| Some years ago David Gibson of CREG estimated the ERP of a Heyphone feeding 4W into a 2 m diameter loop (as sometimes used in cave radio) as of the order of a few microwatts. He also examined the case of a Heyphone feeding a couple of Earth electrodes a few tens of metres apart. In practice the latter gave a range of up to 1.5 km in the French Alps.
I don’t have details to hand for other places, nor of David’s calculations but it all seemed to take place more or less within the near field. David also pointed out that, because of the restricted range in terms of wavelength, one should not speak of cave “radio” – it is in effect an induction system.
The Heyphone and similar technologies have been used for many years in cave rescue and exploration. In the UK, the authorities are well aware of what we do and have been known to issue NoVs allowing the surface end of a cave radio link on 87 kHz to be connected to an HF transceiver. One example of this was to allow cave to cave communications between locations in the North of England and South Wales. As far as I’m aware, the authorities never mentioned licensing the cave equipment.
There is still a requirement, regardless of frequency, for such equipment not to cause undue interference to scheduled services. There was a case where NoVs issued for experiments around 9 kHz included a requirement in some circumstances to liaise with the Met office.
John F5VLF
On 28 Jan 2014, at 12:27CET, Roger Lapthorn < [email protected]> wrote:
Is my reading of the UK frequency allocation chart right?
One can operate without a licence in the old 73kHz band using licence-exempt inductive devices rules as long as the output is less than 72dBuA at 10m? Not sure what sort of ERP this equates to. A few uW I assume?
Perhaps someone can advise/confirm.
73s
Roger G3XBM
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http://www.g3xbm.co.uk
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