5G Router Antenna Guide: MIMO, Bands, Cable Loss and Placement

Antennas are the least glamorous part of a 5G installation and the most important. The same router can deliver 20 Mbps or 400 Mbps at the same address depending on which antennas are used, where they are placed and how they are connected. This guide covers the principles. For the practical side of fitting one, see adding an external antenna.

Why a £400 5G router can perform worse than your phone

It happens all the time. Someone buys a proper business 5G router, installs it in a comms cupboard, and it is slower than their phone held up in the car park. The phone has not got a better modem. It has a better position. It is outside, at head height, with nothing in the way. The router is inside a cupboard, inside a building, possibly inside a metal cabinet.

Fix the position and the router usually wins easily. Its modem has more receive paths and more processing power than a phone, but it can only use the signal that actually reaches its antennas.

MIMO in plain terms

MIMO stands for multiple input, multiple output. The router uses several antennas at once to receive several data streams over the same frequency. The more independent paths the signal takes to reach each antenna, the more data can be carried.

• 2×2 MIMO uses two antennas. Common on 4G and RedCap.
• 4×4 MIMO uses four antennas. Standard for n78 5G and better 4G modems.

MIMO only works well if the signals at each antenna are different from each other. Two antennas next to each other with the same orientation receive almost identical signals, which adds little. That is why antenna spacing and polarisation matter.

Polarisation

Radio waves have an orientation, called polarisation. Mobile networks commonly transmit with cross-polarised antennas, slanted at plus and minus 45 degrees. A good MIMO antenna for a router has elements at different polarisations, so each port receives a different version of the signal.

With simple stick antennas, you get a similar effect by angling them differently, for example two vertical and two at 45 degrees, or spaced apart. Many proper MIMO antennas are designed with this built in, so you just connect the cables in order.

Frequency coverage

UK mobile networks use a wide spread of frequencies. A 5G router antenna needs to cover them all, not just the 5G bands, because the router may use 4G as an anchor or a fallback.

Look for antennas specified from around 600 or 690 MHz up to at least 3.8 GHz, and ideally to 6 GHz. Check the gain figures at the low and high ends, because some antennas perform well at one end and poorly at the other.

Gain explained honestly

Antenna gain is measured in dBi. Higher gain means the antenna focuses more of its sensitivity in certain directions. It does not create signal from nothing. An omnidirectional antenna with high gain is flatter and more focused towards the horizon. A directional antenna with high gain points most of its sensitivity in one direction.

Be sceptical of very high gain claims on small omnidirectional antennas, especially across all bands. A compact antenna claiming 9 dBi from 700 MHz to 6 GHz is unlikely to deliver that at the low end.

Omnidirectional or directional?

Omnidirectional antennas receive from all directions around them. They are simple to install, need no aiming and can use whichever mast is best at any moment. Most installations should start here.

Directional antennas, often flat panels, focus on one direction. They give more gain towards the target mast and reject signals from others, which can improve SINR dramatically in areas with many overlapping cells. They need to be aimed, and if the network changes which mast serves the area, they may need re-aiming.

Cable loss: the silent killer

Coaxial cable loses signal along its length, and loss increases with frequency. At 5G mid band frequencies the loss is significant. Rough figures at around 3.5 GHz:

Every 3 dB lost halves the signal power. Ten metres of thin cable can throw away more than a high gain antenna gains. Connectors add a little loss each too. The rule is simple: keep cables short and thick, or move the router closer to the antenna.

Connectors

Business and industrial 5G routers almost always use SMA female connectors on the router, so antennas need SMA male plugs. Some routers use reverse polarity SMA for Wi-Fi ports, which looks similar but is not compatible, so check which port is which. Outdoor antennas often have N-type connectors, which are more robust, with an adapter or a cable terminated in SMA at the router end. Consumer hotspots sometimes use TS9 or CRC9 sockets, which are small and fragile.

Placement principles

• Get the antenna outside the building or as close to an outside wall or window as possible
• Higher is usually better, but avoid positions that see many masts equally
• Keep antennas away from large metal surfaces unless they are designed to mount on them
• Keep cable runs short and use the lowest loss cable practical
• Connect every antenna port the router has for the cellular side
• Test at several positions before drilling holes

Reading the signal numbers

When testing antenna positions, use the router’s signal readings rather than the bars.

These are rules of thumb rather than hard limits. SINR is often the best predictor of real throughput. A position that improves SINR from 2 dB to 12 dB will typically transform performance, even if RSRP barely changes.

Mounting near metal and in cabinets

Metal is the enemy of a good antenna installation in most cases. A metal cabinet acts as a shield, blocking most of the signal from reaching antennas inside it. Metal surfaces near an antenna distort its pattern and can create dead zones. The exceptions are antennas designed to mount on metal, such as many puck and roof antennas for vehicles and cabinets, which use the metal surface as part of their design.

For cabinets, the standard approach is a puck antenna bolted through the cabinet roof, or a pole-mounted antenna next to the cabinet with cables entering through glands. Avoid magnetic antennas stuck to the inside of a steel door. They may work on a good day, but they are not a design.

Antennas and interference

In busy urban areas, the problem is often not weak signal but too much of it from too many directions. The router hears several cells on the same frequency, and they interfere with each other. RSRP looks fine, but SINR is poor and throughput disappoints.

A directional antenna can help a lot here, by favouring one cell and suppressing the others. So can moving the antenna lower or to a position partly shielded from the interfering masts. It sounds odd to move an antenna somewhere with weaker signal, but if SINR improves, throughput usually follows.

Electrical noise from nearby equipment, such as switched mode power supplies, LED drivers and variable speed drives, can also raise the noise floor. Keep antennas and their cables away from them where possible.

Antenna types you will come across

Stick or blade antennas that screw onto the router. Fine for a router on a desk near a window.

Magnetic mount antennas on short cables, often used on cabinets and vehicles. Convenient but often fitted with thin cable.

Puck or dome antennas, low profile and vandal resistant, popular on cabinets, kiosks and vehicles. Often contain four cellular elements plus GNSS and Wi-Fi.

Wall or pole mount omnidirectional antennas, larger and with better gain, for fixed installations.

Directional panels for weak signal or high interference sites.

For guidance on choosing and fitting one, see adding an external antenna. For outdoor all-in-one units, see outdoor 5G routers.

If you have a site where the router is underperforming and want help working out why, email sales@5grouter.co.uk with the signal readings and a description of where the router and antennas are.