Introduction

This article descibes the technical criteria for planning new FM stations in the UK and then discusses how the number of stations could be increased by relaxing the division into sub-bands and using transmitter synchronisation.

Technical planning criteria

The required field strength to provide a good stereo signal is 54 dBμV/m in rural areas, 66 dBμV/m in suburban areas and 74 dBμV/m in city areas. For good mono reception, the required field strengths are 6 dB lower. However, most popular music stations use lots of dynamic range compression, providing a listenable signal at lower field strengths. However, for good reception, the signal must also be significantly stronger than other signals on the same and adjacent frequencies. The following margins are needed:
Co-channel: 45 dB
100 kHz separation: 33 dB
200 kHz separation: 7 dB
300 kHz separation: -7 dB
400 kHz separation: -20 dB

A negative value indicates that the interfering signal can be stronger. To meet these margins, transmitters on the same and adjacent channels must be some distance apart or heavily screened by hills. Stations 200 kHz apart of similar field strength may be receivable in the same location with different aerial positions. However, official coverage areas can not overlap. If there are hills separating the two coverage areas, 200 kHz spacing can be used for adjacent stations; otherwise there must be a small buffer region. With stations 300 kHz apart, the margin is -7 dB, so it will always be possible to pick up one station or the other, but not always both. This spacing is suited to transmitters broadcasting the same programme to overlapping areas, provided the stonger signal is not subject to multipath interference. It can sometimes be used for different stations serving neighbouring areas, depending on the geography of the overlap region.

A 400 kHz spacing is always suitable for transmitters with overlapping coverage areas, whether they carry the same programme or not, provided reception of each station near the other's transmitter is not required. If both stations are transmitted from the same mast or nearby masts, a 400 kHz spacing is OK. For example, a local station on 97.5 in Nottingham would interfere with Radio 1 from Sutton Coldfield on 97.9, but the same frequency could be used from the Sutton Coldfield mast without disrupting Radio 1. However, if a network radio filler was introduced for Nottingham, it would then be possible to use 97.5 for local radio from the same mast as the network filler as reception of the 97.9 signal would no longer need to be protected.

The final issue to consider in planning FM transmitters is intermodulation products. There are two types: intermediate frequency and cross station. FM radios use a superheterodyne circuit to reduce the mean carrier frequency of the selected FM station down to 10.7 MHz prior to demodulation. As a consequence, intermodulation products within the receiver can prevent reception on a frequency 10.7 MHz below a strong local station. In practice, this only happens very close to the transmitter. In addition, radio receivers transmit a weak signal on a frequency 10.7 MHz higher than they are tuned to - you can demonstrate this yourself using a pair of radios. Consequently, stations serving the same area are never spaced 10.7 MHz apart in the UK. 10.6 and 10.8 MHz spacings are usually avoided as well as cheaper receivers sometimes have off- centre intermediate frequencies, though 10.8 MHz spacing has been used in Bradford and Kettering. Very close to the transmitter or with a poor receiver, intermodulation products of stations on frequencies x and y can be received on frequencies 2x - y and 2y - x, blocking out weaker stations on those frequencies. The use of regular station spacing reduces the impact of intermodulation products and a station covering the same area from a different mast can usually operate 100 kHz apart from a cross station intermodulation product.

More efficient use of the sub-bands

With a handful of exceptions, every transmitter operates within the appropriate sub-band for the station it carries. As a consequence of this, many towns and cities have run out of space for local radio, while spare capacity remains in the national sub-bands. By placing BBC and independent local radio transmitters throughout the FM band, much more efficient use could be made of the spectrum available. Space is available in the national sub-bands in some parts of the country, but not others. However, by moving existing local radio transmitters to the national sub-bands where possible, the local sub-bands could be re-planned, making space available everywhere. In many cases, moving a local radio transmitter to the national sub- bands will increase incoming interference, particularly during temperature inversions when high powered FM transmitters carry much further due to ionospheric reflection. Because of this, lower power transmitters are more suitable for relocation because their powers can be increased to compensate without outgoing interference becoming a problem. A number of case studies are presented on other pages on Frequency Finder - see the side bar.

Synchronisation

Normally, a spacing of 200 kHz is too close to avoid interference. However, if the two transmitters carry the same programme and are time synchronised to within 25 μs, then both carriers will always be on the same side of the centre frequency, preventing the receiver being captured by the unwanted signal (the stereo difference signal is always of equal or less magnitude than the sum signal). Consequently, if the difference in the signal path to the two transmitters does not vary by more than 15 km over the area where they might otherwise interfere, they can be synchronised and operate at 200 kHz spacing. Thus, within the UK, there is scope to move a number of low powered fillers to new frequencies 200 kHz away from the corresponding main transmitters, freeing up space. However, this will increase the transmission cost as a separate feed will be needed instead of simply using the main transmitter's FM signal to feed the relay. This technique has already been implemented for a number of commercial stations in the Netherlands.