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1 Introduction

BBC Southern Counties radio (often abbreviated to BBC SCR), is the BBC local radio stations for Sussex, Surrey and North East Hampshire. By using multiple frequencies the station can cover its target area.

Each individual station (where it has one or more transmitter), uses a system called "Alternative Frequency" which is a sub-function of the R.D.S system (code transmitted with the Frequency Modulated Signal). The basics of this are that when one signal becomes weak then another, stronger signal (of the same station), will be automatically tuned to by the radio (which has the Alternative Frequency function on it). If however, in the alternative frequency list, there is a frequency, which is present, and is not part of the station currently tuned to, the radio will not tune to this signal because the PI Code (a code transmitted with all signals, which have an RDS label) will be different.

Unfortunately there are number of other radio stations very close to BBC Southern Counties Radio and in places these cause interference to BBC SCR as BBC SCR does to adjacent channels.

One of the major problems is XFM (A new rock music station for young people), broadcasting from London. XFM (104.9MHz – 2kW) which breaks into BBC Southern Counties on 104.8 on some areas particularly on high ground to the North of the region. This can be particularly frustrating for the listener of BBC Southern Counties Radio, this also occurs in the shaded area for 104.8 on the map! This is especially frustrating for the older listener who is typically not interested in this kind of music, but more into talk and "older based" music. Also the Q factor and adjacent channel rejection ratio may not be as good on an older set.

Figure 1 – the entire coverage area ^[1]

Map © BBC Southern Counties Radio

Figure 2 - Individual transmitter cover maps which make up the network ^[1]

(Original maps © BBC Southern Counties Radio)

Figure 2, shows the current configuration. Note all Medium Wave frequencies are all un affected.

2 The Frequency Swap

Looking at the transmitters, strangely, 104.8 is close to Wave 105.2 off a mast no more than 25 miles away (running at 10 k W), although not as close as BBC Southern Counties and XFM are. (These two are worst cases due to the style of music!!).

However, the reception is definitely effected by the other mast and vice-versa, with both stations causing interference.

Mike Smith very kindly carried out some unofficial pixel plots of the various transmitters, which clearly show the interference. Whilst these plots do not guarantee 100% accuracy with real-life signals these seem relatively close, particularly the Crystal Palace v Burton Down plot.

Mike says "I did not know the height of the aerial at Burton Down, ….it looks like it must be between 40m and 50m agl, so I have done plots for both heights. As the aerials are simple dipoles I have assumed an Omni-directional aerial pattern. In real life, however, there would be some sort of mast null, but I cannot put this in the plot, and it should be very minor anyway I would have thought. ERP is taken to be 2 kW in the vertical plane.

"Crystal Palace is the same in all plots. ERP is taken as 1 kW in the vertical plane (as it is a 2kW mixed polarisation system), aerial height is 144 agl with a cardioid radiation pattern at 50 degrees. The effect of the cardiod antenna can be easily seen. Unfortunately I cannot specify the antenna type too closely, but the one used has an 8dB reduction to the rear....I have included urban loss factors into the plot."   ^[2]

Figure 3- Mike Smith’s plot of Wave 105.2, from Chillerton Down, showing 54 & 60 dBv/m

Figure 4 shows Burton Down (104.8) plotted red, against XFM plotted Blue, in the current configuration. It shows problems around Cranleigh, Horley and Crawley, and Brighton being a possible half way house too! Note how the yellow pattern (54 dbv/m) shows no real impression in Brighton. ^[2]

Figure 5 – Mike has shown the map here for clarity without text and markings to show more clearly the cross over points

"From the plots that I have done for various sites, I would have to say that I think the representations possibly appear to over-estimate the coverage area for a given contour. At a wild guess I would say that it could be 6dB to 8dB perhaps - I am not sure what other variables can be factored in" ^[2]

Other interference comes in the form of 95.3 (Brighton) which causes co-channel interference on 95.0 (Newhaven). This is again where the 2 waves touch. It will be particularly bad on older sets or sets with a poor Q-factor or sets with a poor adjacent channel rejection ratio. Clearly therefore it is required that 104.8 and 95.3 require swapping or exchanging for different frequencies to minimise the loss due to interference from adjacent channels. This is due to the neighbouring frequencies of 105.2, 104.9 and 95 respectively. Therefore a greater spacing is required to create the best coverage – but possible by using less power (and saving money!).

THIS DOCUMENT ASSUMES THAT THERE ARE NO MORE FREQUENCIES AVAILABLE AND THE ONLY WAY THAT A BENEFIT MAY BE BROUGHT IS BY SWAPPING FREQUENCIES AROUND THE NETWORK. This in reality may not be true there may be other frequencies which could be allocated, however a worse case scenario for change is assumed – as the source below states, spare frequencies in the South and South East are sparse!

"….the whole issue of the allocation of 104.9 / 104.8 in the South East is shrouded in bitterness. The BBC originally had 104.9 allocated for a relay station for BBC Radio Berkshire at High Wycombe [*]. Just before Radio Berks was launched, the then IBA got approval to advertise two more London wide ILR licences, because the application for 102.2 was so oversubscribed. The BBC reluctantly handed over 104.9, and this was given to Melody. (Kiss FM ended up with what would have been Classic FM's Guildford allocation, 100.0).

ISTR that the Burton Down allocation of 104.8 existed at the same time as the 104.9 slot for High Wycombe. It was just that Burton Down came on air after Melody was launched. By this time 105.4 became available in London, so the RA moved Melody, and readvertised 104.9 as a 'not quite' London wide licence.

[*] Even more bizarre, as High W is in Bucks! Mind you Henley-on- Thames is in Oxfordshire, and Radio Berks has a relay for that, as well as R Berks' main Tx site at Hannington, which is in Hampshire." ^[3]

The creation of this frequency swap would resolve most problems. If Burton Down was moved to 95.3 – this frequency would also be more immune to French interference (higher power) there are problems with this frequency in Brighton, although 104.8 seems unscathed from intercontinental sky wave interference. (Major French interference comes mainly in the form of Musiques on 95.6 and Inter on 92.7 MHz, both receivable fairly clear throughout the summer in Brighton – the home of BBC Southern Counties on 95.3 – just 300kHz away!)

104.8, would become Brighton. From there XFM would then not clash and hence not cause interference. However, it makes 104.5 and 104.8 rather close, so moving Brighton’s frequency to 104.9 would be advisable.

Wave105 would not get close to 104.8. (400kHz channel spacing). However its only 300kHz to 104.9, but since this transmitters coverage doesn’t get out of Brighton too much then things should be OK. Ideally Wave is a station aimed at people mainly living in Hampshire and Dorset – however there is a bit of an overspill into Sussex. Alternatively by the maps provided by BBC Southern Counties the Brighton mast could be 104MHz (the same as Reigate), now this is not ideal as the Reigate signal is receivable in Brighton!! For this to work both transmitters would have to be reduced slightly in power – Reigate especially, this would reduce the power bill for the BBC!

104.9 is easier to find from the main frequency of 104.5 – this is where the only city is in Sussex! But also 104.5 does cover most of the area that the Brighton mast covers and hence the listener may not need to retune. There may be a possible adjacent channel problem with the use of 104.5 and 104.8, however this can easily be resolved, (as discussed above), and making one of the transmitters more directional would also help.

Also 95.3 and 95 would not interfere. Since there would be a major gap in between of about 30 miles!. – However BBC Oxford & BBC SCR are now close. So BBC SCR from Burton Down could be made 95.5, 300kHz to BBC Oxford since Burton is the furthest transmitter West and near the edge of the area it should be restricted Westwards; therefore there shouldn’t be problem. BBC London could be made 94.8 but this then makes the channel spacing to Brighton’s radio 4 as only 300kHz with this the case BBC SCR could then be 95.1.

Capital FM, may have cause for complaint with it being at 95.8 MHz. However, the Capital format is re-broadcast out along the coast by a station owned by Capital Radio Group. Since the same type of music as London’s Capital FM is available on the South Coast the listener would not lose out.

95.1 (Horsham) is only 40W and hence there should not be too much of a problem with interference there!

Although Horsham too could have a frequency of 104.8 because there are only a few signals which achieve any kind of coverage in Horsham. XFM is a London station and hence "Horsham" is not theoretically in their coverage footprint – so this frequency could be used.

The Horsham frequency in reality could be almost anything due to the low power of the mast! – 103.8 or 103.7 would be a good frequency as this would not cause too much interference to BBC Solent for Dorset (another 500W transmitter for a coverage null in Dorset.) An test here would be to see which distant signals can not get into Horsham and then re-use one of those frequencies (there could be quite large choice here!), because the Horsham transmitter does not cover a large distance at all – just covering Horsham itself! (It’s a 40W mast!!).

Here’s a graphical representation!

Figure 6 REVISED transmission frequencies,^[1]

Alternatively (since the above causes interference) possible in-between Brighton and Heathfield.

Note now Brighton is assumed to be 104.9MHz – I asked Mike Smith to plot the interference this would cause to XFM and vice –versa which he kindly did. Figure 9 below shows literally no interference at all, which surprised Mike and myself. The downs encases 95.3 in most directions apart from West.

Figure 8 – the effect (or lack of), of making BBC SCR off the Brighton mast 104.9. ^[2]

Note how there is barely any interaction at all between this and XFM.

In Mike’s plots he uses a pixel plot programme (called Radio Mobile). Note in the case of the cardoid he has changed the model shown (slightly).

Figure 10 - Cardoid shape used in Mike’s plots, Mike has rotated this slightly, all other antenna patterns are as standard. ^[4]

Figure 11 – shows Microsoft Excels representation of the Radio Authorities figures on the polarisation of the XFM signal. This is relatively close to the shape used in Mike’s plots if the two components are added together. Mike has taken the ERP as 1 kW in the vertical plane Mike’s cardioid radiation pattern is at 50 degrees. Mike, could not specify the antenna type too closely, but the type he used for simulation has an 8dB reduction to the rear. Note the shaded area of for aerial pattern show the attenuation in each direction, not the power

Figure 12 – shows Microsoft Excels representation of the Radio Authorities figures on the polarisation of the 10kW (5kW Vertical & 5 kW Horizontal) of Wave 105.2 from Chillerton Down. Hence all of the power is directed awayfrom the South.

Unfortunately, the exact polarisation of the BBC transmitters is not known because there is no data available on the BBC web site. Whilst it is easily possible to use the polarisation from the respective ILR stations which use the same masts as the BBC this would be foolish as the BBC runs higher powers (in some cases) and in other cases may run different polarisation. In the case of Burton Down there are no independent local radio stations, which use the same mast. This dramatically hinders calculations, as it can not be said for certain if Mike has plotted the various polarisation set-ups correctly due to the various unknown BBC transmitter variables.

Mike then stated that BBC Oxford was on 95.2, BBC Berkshire uses 95.4 at Windsor running at 500W - these had to be taken into account. Mike carried out some pixel plots to prove if either of the 2 transmitters would suffer a loss of service.

Figure 13 shows the reception of BBC SCR on what would be 95.3 v reception of BBC Oxford on 95.2 the power of this mast at Oxford is 5.8kW.^[2] In this plot Mike is showing Burton Down with 60 / 54 / 48 dB coverage, and BBC Oxford. Mike believes that his 60dB plot of Oxford and Burton Down is far more representative of the actual 54-dB coverage as plotted by the BBC.

"I think this programme way overstates the coverage I really don't think you would ever get a 48 dB field strength were the map predicts" ^[2]

Figure 15 shows the reception of BBC SCR on what would be 95.3 v reception of BBC Radio Berkshire on 95.4 this runs at 500W. ^[2]

By the fact that this mast is running at 500W dictates that is either a restricted mast (e.g. like Brighton, Newhaven etc..), or covers a very specific area where there is a coverage null. Note how the 54dB line is touching the 60dB marker of BBC Windsor, which is a problem. However, as Mike thinks the coverage is overstated this would be a worse case scenario.

These plots indicate the transmitter at Burton Down transmitter could use 95.6 to protect BBC Berkshire at Windsor, this would mean a loss of service in parts for Capital (outside of London) – where the channel separation was only 200-kHz. Because Burton Down gets out well (almost reaching the M25!) from its site it would not suffer too much interference from France Musiques on 95.6. Note BBC SCR interferes with itself on 104.5 and 104.6! (This is due to the 200kHz and 300kHz channel separation respectively between the other high power transmitter and the Guildford in-fill). Getting away from 104.8 would solve this!

Alternatively BBC Radio Berkshire could be moved to 95.5 or 95.6MHz. Making one of these changes is essential – being a 500W transmitter there would be little or no interference to Capital. If need be this could be made into 800W.

If BBC Berkshire was allocated 95.5, it will almost never suffer interference from France Musiques on 95.6 (see section 3), the skip distance is too large.

The BBC Southern Counties radio transmitter at Burton Down should either be restricted in coverage Westward to avoid interference to other nearby stations; this is not a problem since Burton Down is the further transmitter West. Its distance to the edge of BBC Southern Counties broadcast area is relatively small in context to the distance to the edge of BBC SCR’s broadcast area in the other directions (bar South!). Alternatively reducing the power from Burton Down will also solve the problem – this saves money. Reducing down from 2kW down to 1.5kW may be enough.

Figure 17 shows the current configuration, this shows visually the distance between the various transmitters. In the proposed change Burton Down becomes 95.3 or 95.5 and Windsor either stays as it is or becomes 95.5 or 95.6. ^[5]

3 Propagation Conditions

"Propagation conditions" is where a distant transmitter normally well out of range is receivable, and tropospheric scatter is the type of propagation condition, which effects signals under 400MHz. Propagation conditions such as Tropospheric scatter, are less frequent than the nightly Ionosphonic skipping condition which affects M.F. (A.M) or Television propagation conditions, tropospheric scatter however does occur quite a few times a year.

As mentioned above France Musiques is heard in Brighton. This map of France shows the stations occupying adjacent or the same frequencies to that of BBC Southern Counties Radio in England.

NB: - (Note in this picture, the Caen transmitter should be located about 20 miles SW of its actual destination).

Figure 17 shows the transmitters on similar frequencies to BBC SCR in France. ^[5]

Of particular note are the high powered ones, Inter at Amiens is 20kW on 95.4 – which is only 100kHz from the 1.2 kW signal at Brighton. France Musiques on 95.6 is 100kW and is at Caen. 104.7 is re-used at Calais at a power of 1kW for RTL. Europe 1 uses 104.9 with a number of masts less than 5 kW. 104.5 is re-used at Rouen. It can be seen therefore that a large number of transmitters use 95.3, which run in England at a power of 1.2kW from a location in Brighton which does not carry far under normal conditions see figure 2. By swapping this with 104.8, as in figure 3, shows that 104.8 and 104.9 will survive in Brighton even during propagation conditions, whilst the higher powered 95.3 Burton Down transmitter should be apply to cope with the other higher powered French signals during propagation conditions. The above image proves this.

Again using Thierry Vignaud’s research.

Figure 18 – Thierry Vignaud’s research ^[6]

This shows clearly that there are more transmitters using 104.5 than 95.3, than 104.8/9. The average power per transmitter is also higher with 95.3. This means it would make perfect sense to carry out this swap and give the frequency of 95.3 to a better transmitting location and using the higher power (2kW instead of 1.2kW). 104.9 clearly is the best frequency and being nearest to France makes a lot of sense since the chance of a signal from the continent spilling over are lower. If Burton Down was made 95.1, here there are less than 30 transmitters on 95.1 each running less 2 kWs, infact 2 at 2kWs, 2 at 1kW the rest below 1kW. However this would cause a major re-shuffle elsewhere.

Figure 19 – Shows the skip distance that the Amiens transmitter would have to cover to cause interference with BBC Southern Counties Radio in its current configuration.^[5]© Microsoft World Atlas 1998 Edition.

A frying and hissing sound can sometimes be heard on BBC SCR on 95.3 in Brighton (this is known as "co-channel" or "adjacent channel interference" to engineers). With the frequencies swapped, an extra 50 kilometres would have to be covered in order for interference to take place!

Figure 20 – The distance from Amiens to Burton Down, in the new configuration Burton Down would be on 95.3 or 95.5 depending on other adjustments around the South of England to other transmitters on adjacent channels. ^[5]

The Caen transmitters needs only to skip 209 miles in order to make its presence felt in Brighton, which it does. Fortunately it is on 95.6 but if any change were to made by the French Authorities to this mast and the frequency was decreased the problem of interference would be very evident. This is only an offset of just 300kHz! – Infact an offset of 300kHz causes problems between Brighton and Newhaven so this kind of offset will have interference related to it. Note the Mast at Mount Pincon (Caen) is 800 Metres High.

Figure 21 – The distance from Mount Pincon (the Caen transmitter to Brighton). At its nearest point to the coast this is only 198 Miles, particularly in the area around Worthing – which is served by the Brighton mast. ^[5]

Figure 22 – the distance from Caen to Burton Down 214 miles (a mast, which covers a large area and is not particularly restricted by the downs). ^[5]

Figure 23 – the distance from the 2kW 104.9 transmitter to Brighton where the frequency of 104.8/9 is proposed is only 175 kilometres however this signal is only 2kW in strength. ^[5]

This is the compromise, between the closer transmitter at Crystal Palace of XFM, the Caen transmitter and Amiens transmitter causing interference. Or the more distance transmitter of Rouen at the same power that may cause interference during skip conditions, and where the other offsets between the South Coast and French transmitters are larger in the proposed new configuration. This all in all means less co-channel interference and interference from Crystal Palace than there is presently now. Also the weaker transmitters (power wise) are up against the other weak transmitters (104.9) is 1kW (heavily restricted) in London, and a maximum power of only 4kW in France, as opposed to 95.3 which has a French neighbour on 95.4 at 20kW and another at 95.6 on 100kW! From a transmitter which itself is restricted by geographical features – indeed the transmitter has to transmit over the tallest hill in Sussex (Ditchling Beacon) – this serves to remove a large portion of the Horizontal Component of the signal. Hence lowering the chances of good coverage.

The following sources have been used for information regarding this design idea;

• 1 - http://www.bbc.co.uk/england/southerncounties/map.shtml the BBC Southern Counties radio transmission frequency page

• 2 - Very sincere thanks also go to Mike Smith for taking the time and trouble to firstly calculate a number of factors and then to pixel plot the various transmitters which have the potential to interfere with BBC Southern Counties Radio – many thanks Mike! Also for proving my theories through his pixel plots! – Mike should also be thanked for reading through the document making further suggestions and improvements. I must also thank Mike for pointing out BBC Oxford and BBC Berkshire (at Windsor) which initially I had forgotten.

• 3 - I also wish to thank the unofficial source for the interesting historical view on the frequency usage.

• 4 - Mike uses a program called Radio Mobile ([4]) for Windows (version 4.1), for the pixel plots, this program is highly recommended by all who use it, the BBC use a program called Plan-TX.

• 5 - The maps are © Microsoft World Atlas 1998 Edition

• 6 - The most sincere thanks to Thierry Vignaud for all of his time in effort in collating information on details regarding French transmitters, their frequencies and powers. Thierry’s web site is absolutely brilliant, seeming to be the only French web site where actual transmitters of France are depicted and described. http://www.emetteurs.fr.fm/

• Thanks also must go to Martin Watkins for acting as a translator between Thierry and myself and in making such constructive comments on earlier versions.

• The radio Authority with their powers of Independent Broadcasters transmitters (e.g XFM & Wave)

• Mike Brown’s web site at www.mb21.co.uk has assisted greatly with pictures of the U.K transmitters with some site heights – however when the site heights are missing the pictures themselves have been used for a little educated guess work of the transmitter height!

• The BBC web site www.bbc.co.uk, and http://www.bbc.co.uk/reception/radio_transmitters/ contained information regarding BBC local Radio transmitters and Radio stations. Although it did not state exactly the polarisation and aerial patterns (by giving in use aerial patterns). It only stated only an M (Mixed) – also known as circular, V (Vertical) or H (Horizontal), neither did it state the aerial height.