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Broadcasting on 3 metres - The BBC's Experimental AM/FM Tests

From the April 1951 issue of Wireless World

Investigations into the. possibilities of metre-wave were instituted by the Engineering Department of the B.B.C. as far back as 1945. The frequency used being in the region of 90 Mc/s. The two rival systems amplitude and frequency modulation, were employed from time to time.

These early tests were definitely encouraging but the power employed was low and so it was decided to carry out a more comprehensive programme of test transmissions on high power, simultaneously on a.m. and f.m. so that direct comparisons could be made. This plan was implemented last year when a new station was completed on the summit of Wrotham Hill which is about 20 miles south-east of London adjoining the London-Folkestone road. If a more precise location in desired, the national grid reference is 51/594604.

It is ideally situated for v.h.f. transmissions as the hill rises to 730 feet above sea level and it in now surmounted by a 470-ft mast of similar design to that used at Sutton Coldfield, but without the television top section. The aerial in this case is the 110-ft tubular section it the top. which is 6ft in diameter and has 32 vertical slots cut in its circumference. These are arranged in eight tiers of four spaced equidistant round the surface of the mast.

For explanatory purposes, a slot cut in a sheet of metal behaves as an aerial in much the same as would the piece of metal removed if it were employed as a conventional dipole. There are certain differences in general characteristics. however, one is that a vertical slot propagates or responds to horizontally polarized waves and a horizontal slot is, of course vertically polarized Wrotham therefore is horizontally polarized and a horizontal dipole, or other aerial system, should be used for hest reception.

Each slot measures 8ft v 1ft and with this vertical dimension the aerial elements are approximately three-quarters of a wavelength long at the frequencies used. The slot being wide compared with its length behaves as would a dipole of similar shape and exhibits wide-band characteristics, being sensibly "flat" the range of frequencies 87.5 to 95 Mc/s. The power gain of the system in all horizontal directions is 8db.

Each slot is fitted with a quarter-wavelength long vertical rod, so positioned and connected that it has the effect of converting the aperture into a folded slot. the counterpart of a folded dipole, but curiously enough, with a lower input impedance than unfolded, In the case of the Wrotham aerial the slot impedance is 150 ohms. The outputs of the f.m. and the a.m. transmitters are fed simultaneously into the one aerial system.

"FMQ" System of Modulation

The f.m. transmitter is of unusual design in that it embodies a quartz crystal oscillator and the crystal is actually frequency modulated by the audio signals. This system of modulation, developed by Marconi's Wireless Telegraph Company, is known as "FMQ." The frequency deviation at the crystal is not large. being of the order of a few parts in a thousand, but a comparatively, low frequency crystal is employed and its output passed through four frequency multiplying stages before the actual radiated frequency Of 91.4 Mc/s (3.28 metres) is reached. In the case of the Wrotham transmitter. which is made by Marconi's, the multiplication amounts to 24 times.

The frequency deviation, or modulation range, or depth. is multiplied by a similar amount and at this working frequency a deviation of 75kc/s is is obtainable. This is a measure of the depth of modulation and bears no direct relationship to the range of frequencies that can be handled.

Frequency multiplication takes place at a comparatively low power level, the output at the 24th harmonic being of the order of a few watts only. Six stages of amplification at the working frequency are consequently employed. the first two are conventional push-pull amplifiers and the final four are single-ended earthed-grid stages with co-axial line tuning elements. The final. or output, stage consists of a pair of giant BR128 air-cooled valves operating in parallel and giving an r.f. output Of 25 kW. an unusually high power for v.h.f. equipment of this kind.

The r.f. portion of the a.m. transmitter is identical to the f.m. set, the only difference being that the "FMQ" circuits are rendered inoperative. The quartz crystal is chosen to give a frequency, after 24 times multiplication, Of 93.8Mc/s (3.2 metres) and the r.f. power output is. in this case 18kW. These are quiescent carrier figures; while the aerial power does not change in the case of the f.m. transmission it undergoes considerable variation in the case of a.m. An increase of anything up to 50 per cent is possible depending on the depth of modulation and the waveform. In this equipment the final r.f. amplifier, again a pair of BR128 valves, is modulated by a class "B" push-pull stage fitted with two ACT14 valves.

Feeder System

Both transmitters are controlled and monitored in a single room adjacent to, and with windows looking into, the transmitting hall. In addition to switches and meters for controlling and monitoring the voltages and currents at each stage of the transmitters,
there is also included special apparatus for measuring the frequency deviation (modulation depth) and for checking any shift in the mean carrier frequency.

A concentric feeder system connects the output from each transmitter, via an harmonic filter, to a combined filter unit which prevents power from one transmitter being fed into the other, but diverts it into a common concentric feeder and thence to the aerial. The aerial system is common to both transmitters, as explained earlier.

Various methods have been evolved to enable one aerial to be used simultaneously for two or more transmitters. The diplexer as used at Sutton Coldfield is one and the Wrotham system, which consists of sections of concentric line, is another. The feeder is a large copper tube of some 6 to 7in outside diameter and with a surge impedance of 51 ohms. Dry air is pumped into it and into the combined filter in order to exclude moisture. which. if allowed to accumulate, would change the characteristics of the feeder system to such an. extent that it would upset the loading at the transmitters and very likely cause serious damage to the output valves unless a drastic reduction in power were made.

The slotted aerial system was designed by the Engineering Research Department of the BBC, the associated feeder system was developed by Marconi's and the mast designed and erected by British Insulated Callender's Construction Company.

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