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
What about other tunnels ?
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
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
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.
Radio Tests on the Nene Valley Railway undertaken in
Tunnel Communications Exercise 26th November 2006.
North Wiltshire and Gwent RAYNET Groups in conjunction with Wiltshire
Fire & Rescue service and Network Rail.