Saffron Green is a 4 mast directional MF array near Barnet in North London which superceded Lots Road, a temporary site established earlier to allow Capital and LBC to get on air by their deadline in 1973. Saffron Green was constructed in 1975 to allow the station to radiate high powers towards London on 1548kHz and 1152kHz whilst radiating virtually no power at all towards Birmingham and Liverpool where the frequencies are used by other ILR stations.
Roger Piper writes :
The original Capital installation at Saffron Green consisted of four 10kW linear amplifiers, Marconi if I remember correctly, one on standby and three in use to give the 27.5kW needed to achieve the licensed erp of 100kW maximum. Initially the transmitter failed to produce the expected power output. IBA staff traced this to the coaxial cables from the drive unit to the individual amplifiers each having been routed most conveniently so they were not equal lengths.
With the coordinated frequency changes in 1978 Saffron Green Capital's MF transmitter had to be changed from 1546kHz to 1548kHz. At first all seemed well, but driving around away from the main lobe there seemed to be audio distortion not there before the change. Apparently the initial changes had resulted in the directivity of the antenna array being frequency dependant across the 9kHz wide channel, resulting in incorrect sidebands to carrier relative levels. A complaint to the IBA resulted in an acceptable change.
I asked Ray Hills, former IBA Assistant Director of Engineering, what he remembered about the problems of finding a permanent London MF site and have the following reply which may be of interest.
"At that time I was not responsible for service planning and site finding so I don't know the details. As far as I recall, Brookmans Park was ruled out because of the high powers being used there and because, unlike TV, radio site sharing was not mandated by Government. I don't think we shared any radio sites with the BBC. Even the VHF ones were on existing IBA masts (or new sites of course). The incinerator site was, I believe, somewhere near Heathrow."
"I don't know how we achieved the Saffron Green site but it is not beyond possibility that some pressure was applied at a high level."
As mentioned in the mb21 Transmission Gallery, on the evening of 1-Jul-1985 a serious fire occurred at Saffron Green. Originating in the transformers of a reserve transmitter for LBC, the fire spread and affected other parts of the building, and resulted in the loss of the AM service from Capital Radio. IBA maintenance staff worked throughout the night and had restored both services by 6.00am the following morning. The extensive damage to the building meant that transportable radio transmitters had to be brought to the site and special measures taken against chemical hazards. Permanent replacements for the LBC transmitters were due to be installed in April 1986.
Polar diagram and interference
In a discussion in September 2020, Dan Goldfarb wondered whether the null in the 1548kHz polar diagram at 67 degrees was designed to protect an adjacent transmitter coverage such as Lithuania on 1557kHz. Mark Carver said that all of the IBA built arrays, only had deliberate nulls to protect co channel and not adjacent channel transmissions.
Saffron Green was designed to generate nulls towards the other UK 1548 kHz sharers, Bristol, Liverpool, Sheffield, Teesside, Edinburgh (possibly), Wolverhampton (never used). Given 1557 was later used by the IBA in Southampton, and Northampton, (without problems) he really couldn't see why 1557 in Lithuania would have required, or even qualified for protection, particularly as it was way off beam anyway.
Martin Watkins added that planning just didn't work like that. The main beam on 1548 kHz goes on and straight through Nice, which also used 1557 kHz. When he was on holiday in the French Alps in the 1970s the early morning signal from Capital knocked the Nice signal (300 kW) into a cocked hat.Before the Second World War he believed that - with stations often having audio up to 8 kHz on a good day, transmitters with overlapping areas (eg France and Britain) would be allocated frequencies at least 18 or 20 kHz apart, as otherwise the sidebands would indeed have caused mutual problems. But all that went out of the window by the 1960s, and older valved sets with wider selectivity often displayed the spitching effect on sibilants from a station next door to the one you were listening to. Again - in the French Alps - Roumoules on 218 kHz constantly spitched across Droitwich 200 kHz; one signal was incredibly weak, the other practically lit the valves on his Philips mains radio on its own.
He questioned whether that null on 55 deg was simply the result of a similar null on 265 deg, (which is needed), as a result of basic symmetry?
Try it: main beam 160 deg, important and necessary null on 265 deg, angular difference + 105 deg. If you go round the other way 160 - 105 deg = 55 deg. Ah, there's the other null, he commented.
Roger Piper added that he couldn't think of any reason to give protection to a station on adjacent channel. The audio bandwidth was, of course, restricted.
As far as freak reception is concerned he could only repeat what is, he believed, already well known which is that the standby antenna was a sloping wire hung off the front mast so would have little directivity compared to the four mast array.
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