Antenna Separation on Multi-Antenna Vehicle Roofs

Modern emergency and fleet vehicles carry more radios than ever. A single roof might hold TETRA, 4G or 5G with MIMO, GNSS, Wi-Fi and sometimes a VHF or UHF antenna for a second network. Each one works fine on its own. Put them close together and they start to interfere with each other.

What goes wrong

• Desensing. A strong nearby transmitter overloads a receiver, so it struggles to hear weak signals even on a different frequency.
• Lost GNSS lock. GNSS signals are extremely weak, and a TETRA or cellular transmitter close by can swamp the receiver while keyed.
• Pattern distortion. Antennas close together couple with each other, changing each other's radiation pattern and VSWR.
• Shadowing. Light bars, roof boxes and ladders block signal in some directions.

Start with a roof plan

Draw the roof to scale before drilling. Mark the light bar, roof rails, sunroof and any structure that cannot move. Then place antennas in order of priority, giving the most critical service the best position, which is usually the centre of the largest clear area of metal.

Spacing rules of thumb

• Keep antennas at least half a wavelength apart at the lowest frequency involved. At TETRA frequencies that is roughly 40 cm.
• Put the most distance between the highest-power transmitter and the GNSS antenna.
• Keep MIMO elements apart from each other, or use a single housing designed for MIMO so element spacing is controlled.
• Avoid placing antennas directly in front of or behind a light bar.

Treat these as starting points, not guarantees. Measure on the vehicle before signing the build off.

Combine where it makes sense

Multi-element housings reduce the number of holes and fix the spacing at the factory. A unit such as the 2J6B86BGFf combines 4x4 5G MIMO, 4x4 Wi-Fi and GNSS in one roof housing. The TELESAT-8 covers cellular, Wi-Fi and GNSS in a rugged low-profile design. Keep the TETRA antenna separate, as it is usually the highest-power transmitter on the vehicle. Browse the full combination antenna range.

Test with everything transmitting

Bench tests with one radio at a time will not show interference. Test with all radios powered, then key the TETRA radio while watching GNSS position and cellular throughput. If GNSS drops or data stalls, move antennas further apart or add filtering before rolling the layout out across the fleet.

Frequently asked questions

How far should a GNSS antenna be from a TETRA antenna?

As far as the roof allows. Half a wavelength at TETRA, roughly 40 cm, is a minimum starting point. Test with the TETRA radio keyed to confirm GNSS holds lock.

Can a light bar affect antenna performance?

Yes. Light bars block and reflect signal, and some generate electrical noise. Keep antennas clear of them and avoid placing antennas directly in front of or behind them.

Is it better to use one combined antenna?

For cellular, Wi-Fi and GNSS it often is, because spacing between elements is fixed by design. Keep high-power TETRA or VHF transmit antennas separate.

For more on mounting and earthing, read our vehicle mounting guide. To plan a roof layout for a fleet, contact sales@renair.co.uk.