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As with any other type of broadcast electronic signal, microwave transmission is also very carefully regulated in the UK, with only three specific frequency blocks currently available for ‘licence exempt’ short range commercial use; these being 1.394 GHz ( 1.4 GigaHertz) 2.4GHz & 5.8 GHz. There are actually other microwave frequencies available for broadcast use, fixed commercial operations and security agencies (particularly police covert operations), but these all require to be licenced or notified, and are not available for use by the general public. The two frequencies that are, essentially are limited to a low radiated output from the transmitter, with ‘line of sight’ operation of up to 5 kilometres on the 1.394GHz frequency, and generally up to 1 kilometre on the 2.4GHz block. In practice, whilst the radiated power output on the transmitter must be within strict guidelines, there is no obvious problem with using a higher performance aerial on the receiver to increase the overall range. In practical terms, the 2.4GHz band is generally the preferred option, particularly as four seperated frequencies have been allocated, which will allow four independent video links to be used at the same location, without any risk of ‘crosstalk’ (co channel interference). It is important to remember that any radiated signal (unless scrambled) can just as easily be ‘picked up’ by a suitably equipped ‘eavesdropper’; indeed if two neighbouring commercial units decide to use this transmission technique to link their buildings on opposite sides of the road, if both attempt to use the same specific frequencies, the results may be interesting to say the least! Unlike ‘laserlinks’ which are truly ‘line of sight’, microwave transmitter and receiver aerials need to be unhindered in order to work at maximum efficiency, and yet the fact that residue signals will bounce off buildings and structures (and even pass through walls) means that this technique is actually quite flexible for relatively short range use (particularly in covert operations), but the integrity and security of the link must be carefully assessed in each individual situation. The latest generation digital links use a technique called COFDM, which actually exploits the effect of reflected signals in order to provide a highly efficient transmission system, particularly in densely developed metropolitan locations. It should be mentioned that where licence exempt microwave is used in densely populated areas, there is a significant likelyhood that other local users can also pick up the transmissions, so that's one very good reason why wireless CCTV should not be used in a residential setting, particularly if there are neighbour disputes. With short term covert operations, although microwave presents a relatively simple and attractive solution, Doktor Jon would rarely if ever consider it as the primary option, without first exploring all the possible alternatives. | ||||||||||||||||||||||||||||||
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