Underground Radio Tests on the Nene Valley
This report has been compiled by John Rabson, with
additional contributions from Rob Gill, Chris Trayner and Roger
Tests were conducted in a railway tunnel to establish the
best frequencies for radio communications. Best results were obtained
at UHF, next best being an LF induction/earth current system. VHF was
of somewhat marginal performance, and mobile telephones did not work
any useful distance into the tunnel.
Caves are not the only holes we find in the ground. There
are railway tunnels in many parts of the British Isles. We do not
usually venture into these but in an emergency RAYNET or similar
organisations might be asked to provide communications in and from
them. It is a good idea to do our homework beforehand. Enquiries about
previous work revealed that one or two things had been tried but not
written up. Some years ago tests were done in a mile-long tunnel on
BR's West Coast Main Line. UHF was effective but VHF was not. Some
tunnel tests were also done in Essex a few years ago with similar
findings, but no report seemed to have been published. We felt is was
time to stop theorising, find a tunnel and do some experiments.
Unfortunately, it is not easy to get permission to use
railway tunnels since many of them are regularly used: the light at the
end of the tunnel tends to indicate an approaching train rather than a
breakthrough! I sought advice from railway enthusiasts and ended up
arranging to use Wansford Tunnel on the privately owned and run Nene
Valley Railway in Cambridgeshire, on Monday 9th February 1998 with
Chris Trayner, Rob Gill and Roger Goodchild.
Wansford Tunnel is 550m long and straight, 7m high and 7.5m
wide. It runs through oolitic limestone and is lined with Victorian
brick. It contains two standard gauge railway tracks, one of which runs
only about 150m into the tunnel and is used in winter to store rolling
stock in the dry. The notional markings on the sleepers, which were
supposedly at intervals of one chain (22 yards or 20m) were not always
accurate. We therefore took our own measurements of distance into the
We tried four different frequency bands: LF, VHF (145MHz)
and UHF (433MHz amateur band) and 934MHz CB2.
On LF we used John Hey's cave radios. These worked well from
one end of the tunnel to the other, using loop aerials. It made no
difference whether the loops were placed between the rails or at the
side of the tunnel, or in the 6 foot space between the two tracks.
We then moved one station to the surface above the point
where the tunnel was at its deepest , about 20m below the surface, and
replaced its loop aerial with an earth current system. This used a 40m
wire lying on the ground and running at right angles to the tunnel,
with a step-up transformer between the transmitter and the earth wire.
(It was not possible to run the wire along the line of the tunnel
because of the layout of the roads). This configuration worked
satisfactorily for a distance of 250m along the tunnel, centred on the
point vertically below the surface station. Reception was noticeably
better with the loop vertical than with it horizontal.
A pair of 433MHz hand-helds with rubber duck aerials worked
perfectly satisfactorily from one end of the tunnel to the other, as
did the 934MHz mobile equipment which used a rubber duck aerial at one
end and a 10 element beam at the other. We also checked how far into
the tunnel it was possible to get service from the local GSM900 mobile
telephone cell. The answer was about 10m from the eastern portal and 4m
from the western portal: GSM1800 was not tested.
VHF hand-helds were much less effective than UHF ones. We
got rather less than half way along the tunnel before we lost our
signal altogether. Replacing the rubber duck aerial at the tunnel mouth
with a 4 element Yagi gave us only a few metres increase in range.
Replacing the rubber duck in the tunnel with a mobile whip on a
rucksack gave a large improvement. We repeated the tests using a field
strength measuring receiver with the beam horizontal (vertical was not
so good). The results are shown on the graph.
The received signal strength varied very little for the
first 100 metres into the tunnel probably an artefact due to receiver
saturation. Thereafter the signal fell off steadily until, at the mid
point of the tunnel, it was more than 55dB weaker than at the entrance.
At about this point it fell below the FM threshold of the receiver and
therefore was unusable further along the tunnel. We thought at first
that the presence in the tunnel of rails with rolling stock on them
might have enhanced the propagation at VHF; but the signal strength, as
we said, started falling off about 100 metres into the tunnel, while
the rolling stock and rails it was on extended to nearly 150 metres
into the tunnel. This suggests that the presence of all this metal had
only a minor effect on the propagation. We would welcome comments from
experts in wave-guide matters as to whether we are on the right lines
in our reasoning.
We then tried an earth current system on the surface as
before, but on 145MHz. The earth spikes were 2 metres apart and fed by
a pair of wires about 2 metres long, connected to the transmitter via a
4:1 balun. The transmitter output power was 1W. This station could just
be received in the tunnel, provided the tunnel set was below the
surface station and had its rubber duck pressed firmly against the wall
of the tunnel.
From these results we conclude that the best choice of band
for working in tunnels is likely to be UHF. Second choice is an LF
induction/earth current system. VHF seems to be of somewhat marginal
performance, and mobile telephones do not seem to be much use in
tunnels. If anyone else is thinking of doing such tests, they must
obtain permission from the railway company concerned. We found the Nene
Valley Railway very helpful and even brought in two cheerful volunteers
to provide refreshments. The exercise did, however, cost us a £100
donation to their funds, which was met in part by contributions from
some of the participants and in part by sponsorship from The Word
Factory, Eyke, Limited.
If anyone would like to know more about these tests, please email me.