a full 3D pattern.
To measure gain, compare the received signal from the AUT to that of the reference antenna under identical conditions. This process uses the gain transfer method to figure out the AUT’s gain in dBi.
After collecting the data, you process it to remove noise and normalize the values. Results show up as polar plots, Cartesian plots, or 3D visualizations. You’ll usually extract key parameters like main lobe direction, beamwidth, side lobe levels, and cross-polarization levels.
Common Measurement Errors
A few things can mess with your accuracy:
| Error Source | Effect on Results | Mitigation |
|---|---|---|
| Misalignment | Shifts beam direction, alters gain | Use precise positioners |
| Reflections | Distorts pattern shape | Use anechoic chamber or far-field range |
| Cable Loss | Reduces measured power | Calibrate with cables in place |
| Temperature Drift | Changes equipment response | Stabilize environment |
| Improper Calibration | Incorrect gain values | Verify before each test |
You might also run into trouble with environmental noise.
Multipath interference can sneak in and throw off your numbers.
Unstable equipment settings don’t help either.
If you stick to consistent procedures and keep your conditions under control, you’ll get much more reliable antenna measurements.