An oddity on the summit of Pike O'Blisco was strong mobile phone signal on one end of the summit gully and no signal 5 metres away at the other end. The summit rocks might be 10 metres high. They seem to block the electromagnetic waves. Waves diffract around objects that are the same size as the wavelength. The average mobile phone wavelength in the UK is 30cm
http://www.creative-science.org.uk/mobile_LED.html The summit rock is much bigger than this. The waves cannot diffract around them.
