The antenna decides whether a connected device can connect to the mobile network, and how reliable that connection is. This guide explains the main types of IoT antenna, what gain and MIMO mean, and how we choose and fit the right antenna for cellular routers and IoT devices.

An IoT antenna is the part of a connected device that sends and receives its radio signal. Every wireless IoT device has one, from a smart meter with an antenna printed inside its case to a cellular router with four external antennas on a cabinet roof.

At Millbeck, we supply IoT antennas alongside the cellular routers and IoT SIMs we connect, and the antenna is something customers frequently ask us about when a site under-performs.

This guide covers antenna types, omnidirectional and directional patterns, gain, MIMO, frequency bands, cable loss and mounting, and ends with the six questions we ask before we specify an antenna.

What Is an IoT Antenna?

An IoT antenna is the component that turns a device's electrical signal into radio waves and back again, so a sensor, router or tracker can connect to a mobile, Wi-Fi or LPWAN network, or receive GNSS positioning signals.

The antenna works in both directions. It carries the device's transmissions out to the mast, and it collects the signal coming back. The same antenna behaves the same way when sending as when receiving, so an antenna that performs well on one does well on the other.

The antenna sets a ceiling on what the rest of the hardware can do. A router with a capable modem and a SIM on the strongest network in the location will still perform poorly if the antenna cannot pick up a clean signal.

What Are the Main Types of IoT Antenna?

IoT antennas fall into two groups: embedded antennas built inside a device, and external antennas that screw onto a device or connect to it by cable.

Embedded Antennas

An embedded antenna is an antenna fitted inside the device housing. The three common forms are the chip antenna, a small ceramic component soldered to the circuit board and used in wearables, trackers and medical devices; the PCB antenna, a conductor pattern printed on the circuit board itself; and the flexible PCB antenna, printed on a thin film that sticks to the inside of the case.

Embedded antennas suit products made in volume, where size and cost matter more than peak performance. The trade-off is that an embedded antenna sits close to the battery, circuit board and casing, and all of them absorb or distort its signal. Placement inside the housing decides much of how an embedded antenna performs. Our article on the Panorama TF range covers flexible embedded antennas in more detail.

External Antennas

An external antenna is either screwed straight onto the connector or mounted elsewhere and linked to the router or gateway by coaxial cable. External antennas are what most cellular routers and gateways use, because the antenna can then go wherever the signal is best rather than wherever the router happens to sit.

Common external types are whip or stick antennas that screw onto the router, low-profile puck antennas that bolt through a cabinet or vehicle roof, and directional panel or Yagi antennas that point at one mast. Many puck antennas are combination antennas, with several cellular elements plus GNSS and Wi-Fi elements in one housing, each on its own cable.

Some external antennas need a ground plane. A ground plane is a metal surface beneath the antenna that acts as the other half of the antenna. A monopole design, such as many roof-mount pucks, is built to sit on a vehicle roof or metal cabinet lid, and it will under-perform on a plastic enclosure or a wooden post.

Where Each Type of IoT Antenna Fits

Omnidirectional or Directional: Which IoT Antenna Do You Need?

An omnidirectional antenna receives from every horizontal direction and suits most sites. A directional antenna concentrates on one direction and suits fixed sites where the nearest usable mast is a long way off.

An omnidirectional antenna covers the full 360 degrees around it. That lets the router's modem choose between several masts and move to another one. Anything that moves, such as a vehicle, a trailer or a temporary CCTV tower, needs an omnidirectional antenna, and it is our default for most fixed sites too.

A directional antenna focuses its energy towards one point. The benefit is reach: a panel or Yagi antenna aimed at a distant mast can turn an unusable signal into a usable one. The cost is flexibility. A directional antenna has to be aimed during installation, it ties the site to the masts in that direction, and if that mast is taken out of service the router may have nothing else to connect to. Where a fixed site is a long way from the nearest usable mast and the installation allows it, we suggest a directional antenna.

What Does Antenna Gain Mean?

Antenna gain is a measure of how strongly an antenna concentrates radio energy in its best direction, compared with a reference antenna. Gain does not add power to the signal.

Gain is quoted in dBi or dBd. dBi is gain measured against an isotropic antenna, a theoretical antenna that radiates equally in every direction. dBd is gain measured against a half-wave dipole, which has a gain of 2.15 dBi, so adding 2.15 to a figure in dBd gives the figure in dBi. Comparing a dBi figure with a dBd figure without converting makes one antenna look 2.15 dB better than it is.

Energy an antenna concentrates in one direction is energy it takes from another. On a directional antenna, higher gain gives longer reach over a narrower angle, so the antenna needs more careful aiming. On an omnidirectional antenna, higher gain flattens the pattern into a thin horizontal disc that reaches less far up and down.

A flatter pattern is not always better. A high-gain omnidirectional antenna on open, level ground reaches further. The same antenna on a tall building, or close to a mast that sits well above it, can miss the signal it needs. Gain also varies by band, and datasheets often print the peak figure for the best band on the front, so we read the figure for the bands the site will use rather than the headline number.

Why Do Cellular Routers Have More Than One Antenna Port?

Cellular routers have two or four mobile antenna ports because 4G and 5G use MIMO, which carries more than one data stream at once over separate antennas.

MIMO (multiple input, multiple output) is a radio technique that uses several antennas at each end of the link to send and receive parallel data streams. The streams arrive over slightly different paths, and the modem uses those differences to separate them. More antennas give more streams and a more stable connection.

How a 4G Router Uses Its MAIN and AUX Ports

A 4G router usually has two mobile ports, labelled MAIN and AUX. The MAIN port sends and receives. The AUX port is a diversity antenna, a second antenna the modem uses to recover signal the first received poorly, and on 4G it also acts as a second receiver that lifts download speed and connection quality. A 4G router with only the MAIN antenna connected will work, but it works worse.

How a 5G Router Uses Four Ports

A 5G router usually has four mobile ports, and all four act as main antennas. On the Teltonika 5G routers we supply, typically ANT0 and ANT3 send and receive, while ANT1 and ANT2 receive. We connect all four as standard. If a site genuinely has room for only two, ANT0 and ANT3 are the best ports to use.

Why MIMO Antennas Are Often Cross-Polarised

A cross-polarised antenna is a MIMO antenna whose elements are set at different angles, usually +45 and -45 degrees, so each element receives a slightly different version of the signal. That difference is what lets MIMO separate the streams. The antenna does not need to match the mast's polarisation, because reflections from buildings and terrain mix the signal's polarisation long before it reaches the site.

Where a router uses separate whip antennas instead of one MIMO housing, the same principle applies. Setting the whips apart in a V shape, so each sits at a different angle, gives a small improvement, and SINR and throughput on the router's status page show whether it has helped. Antenna needs are also changing with 5G: a 5G RedCap device can run on one or two receive antennas, compared with four on a standard 5G router.

Does the Antenna Cover the Right Frequency Bands?

A cellular antenna for a UK deployment should cover every band UK mobile operators use, which run from 700 MHz to 3.8 GHz.

An antenna works well only across the frequencies it was designed for. An antenna rated 698 MHz to 2,700 MHz covers most 4G bands but misses the 3.4 GHz to 3.8 GHz band that carries much of the UK's 5G capacity. For a 5G router, we specify an antenna rated to at least 3.8 GHz, and many of the cellular antennas we supply now cover 698 MHz to 6,000 MHz.

LTE-M and NB-IoT run inside the operators' standard 4G bands, so a standard cellular antenna covers them. They use much narrower channels than standard 4G, 1.4 MHz for LTE-M and 200 kHz for NB-IoT, and are designed to reach devices deep inside buildings and underground. LoRaWAN is different: it uses the licence-exempt 868 MHz band in the UK and needs an antenna tuned for that band. GNSS positioning and Wi-Fi each need their own antenna or antenna element, connected to their own port on the router.

How Much Signal Does the Antenna Cable Lose?

Coaxial cable between the antenna and the router loses signal on every metre, and the loss rises with longer cable, higher frequency and thinner cable.

Cable loss is measured in decibels (dB), and a loss of 3 dB halves the signal power, so a few decibels matter. We have worked out the figures below from the manufacturer's published attenuation for two common cable types: LMR-195, a thin, flexible cable often fitted to antenna tails, and LMR-400, a thicker low-loss cable for longer runs.

Ten metres of thin cable at 2500 MHz loses more than 6 dB, which can cancel much of the benefit of a high-gain antenna. Losses on the 3.4 GHz to 3.8 GHz 5G band are higher again. For a long run, we specify low-loss cable, or move the router closer to the antenna and run Ethernet back to the equipment instead.

Connectors

Cellular routers normally use SMA connectors for their mobile antennas. Larger outdoor antennas and thick low-loss cables often use N-type connectors, which are bigger and weather better. Every adaptor between the two adds a little loss and another joint where water can get in, so we match the antenna's connector to the router wherever we can.

Where Should You Mount an IoT Antenna?

Mount an IoT antenna outside any metal enclosure, as high as practical, clear of nearby metal, and with the shortest cable run that reaches.

• Outside the metal box. A metal cabinet blocks most of the signal. An antenna inside it, however good, starts at a disadvantage.
• High and clear. Height and a clear view of the horizon matter more than extra gain. Metal, concrete and tinted glass all block signal.
• Away from other metal and other antennas. Keep the antenna clear of railings, poles and other radio equipment that can distort its pattern.
• Sealed against the weather. Choose an antenna with an IP rating suited to the site, seal the connectors, and loop the cable below the entry point so water runs off.
• Aimed by signal quality, not signal strength. When aiming a directional antenna, watch SINR, the ratio of wanted signal to interference and noise, as well as RSRP, the strength of the received reference signal.

Many installers miss the last point. The strongest signal is not always the most usable one, and we explain why in Why Good Signal Strength Does Not Mean Good Connectivity. Sites with no outdoor mounting point, such as basements and underground car parks, need a different approach, covered in our guide to connectivity for underground car parks.

How We Choose an IoT Antenna: Six Questions

We choose an IoT antenna by answering six questions about the networks, the router, the site and the environment, and we ask customers the same six before we recommend anything.

1. Which Networks? 4G, 5G, GNSS, Wi-Fi or LoRaWAN. Each needs an antenna or element rated for its bands, up to 3.8 GHz for UK 5G.
2. How Many Ports? Match the antenna to the router: two mobile connections for most 4G routers, four for most 5G routers, plus GNSS and Wi-Fi if used.
3. Where Is the Mast? Several masts in reach, or a moving asset: omnidirectional. One distant mast on a fixed site: consider directional.
4. Where Will It Mount? Cabinet roof, wall, pole or vehicle. Check whether the antenna needs a ground plane, and keep it outside metal.
5. How Long Is the Cable? Keep the run short. Over a few metres, specify low-loss cable or move the router closer to the antenna.
6. What Is the Environment? Weather, temperature, vibration and vandalism. Choose the IP rating and housing for the site, not the showroom.

What Is Changing in IoT Antennas?

IoT antennas are moving towards wider frequency coverage, more functions in one housing and smaller embedded designs, driven by 5G and by devices that keep getting smaller.

• Wider bandwidth for 5G. Antennas that stopped at 2.7 GHz are giving way to designs rated to 3.8 GHz and beyond, to cover the UK's main 5G band.
• Combination antennas. MIMO cellular, GNSS and Wi-Fi elements now share one housing, which cuts the number of holes in a cabinet or vehicle roof.
• Embedded antennas that need less from the host. Flexible embedded antennas such as the Panorama TF range are designed to work without a metal ground plane in the product, which makes them easier to design in.
• Fewer antennas on some 5G devices. 5G RedCap allows one or two receive antennas, which suits small, lower-cost 5G devices.

There are also adaptive antennas that steer their pattern with beamforming, and antennas that harvest energy from surrounding radio signals. Both are at an early stage. Neither is a feature of the cellular routers and IoT devices we supply today, and we would treat claims about them with care until products are proven in the field.

Frequently Asked Questions