Underground Radio Operations


Underground structures such as railway tunnels or even naturally formed caves are not where you usually expect to find RAYNET operators but in an emergency RAYNET or similar organisations might be asked to provide communications in and from them. If your group has any risk of being asked to provide communications underground it is always a good idea to do your homework beforehand. Opportunities to undertake tests may be rare to non-existent and there are many safety and technical considerations to be borne in mind beforehand.

The Cave Radio Electronics Group are extremely experienced in underground working and the guidance presented here is based on their findings from many tests and exercises, some with RAYNET groups over many years.

Suggestions from Prior Tests

For single bore tunnels (i.e. around 4m high/wide).

The minimum reliable range for a 145MHz signal of 2-5W output power is about 150m. Increasing diameter or favourable layout may increase this to as much as 300m.

The mininum reliable range for a 430-440 MHz signal is about 500m. Tunnel straightness & bore size again affects range, which may be as much as 3km.

The limited availablity of 1296MHz equipment has limited tests on this band, however it has been found to be much more effective in tunnels with ranges of 5km being achieved in some cases.

In all cases the range is relatively power independant, with a PMR446 radio providing similar results to a Amateur Service handheld. The efficiency of the system may be improved by positioning an aerial a few meters inside the mouth of the tunnel and on the centre line of the tunnel bore. This is understandable since it may be thought that depending upon the composition of the tunnel and surrounding rock forms a (rather large) waveguide.

RAYNET members anticipating undertaking operations or tests in tunnels should bear the above minimum effective ranges in mind, to get the best from what may be limited testing time and to adequately provide for the safety of those doing the tests by having an appropriate fallback network in place should the main frequency under test fail.

What about other tunnels ?

If we follow the premise that we are using the tunnel as a waveguide then waveguide theory can help to guide our frequency selection.

The IET Publication 'Propagation of Radiowaves' (2nd ed.) provides the following guidance; "Dependant upon the propagation mode, the critical wavelength for the guide will be betwen 0.82 and 1.7 times the tunnel diameter and the maximum wavelength for propagation will be about two-thirds of this". [Section 9.4 Page 160].

Current tests have focused on VHF/UHF operations and these have generally been above the critical frequencies for the tunnels concerned but the guidance provided may prove useful for other structures, even those above ground, where a waveguide may be formed by the nature of construction.

Safety and Permission

It is vital (in more than one sense) that anybody wanting to conduct tests in a tunnel or other underground structure should first of all get in touch with the owner/operator, both to obtain permission and to find out what safety regulations must be obeyed. Railway authorities in particular are extremely strict, for good reason.

Railway safety is now the remit of the Office of Rail Regulation (ORR).The Office publishes a range of safety guidance which should be incorporated into risk assessments. Whilst any work on railways is usually undertaken where lines are blocked to traffic so eliminating the hazard from moving trains, the information contained in the 'Railway safety principles and guidance (RSPG)' documents on the ORR website provides a wealth of valuable background information in understanding the design and safety features in use on the railway.

There are other hazards to be borne in mind as well, underground structures may also be considered as 'confined spaces' which are defined as places which are substantially enclosed (though not always entirely), and where serious injury can occur from hazardous substances or conditions within the space or nearby (e.g. lack of oxygen). Further information on Confined Space working can be obtained from the Health and Safety Executive website here


The ranges available in tunnels and underground structures can very widely depending upon the internal dimensions and construction. Generally though 'higher is better' in terms of frequency selection with upper frequency ranges still to be determined when equipment becomes more generally available.

For further advice please contact the technical team or the Cave Radio Electronics Group. Groups are also encouraged to share their reports of underground radio exercises so that others may benefit from the lessons learned. Reports will be posted on this website as received.


Reports Received

Underground Radio Tests on the Nene Valley Railway undertaken in 1998.

Box Tunnel Communications Exercise 26th November 2006. North Wiltshire and Gwent RAYNET Groups in conjunction with Wiltshire Fire & Rescue service and Network Rail.