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Further to your report on the University of Essex's study into the health dangers of terrestrial trunked radio, or Tetra, "a type of phone mast" ("Hold the phones, it's fine", 15 April), it may be relevant to note that there are differences between Tetra and ordinary mobile phone signals.

In particular, the Tetra masts typically produce a number of narrow beams of comparatively high radiation that some sensitised people claim they can feel.

Michael Bevington, Buckingham.