Antenna Pattern Measurement
Understanding Antenna Pattern Measurement
When an engineer designs a directional antenna, the CAD software promises a beautiful, razor-sharp beam of energy. But manufacturing defects, metal warping, and cable interference constantly ruin physical antennas. To prove the antenna actually works, it must undergo an Antenna Pattern Measurement. This is the exhaustive process of physically mapping the exact intensity of the invisible electromagnetic field at thousands of different angles surrounding the antenna.
Inside an anechoic chamber, the Antenna Under Test (AUT) is bolted to a robotic pedestal. A fixed "Source Antenna" at the other end of the room fires a constant RF tone. The VNA measures the received power. The robot rotates the AUT by 1 degree. The VNA measures the power again. This repeats for a full 360 degrees. The resulting data is plotted on a Polar Graph. This graph instantly reveals the Main Beam (where the energy is meant to go), the Beamwidth (how sharp the beam is), and the dreaded Sidelobes (wasted energy leaking out the sides).
Principal Planes: E-Plane and H-Plane
To map a full 3D sphere takes thousands of measurements and hours of robotic movement. For rapid engineering, technicians usually only measure two specific 2D slices of the sphere, known as the Principal Planes. If the antenna generates a Vertically polarized electric field, the E-Plane measurement is taken by slicing vertically through the antenna. The H-Plane measurement is taken by slicing horizontally through the magnetic field. These two 2D cuts are usually enough to verify the core physics of the antenna.
1. Peak Gain (θmax, φmax): The absolute highest dB value on the graph.
2. Half-Power Beamwidth (HPBW): The angle width where the main beam drops exactly -3 dB from the peak. Defines how "sharp" the antenna is.
3. Sidelobe Level (SLL): The dB difference between the peak of the main beam and the peak of the largest unwanted side-lobe.
4. Front-to-Back Ratio (F/B): The dB difference between the main beam (0°) and the energy leaking directly backward (180°).
Comparison
| Graph Type | Data Presentation | Primary Advantage |
|---|---|---|
| Polar Plot | A circular graph showing dB intensity vs Angle. | Highly intuitive. It literally looks like a beam of energy. |
| Cartesian (Rectangular) Plot | A standard X-Y line graph (Angle vs dB). | Much easier to accurately read the deep -40 dB nulls and sidelobe levels. |
| 3D Spherical Surface | A colored 3D topographical globe. | Reveals hidden asymmetrical sidelobes that 2D planes miss. |
Frequently Asked Questions
Why do we measure in decibels (dB) instead of Watts?
Because the human brain cannot comprehend a linear power graph of an antenna. The main beam of an antenna might be 1,000 Watts, but a deep 'null' (blind spot) right next to it might be 0.001 Watts. If you plot that on a linear scale, the null just looks like a flat zero, hiding massive amounts of important physics. A logarithmic (dB) scale compresses the massive main beam and magnifies the microscopic nulls, allowing the engineer to see the entire massive dynamic range of the pattern on a single piece of paper.
What is the difference between Co-Polarized and Cross-Polarized patterns?
When you measure an antenna, you actually measure it twice. First, the Source antenna is perfectly aligned vertically with your AUT. This maps the 'Co-Polarized' pattern (the intended, good energy). Then, you physically rotate the Source antenna 90 degrees so it is horizontal, and measure the entire 360-degree circle again. This maps the 'Cross-Polarized' pattern (the accidental, leaking, garbage energy). A great antenna has a massive Co-Pol beam and a microscopic Cross-Pol beam.
Can a pattern measurement detect if the antenna is physically broken inside?
Absolutely. A perfectly symmetrical antenna (like a parabolic dish or a circular patch) MUST produce a perfectly symmetrical main beam. If you look at the Polar plot and the main beam is 'squinting' (leaning 5 degrees to the left), or if the left sidelobe is -20 dB but the right sidelobe is a massive -10 dB, it is irrefutable mathematical proof that the antenna is physically deformed, a cable is broken, or a phase shifter has failed.