MIMO Antennas Explained — 2×2 and 4×4
What MIMO means for IoT antenna selection, router compatibility, cross-polarisation, and why you cannot use one MIMO antenna port for two connections.
What is MIMO?
MIMO stands for Multiple Input, Multiple Output. It uses multiple antenna paths simultaneously to increase data throughput without requiring additional bandwidth. The transmitter sends multiple independent data streams, and the receiver reconstructs them using the spatial differences between signals arriving at separated antennas.
2×2 MIMO means 2 transmit antennas and 2 receive antennas. In ideal conditions it doubles throughput compared to a single antenna (SISO). 4×4 MIMO doubles again over 2×2. In practice, real-world gains are less than theoretical maximums due to channel conditions and correlation between antenna paths.
MIMO is standard in all LTE Category 4 and above devices, and in all 5G NR devices. If your IoT router is LTE Cat-4 or higher, it uses MIMO. If it is Cat-1 or Cat-M1, it likely uses 1 or 2 antenna ports for diversity rather than true MIMO.
Cross-Polarisation: Why It Matters
For MIMO to work properly, the two antenna elements need to be uncorrelated — they need to see the RF environment differently enough that the signals they receive are distinguishable from each other. Physical spacing of half a wavelength is one approach, but at 800 MHz that is 19 cm — impractically large for most installations.
Cross-polarisation solves this with a compact form factor. One element is vertical polarisation, the other is horizontal (or ±45°). The RF environment interacts differently with each polarisation, providing the decorrelation needed for MIMO to work effectively, without requiring large physical separation.
An outdoor MIMO antenna labelled as cross-polarised (like the Poynting XPOL series) will outperform two physically separated but co-polarised antennas in most environments.
MIMO Port Labelling on Routers
| Port Label | Function | Antenna Required |
|---|---|---|
| MAIN | Primary transmit/receive | Always required |
| AUX / DIV | Receive diversity or MIMO | Required for full MIMO performance |
| MAIN1 / MAIN2 | 4×4 MIMO primary paths | Both required |
| AUX1 / AUX2 | 4×4 MIMO secondary paths | Both required for 4×4 |
Never leave a MIMO port unconnected on a router with an exposed SMA port. An open SMA port is a potential ingress point for moisture and can affect impedance matching on adjacent ports. At minimum, fit a 50-ohm termination cap. See the compatible routers guide for specific antenna port configurations.
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