Transmission Projects - Reminiscences
|Lisnagarvey and the Blaw-Knox mast
By Aubrey McKibben (Australia)
New: See the contribution at the end of this article by John Peacocke.
BBC ‘Lisnagarvey Transmitting Station’ was opened on the 20th March 1936 in a rather overpowering ceremony held - unbelievably - at the actual transmitter site! It was attended by various high-ranking BBC and local dignitaries such as A.C. Norman chairman of the BBC, Sir John Reith Director General, and BBC regional director G.L. Marshall. Civil dignitaries included Lord Craigavon Moderator of the Church of Ireland, the Archbishop of Connor and many others. In today’s world a gathering such as this is inconceivable for the opening of a ‘mere’ broadcast transmitter, however it must be borne in mind the political significance of Northern Ireland as part of the ‘United Kingdom’ and the demand for broadcasting at the time.
The station was to be part of the BBC’s ‘Regional Scheme’ conceived by P.P. Eckersley in 1924, the aim of which was to give listeners a choice of programmes of good signal strength, covering most of Britain. Transmitters needed to provide good reception on inexpensive receiving apparatus within about an eighty miles radius of the site. Northern Ireland became the 6th Region to have a new ‘high power’ transmitter installed. With its 100KW ‘Marconi built’ transmitter, Lisnagarvey was the most powerful medium wave transmitter in the U.K. at time of commissioning. Prior to this Belfast had been served by a low power 1KW transmitter (callsign was 2BE) operating from the Belfast Power Station, which closed the day of Lisnagarvey’s opening.
The new ‘high power’ transmitters of the ‘regional scheme’ produced very high local field strengths ‘blanketing’ the wireless front ends of the era meaning that nearby listeners had difficulty tuning in any other stations! High power sites like Lisnagarvey were therefore best located in sparsely populated areas.
"Five sites were considered" (Pawley p93) and the ‘townland’ of ‘Blaris', just outside the ‘Irish Linen’ town of Lisburn, was chosen thereby putting the station 9 miles southwest of the densely populated Belfast. A ‘Marconi’ 100KW transmitter "was ordered in mid 1935" (Pawley p93) and Lisnagarvey was to be distinguished by the erection of what was to become the U.K.’s only ‘Blaw-Knox’ style of mast. The massive new mast radiator aerial had an overall length of just over 500 feet and made a strong visual statement in the local countryside. It still stands out boldly against the sky, and one is strongly impacted by its ‘cigar-shape’, being considerably ‘thicker in the middle’ than at either at either of its ends - like two elongated diamonds.
The true reason for the mast's anti-fading characteristic
One factor alone imparted the anti-fading characteristic. Quite simply: its electrical length, which was ‘adjustable’! It was the first BBC vertical radiator that had an easily adjustable electrical length. At this point of the story, it is helpful to consider the evolution of the vertical broadcast radiator with respect to anti-fading characteristics.
In December 1924, Stuart Ballantine of the Harvard University Physics Department released two historic papers to the Institute of Radio Engineers (IRE). His second paper is particularly relevant because he specified a height of 0.64 wavelengths (commonly accepted as 5/8 or 225 degrees of a wavelength) for producing the maximum radiation in the horizontal, broadside direction.
Towers henceforth erected at Ballantine’s suggested height were to prove his theory of ‘increased radiation in the horizontal direction’, as he had predicted.
Confluence of two factors
Due to a most unfortunate confluence of two hitherto unknown and unrelated factors, the broadcasting industry soon uncovered problems with the early 5/8 Blaw-Knox towers.
With all of this in mind, now let’s review the Lisnagarvey radiator's dimensions and design criteria.
Lisnagarvey's dimensions and design criteria
The overall length of the main tower body was 475 feet (BBC Annual 1936 p75) to the apex of the top ‘elongated pyramid’. Then there was a further extension; "a sliding top mast which has a maximum height of 75 ft" (BBC Annual p75). Further; "At the top of the mast was a capacity ring mounted on a supporting mast, which could be raised or lowered to alter the electrical height of the whole mast" (Pawley p94).
This means that, overall, the mast's length was very close to a ‘half-wave’ in length physically. However, because of the ‘capacity hat’ the ‘electrical length’ of the radiator would, by very design, have been considerably longer than the visible (and easily verifiable) ‘physical half wave length’.
So, it was to be this ‘adjustable electrical length’ that was the only factor imparting any of the magic ‘anti-fading’ characteristics, despite its interesting cigar shaped appearance. It would have allowed engineers to experiment with the ‘electrical length’ thereby fulfilling the overall aim of installing such a massive and expensive antenna i.e. to obtain the greatest coverage area!
To quote from the ‘BBC Annual of 1936’: "The height of the top mast is used to adjust the electrical length of the mast to suit the operating wavelength" (page 76)
What ‘electrical height’ would they have been experimenting with prior to commissioning?
In the months prior to Lisnagarvey’s commissioning in March 1936, engineers must have spent quite a bit of time ‘adjusting the sliding mast’ for what they considered to be the best coverage area.
The Blaw-Knox towers in the USA, which Lisnagarvey was very much based on, had a "a height in conformance with Ballantine’s ‘optimum-height’ formula, which was five-eights wavelength" (Laport p79).
RCA’s George Brown had been intimately involved with the early radiators and therefore had authority to state with some accuracy: "Conventional antenna theory of the period [the early 1930’s] - had shown that for the largest service area free of night-time signal fading the transmitting antenna should be fifty-nine percent of the operating wavelength in height." (Brown p43)
The early towers of WSM and WLW were exactly ‘fifty-nine percent’ of a wavelength long when first erected, being shortened in later years.
It may well be, therefore, that BBC engineers based their original experiments on ‘fifty-nine percent’ of a wavelength as Lisnagarvey’s nominal ‘electrical length’ as Brown described.
Indeed, would BBC antenna engineers have been aware of the need to use a radiator adjusted to an ‘electrical length’ of 190 degrees (53%) considering the context of a ‘mid-thirties’ timeframe?
In his book published in 1952, Edmund Laport states that "By about 1934, the modern broadcast radiator had evolved to its present state" (Laport p81). This statement seems to indicate that knowledge of the requirement for a 190 degree radiator may have been known by the time the Lisnagarvey radiator was in the planning stages?
Without access to the station's original engineering documentation, it is difficult to know precisely what ‘electrical wavelength’ they were experimenting with. Nevertheless an accurate range of wavelengths can be still be extrapolated from available information. For instance, we have determined that the tower would have been electrically longer than a half wave (180 degrees) and likely to have been adjustable to be as long as a ‘5/8 of a wavelength’ (62.5%) ‘electrically’
In the United States it appears that ‘190 degree modifications’ were not carried out to the original ‘5/8’ Blaw-Knox towers until the late 1930’s.
Referring to the WSM Blaw-Knox still standing in Brentwood near Nashville, Chief Engineer Watt Hairston states that "by 1939, it was determined the tower was electrically [long]" (Hairston). The WLW 5/8 tower in Mason Ohio was also modified back to 190 degrees at some stage.
The time setting for commissioning the Lisnagarvey radiator
It is reasonable to surmise that site plans for the Lisnagarvey site would have been under way by 1935, or possibly even 1934 considering the extensive mechanical and electrical engineering of the site. Brown and Gihring’s paper criticising the design of these radiators was only released in April 1934 by which time the Lisnagarvey radiator may have been decided upon. The paper's ‘findings’ may have taken some time to disseminate within the broadcast engineering industry worldwide, therefore BBC engineers were most likely unaware of this ‘most recent of papers’.
Considering that "it was not repeated for the BBC" (Pawley p94) the very least one can assume is that there were not enough reasons for the BBC to continue using such elaborate towers. The Lisnagarvey mast was expensive to build and undoubtedly ‘field tests’ indicated a lack of any special attributes over ‘normal straight towers’. Perhaps they even discovered deficiencies with predicted field patterns as predicted by Brown and Gihring?
With time, Brown and Gihring’s 1935 ‘findings’ would have eventually made it into the hands of BBC ‘Research’ for them to mull over before ordering any more towers of the same design for the BBC. Indeed, when one considers all the factors – very high erection costs, confusing field pattern results, and in time, papers proving inherent design flaws - it is easy to understand why the Lisnagarvey radiator was ‘not repeated in the U.K’.
The BBC's first - and last - Blaw-Knox mast
As one BBC engineer commented to me: "As far as I can recall, marketing had quite a lot to do with their popularity in the United States. [The Blaw-Knox company really pushed their towers!] - I’d be prepared to bet that someone believed the hype and we tried one. It would also be logical to assume that [BBC] ‘Research Department’ checked the performance – and that would be why we never bought any more (rather a lot of unnecessary steel work)"
It’s interesting to note that the Lisnagarvey style of radiator was the only one of its type to be erected in the whole of the U.K.! Burghead, the Scottish Regional Site commissioned about 7 months later, had a ‘normal’ ‘straight’ sided radiator!
A somewhat sad looking, ‘decapitated’ radiator still exists today on the original Lisnagarvey site. About a quarter of the ‘top pyramid’ of the mast has been removed as the result of a number of frequency changes at the station over the years. Without this severe pruning, the radiator would be way too long for its present electrical wavelength/frequency. This would increase the emission of unwanted ‘high angle’ sky-waves radiation thereby reducing the desired coverage area.
It perhaps also demonstrates that RF antenna engineers have absolutely no sense of aesthetic sensibility having initiated such barbarism to this exceedingly grand old lady from 1936! J
Maxwell, M.W (1990) Reflections: Transmission Lines and Antennas.
Newington, CT: American Radio Relay League.
Edward Pawley: BBC Engineering 1922 – 1972.
E.A. Laport 1952: Radio Antenna Engineering.
BBC Annual 1936.
George H. Brown: ‘and part of which I was’ - Recollections of a Research Engineer:
(courtesy of Jim Hawkins' Radio and Broadcast
New: Further information from Aubrey McKibben:
Many Blaw-Knox towers, of both conventional (uniform cross-section) and diamond design, remain in use in the United States. Few of the diamond towers were built, and several remain; all transmit AM radio signals:
Cincinnati, Ohio: 747 ft (227 m); originally 831 ft (253 m); tower located in Mason, Ohio.
Charlotte, North Carolina: three towers, 428 ft (130 m) each (one original, two reproductions from the original plans after the originals were destroyed in a hurricane).
Manchester, New Hampshire: 400 ft (121 m) Columbus, Ohio: 380 ft (116 m)
Several additional such towers are in use at stations in California but are less well-known.The following Blaw-Knox diamond-cantilever towers remain standing in Europe:
Lakihegy Tower (constructed: 1933, height: 314 metres/1030 feet) at Budapest-Lakihegy, Hungary
Vakarel Transmitter (constructed: 1937, height: 352.9 metres/1158 feet) at Vakarel, Bulgaria
Stara Zagora Transmitter at Stara Zagora, Bulgaria
Riga LVRTC Transmitter at Riga, Latvia
Further information from David Porter:
There was another BBC Blaw Knox site. It was at Droitwich and was erected in the War. After the war it was used on 1088kc/s and the matching required to the 120R feeder was just a few micro Henry of L in series.
For 1052 kHz in 1972 a tweak was required. The tower was of the shape more akin to a pylon, certainly not the double pylon of the LIS configuration.
It was dismantled in the mid 80's when the two MF masts were erected to the north of the site to gain directivity more easily.
Further information from John Hettish:
John has over fifty photos from all angles taken around and on the WSM
Blaw-Knox tower posted on his web site
New: Further information from John Peacocke:
The part played by Lisnagarvey staff in the critical period of the Ulster (Electrical Power station) Workers` Strike in 1974 should get a mention. The Marconi transmitter was for much of the country at certain times the only communication between Government and people who were trying to keep businesses, farms and schools alive. This was because the high powered Diesel generator and MW transmitter could be kept going and could always be received on battery portables. Engineer [name removed] was effectively locked in for days and hostile pickets made it very unpleasant for the engineers who came to relieve him.
I was the Site Engineer at Lisnagarvey in 1981, when TCPD REPU set about demolition of the old original transmitters. The Marconi railway carriages and associated crypt full of cooling coils were junked and a Doherty BBC design supplied its replacement. The mighty H &W marine Diesel was dragged away to finish its days in a sawmill. The High Tension rotary generator was cut up for its brass and copper. Harris 10 KW units were imported and installed with a new two masted centre fed T array. The Blaw Knox aerial was jacked up and its ceramic high voltage supporting insulators were washed and put back again. The result was a station which required a very minimum of maintenance, with a resident mobile maintenance team who serviced TV and FM transmitters in conjunction with a similar team based at Sheriff`s Mountain Londonderry. The place was full of ghosts and echoes, there were archaeological moments such as a magpie`s hoard of small gold and silver items, and a highly polished light switch which for generations had afforded a periscopic view of the Senior Engineer`s approach. There was a glorious Imperial themed sculptured facade to the building, with flying boat and steel tracery. I believe it was taken away subsequently and preserved. I worked for a while in NE England, and I can confirm that the sky wave coming from Lisnagarvey did actually provide useful coverage there. I recall that the BBC N.I material in the 60`s was very much in plummy Received Pronunciation and would have been quite intelligible anywhere in Britain. Not really so any more. JOHN PEACOCKE E110934F.
Aubrey McKibben would be pleased to hear from you if you would like to
discuss this or related topics.