Cross-Pol Pattern
Understanding the Cross-Pol Pattern
When you design an antenna for a cell tower, you typically design it to transmit a Vertically polarized electromagnetic wave (the electric field points straight up and down). In a perfect universe, 100% of the energy leaves the antenna vertically. In the physical universe, manufacturing defects, surface currents on the metal chassis, and edge diffractions cause the antenna to accidentally radiate a small amount of energy Horizontally. This accidental, orthogonal radiation is plotted on the Cross-Pol Pattern.
During an Antenna Pattern Measurement inside an anechoic chamber, the engineer first measures the intended Vertical beam. This is the Co-Polarized (Co-Pol) Pattern. The engineer then physically rotates the receiving probe exactly 90 degrees so it is Horizontal, and runs the entire 360-degree sweep again. This maps the Cross-Pol Pattern. The Cross-Pol pattern represents pure interference and wasted energy.
The Importance of XPD in 5G
In modern 5G networks and Satellite links, bandwidth is so expensive that engineers use "Polarization Diversity." They literally transmit two completely different data streams on the exact same frequency at the exact same time—one stream is Vertical, the other is Horizontal. The system relies on the physics of orthogonal polarization to keep the two streams from interfering with each other. If your antenna has a terrible Cross-Pol pattern, the Vertical data stream will accidentally bleed into the Horizontal data stream, completely destroying both signals in a massive wave of interference.
XPD (dB) = Peak_GainCo-Pol - Peak_GainCross-Pol
Industry Standards:
A cheap Wi-Fi antenna might have an XPD of 15 dB.
A high-end satellite dish using polarization diversity requires an incredible XPD of > 30 dB to prevent data corruption.
Comparison
| Pattern Type | Physical Meaning | Design Goal |
|---|---|---|
| Co-Polarized (Co-Pol) | The intended, desired electromagnetic field. | Maximum Gain, sharp beamwidth, low sidelobes. |
| Cross-Polarized (Cross-Pol) | The accidental, orthogonal leakage. | Must be completely crushed (-30 dB below Co-Pol). |
Frequently Asked Questions
Why does the Cross-Pol pattern often look like a butterfly?
In many common antennas (like a rectangular patch antenna), the accidental cross-polarized energy is generated by surface currents running along the diagonal corners of the metal. Because these currents flow in opposite directions, their radiation violently cancels out directly in the center of the beam (creating a deep null at 0 degrees), but radiates strongly out to the sides (at 45 and -45 degrees). This creates a distinct four-lobe 'butterfly' shape on the polar graph.
Can the Cross-Pol be higher than the Co-Pol?
Yes, and if it is, you have catastrophically failed. If your antenna was designed to be Vertical, but the Horizontal (Cross-Pol) pattern is louder than the Vertical pattern, it means you have accidentally excited the wrong resonant mode in the antenna. Your antenna is physically radiating the exact wrong polarization, and the intended signal has been completely suppressed.
How do engineers fix bad Cross-Pol?
Usually by altering the physical symmetry of the antenna. They might cut tiny notches in the corners of a patch antenna to disrupt the diagonal surface currents. For massive satellite dishes, they might place a 'Sub-reflector' or specialized corrugated rings around the feed horn to trap and choke off the accidental orthogonal waves before they can bounce off the main mirror.