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1936 ANGLO-FRENCH MICRO-RAY LINK, THE FIRST UHF DATA LINK.
Peter Butcher, DEHS.
Scope.
This paper discusses the generation of UHF frequencies by ‘valve oscillators’ before going on to give
details of their first commercial use in a cross-channel link between the airports at Lympne, in Kent
and Saint - Inglevert in France.
Early UHF oscillators.
As the quest for higher frequencies with valve oscillators progressed, the transit time (the time taken
for the electron stream to pass from the grid to the anode) became important. In a triode at UHF the
anode current lags the grid voltage due to transit time. Now consider a simple diode connected to a
battery via an RF choke. At time to a certain amount of charge leaves the cathode and after a time, the
transit time, appears at the anode. The pulse of current leaving the cathode at t o depends on the anode
voltage at that time, i.e. the anode current is a function of the anode voltage at the beginning of the
transit time. There is a lag between the effect of the anode voltage and the anode current. With a
disturbance in the circuit, given the right choice of anode voltage and valve dimensions it is possible
to have anode voltage falling as anode current is rising, giving a negative resistance and the circuit
can oscillate, albeit at very low power. For a 3mm cathode to anode gap and 100V on the anode,
oscillation occurs at about 500MHz. Clearly transit time can be reduced by reducing electrode
distances and by increasing anode voltage, but ultimately the two are mutually exclusive.
In a triode, electrons leaving the cathode,accelerate towards the high potential anode, passing
through the grid wires, which are normally biased slightly negative. Plotting the voltage seen by the
electrons as they pass from cathode to anode shows a rising voltage, with a dip in the curve to a
negative value in the vicinity of the grid. Power can be dissipated in the grid circuit by energy
exchange with passing electrons. Neglecting bias on the grid, if the grid is going positive as an
electron approaches, the electron accelerates and gains energy, but as it passes the grid it is slowed
down and looses energy. If the grid potential remains static, the net loss in energy is equal to the net
gain. However, if the grid potential changes during the time the electron is passing, then there can be a
net loss or gain in electron energy, depending on which way the grid potential moves. This
phenomenon gave rise to a whole new class of oscillators.
Barkhausen – Kurz Positive Grid Oscillator. Heinrich Barkhausen studied at Munich and Berlin
Universities before earning his doctorate at Gőttingen in 1907. He joined Siemens and Halske in
Berlin, but in 1911 accepted the professorship in the Communications branch of Electrical
Engineering at the Technical Academy in Dresden. He worked on theories of spontaneous oscillation
and non-linear switching elements.
However, possibly his best known legacy to science was the Barkhausen Effect in magnetisation, that
of magnetic domains in magnetic material, discovered in 1919.
In 1920, together with Karl Kurz, he developed the Barkhausen- Kurz positive grid oscillator. The
oscillator used a triode valve, with the grid biased to + 200 V and the anode at about – 2 V with
respect to the cathode. Electrons emitted by the cathode were accelerated by the high potential on the
grid and most passed through the grid wires and approached the anode where they decelerated due to
the negative potential, came to rest and were accelerated back towards the grid by its high positive
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potential. Again, many passed through the grid wires, came to rest near the cathode, then accelerated
again towards the grid. Some electrons oscillated between the cathode and anode until caught by a
grid wire. As electrons moved past the grid structure they induced opposite charges on the grid and
oscillating electrons could give energy to the grid,
therefore ac current flowed in the grid lead. Energy
given to the grid was at a frequency controlled only by
the transit time of the oscillating electrons, not the
resonant frequency of an external circuit used to
extract it. Figure 1 shows a basic circuit for such an
oscillator. A Lecher line, tuned by a sliding short
circuit, is tuned to the oscillation frequency to couple
the power out. A blocking capacitor at the anode
isolates the high grid potential of 200V (+u).
The frequency is given by:
f = 1/d√[eu/2m].
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f = frequency.
d = anode to cathode distance.
e = electron charge.
u = anode to grid voltage.
m = electron mass.
Figure 1: Basic Barkhausen – Kurz Oscillator Circuit.
Since the frequency depends only on the electron mass, such an oscillator has been used to verify the
influence of an electrostatic potential on the inertial electron mass, as predicted by Weber’.s
electromagnetic theory [1].
Barkhausen - Kurz oscillations in zig-zag filament Tungsten lamps was reported as a source of
interference to 405 Line Television in the 1950s [2]. Later, Gill and Morell found a positive grid
oscillation where the frequency was determined by an external resonant circuit, usually a transmission
line. It is probable that the Gill - Morell oscillator found a more practical use than the original
Barkhausen - Kurz oscillator and was used in the Micro- Ray link to be described in this paper.
Following the original work, there was an intense amount of research, both theoretical and
experimental, aimed at producing specialised devices (valves), antennas, methods of introducing
modulation and detection.
Specialised Devices. The valve nomenclature of grid and anode were soon replaced by more
descriptive terms ‘oscillating electrode’ and ‘reflecting electrode’. Special valves were
developed, with the ‘oscillating electrode’ taking the form of a wire helix surrounding the
filament and having a lead-out at each end, connected to a transmission line from which the
power was taken. The ‘reflecting electrode’ was a cylinder surrounding the helix. The
filament was a straight Tungsten wire.
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Figure 2: Construction of specialised ‘Micro-Ray’ valve.
The two valves shown in Figure 3 have
an internal construction shown in Figure
2, with the ends of the ‘oscillating
electrode’ brought out through the
envelope as close as possible to the
electrode to avoid radio frequency losses
that would be incurred through the valve
base with its pinch and long leads. The
axial filament is a pure Tungsten wire.
The ‘oscillating electrode’ is 20 turns of
Tungsten wire and the ‘reflecting
electrode is a Molybdenum cylinder.
Figure 3: Micro-Ray valves, scale on the left is 14 centimetres.
Antennas. The frequency used in the first Link was 1,724 MHz, wavelength 17.4 cm, so the
use of optical-like beam forming was investigated. Possible antenna systems investigated
were; Wire Arrays, Lenses, Zone Plates, Parabolic and Paraboloidal Mirrors and Echelon
Gratings. The final method chosen was a hemispherical reflector in front of the half-wave
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dipole antenna at the focus of a paraboloidal aluminium mirror. The antenna gain was 28 dB
with a further 3 dB from the hemispherical reflector. [3].
Modulation. It was found by experiment that the same frequency could be generated (given a
constant load) by different pairs of ‘oscillating’ and ‘reflecting’ electrode voltages, although
with different output powers. Plotting these voltages showed that amplitude modulation with
a substantially constant frequency was possible by simultaneously varying the ‘oscillating’
and ‘reflecting’ electrode voltages. [4].
Detection. Although detection of the received signal was possible using a crystal or any other
normal means, using the Micro - Ray valve gave amplification, important for the weak signals
received, as well as detection. The valve was used in a non-oscillating mode, with the signal
applied to the ‘oscillating electrode’. Two sets of voltages for the ‘oscillating electrode’ and
the ‘reflecting electrode’ were found, depending on whether the amplifying or detecting
action was to dominate [5].The amplifying condition was accompanied by a marked
selectivity for the frequency to which the ‘oscillating electrode’ circuit was tuned and thus
was the chosen setting.
Practical Links.
The first step was in March 1931, with a public demonstration of a Micro-Ray duplex radiotelephone
link across the English Channel from Dover to Calais [6]. This experimental link had been established
in February 1931, with the English side located in two wooden huts about 80 yards apart on the cliffs
about a mile north of St Margaret’s Bay. The huts were about 80 yards from the edge of the 200 ft
high cliff. On the French side the huts were situated on Cap Blanc Nez, about 13 km South West of
Calais, at Escalles. The huts were on high ground, 450 ft above sea level, about 500 yards back from
the cliff edge. The two stations were 35.7 km apart. The demonstration was given by STC Ltd and Le
Matériel Téléphonique (both I.T.T. Companies).
Each transmitter and receiver employed a 3m diameter paraboloidal reflector, with the antenna
doublet, 2cm long coupled to the active valve by a 4 cm transmission line. It was found that the
transmitted waves were not plane polarised, as would be expected, probably due to the fact that the
transmission line to the doublet and the active device were all transmitting. This problem was
overcome by using a half wavelength radiator and a coaxial transmission line feeder with the active
device mounted behind the reflector, thus shielding the active device from the reflector.
The choice of sites for each end of the link was an important factor influencing performance. Ideally
there should have been an uninterrupted straight line of sight between the stations, with the
transmitted beams passing as high above ground as possible. At St Margaret’s, however, the
transmitted beam only cleared the top of the cliffs by about a foot, whilst the incoming beam from the
French side cleared the cliffs by about 14 ft. Measurements showed that there was an 8dB gain in
signal received at Escalles if the St Margaret’s site transmitted from its normal receiving hut where
the beam cleared the cliffs by about 14 ft.
Observations were made on the St Margaret’s experimental link February to June 1931, May to
December 1933, May to August 1934 and May to July 1935. On the Lympne to St Inglevert link, once
in operation, observations were made from January 1934 to July 1935, although operation was not
continued long after sunset. Only limited observations were made after dark, but no appreciable
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difference in fading was noticed between day and night time. Few observations were made on the
experimental link during winter, owing to the exposed position at Escalles. Atmospherics of the
normal type were never heard on either links. Much data was collected from the operation of both
links.
Analysis of the data showed:
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The most stable conditions occured with stable air temperature and pressure, although
the actual values were unimportant.
With stable temperature and pressure, rain, hail, snow, or fog did not affcct the link.
Excellent operation was obtained during thundery periods.
A high wind almost invariably gave good performance.
A settling fog bank gave sever fading until the fog ceased to move.
During summer, extremely violent fades of short duration, 1 to 2 minutes, occured.
The British and French Air Ministries were impressed by the demonstration and saw such a link as
being able to control air movements of commercial cross-channel flights to France. With the increase
in flying speeds the existing control method of a landline to Croydon, then a 1,380m Radio Link to
France meant that often aircraft arrived before the message saying they were on their way! [6].
Colonel Sosthenes Behn, President of ITT offered an advantageous price for the link, no doubt seeing
in it a publicity coup, and a system was ordered by the Air Ministry.
On 26 January 1934 a Micro-Ray link was opened for commercial services between the aerodromes at
Lympne in England and St Inglevert in France, 56 km apart, using a wavelength of 17.4 cm, providing
a duplex service on radiotelephony, teleprinter, or Morse telegraphy. In order to facilitate duplex
operation, slightly different wavelengths were used in the two directions. The difference was trivial,
about 0.5 mm, giving a frequency separation of 5 MHz. [7]
The Equipment. A simplified circuit for the transmitter is shown in Figure 4. The salient
points to note are the following:
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Capacitors C1, 4 µF, block dc from the modulation potentiometers.
Capacitors C2, a few pF, block dc from the transmission line feeding the antenna.
Chokes L1 block the modulation voltage from the power supply.
Transmission Line TL1 feeds the transmitting reflector and parabolic mirror antenna.
Transmission Line TL2 feeds a thermo-couple radiation monitor for the transmitter.
A1 is an audio amplifier for the modulation.
The transmitting valve had a grid dissipation of 20 Watts and ran at a white heat, thus
a special method of construction was necessary.
The capacitors C2 were formed by the capacitance between a nut on the two
oscillating electrode extension rods and the Transmission Line terminal plate, with a mica dielectric.
The power supplies and control equipment were located a considerable distance from
the transmitter at Lympe, 250 ft, and were connected by a lead sheathed cables, two core for high
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tension, two core for the radiation monitor and two core for modulation. The high tension leads also
carried the modulation signals. The low tension leads did not require lead sheathing.
Figure 4: Simplified Circuit Diagram of Micro-Ray Transmitter at Lympne.
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Figure 5: Simplified Circuit Diagram of Micro-Ray Receiver at Lympne.
The salient points to note for the receiver are:
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
P2 is the demodulator potentiometer.
T2 is the demodulator transformer.
Chokes L2 block the 500kHz demodulator (quench) signal from the power supply
and the audio amplifier.
A2 is the audio frequency amplifier.
TL3 is the Transmission Line from the antenna to the receiver.
The circuit was an externally quenched super-regenerative receiver, thus achieving a high degree of
amplification of the weak input signal. The valve was set to be on the point of oscillation, with the
input signal triggering oscillations, which grew exponentially, thus giving a high gain. The 500kHz
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oscillator periodically quenched (interrupted) the oscillations, which started again at the end of each
quench cycle. (Gains of around a million were possible with such receivers). The voltages on the
valve electrodes were set to optimise the gain, via P2.
The power supplies and control equipment were located about 70 feet away at St Inglevert and four
single lead sheathed cables were used to carry the valve heater and high tension supplies. C3, L2, T2,
P2, A2 and the 500 kHz oscillator were located in the control equipment rack.
Figure 6: Layout of Micro-Ray Control Bays at Lympne airport.
Figure 7: Control Bays at Lympne, front and rear views.
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Figure 8: Micro-Ray transmitter and antenna dipole assembly.
The combined transmitter and antenna dipole assembly was located behind the parabolic reflector.
The half-wave dipole is barely visible on the right of the picture [8]. Note also the barely visible
dipole on the radiation monitor below the antenna feeder.
The Transmission Line coupling the transmitter and receiver valves to the dipole was of tubular
construction, with a section adjustable in length in order to adjust the matching between the valve
impedance and that of the dipole.
The antenna system used had the advantage that the plane of polarisation of the signal was uniquely
dependent on the plane of the dipole. Thus the Lympne to Saint Inglevert link operated with vertical
polarisation, whilst the reverse link used horizontal polarisation.
The radiation monitor antenna was coupled to the monitor thermocouple via a tubular Transmission
Line, again with an adjustable section to allow matching of the impedance of the dipole to that of the
thermocouple. The thermocouple was connected to a micro-ammeter in the control console to indicate
the power output of the transmitter.
Stability of the power supply to the ’oscillating‘and ‘collecting’ electrodes for the valves was crucial
to maintain frequency stability. Initially this proved to be a problem at Lympne. The mains supply to
the Lympne site was 220V dc from a small local power station. However when the airport beacon
flashed the Morse letter A every two seconds the abrupt large change of load proved too much for the
regulation of the mains supply. Use of a motor alternator with an automatic regulator was not a
solution, as the period of the beacon flashing was too close to that of the automatic regulator. This
problem was finally solved by using a bank of small 3 Ampere- hour accumulators; float charged by
rectifiers from a mains driven supply. As the batteries provided no power, merely acting as a
reservoir, maintenance was minimal. Heater current stability for the Micro-Ray valves was very
important and heaters were run from accumulators on a ‘charge-discharge’ cycle. Float charging was
not employed, since the charge circuits could introduce noise into the system. During operation a
stabilisation period was necessary for the filament supplies to settle after accumulator charging.
At St. Inglevert, battery supplies were used. A 340 Volt battery supplied the ‘oscillating electrode’,
with a tapping at 130 Volts for the auxiliary amplifiers and oscillators. A 60 Volt battery supplied the
‘reflecting electrode’. Fine adjustment of applied voltages at both sites was by potentiometers in the
Control Bays. Two 6 Volt accumulators supplied the heater voltages.
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All communications on the link were tone modulated: telephony, or 3.5 kHz tone keyed for Morse
telegraphy or controlled by the signalling contacts of a Creed teleprinter. The tone oscillator
frequency was unusually high, chosen to permit a possible combination of speech and teleprinter
working at a later date.
Figure 9: Antennas at Lympne.
Figure10; St Inglevert antenna, close up/
Figure11: Antennas at St. Inglevert supported on a single steel
tower 66 feet high.
Separated transmitting and receiving antennas were used at
Lympne, 43 feet above ground on towers built atop the butresses
of the main hanger. In both installations, absolute rigidity against
the weather was necessary once the two stations had been aligned
on each other. In December 1938 the British and French Post
Office administrations placed an order with STC and Le Matériel
Téléphonique for an 18 channel system, but this was never built.
The success of the link was in no doubt and an interesting
observation was made that maritime traffic in the channel
could be observed where the beams were close to the water
surface. This phenomen was not pursued !
Sadly, the Second World War put an end to the link and St.
Inglevert became a German Fighter Station for a squadron of
Messerschmidt Bf 109s commanded by the German ace Werner
Mölders
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References.
1
Mikhailov, V. F , ‘Annales de la Fondation Louis de Broglie, 26, No 4, pp 633-638.
2
‘Phosphor’, Radio Bygones, 100, pp 26-27, 101, pp 33-34.
3
McPherson,W. L. and Ullrich, E.H. Micro-Ray Communication, Read before the I.E.E.
January 1936, p 637.
4.
Ibid, pp 631, 632.
5.
Ibid, p 632.
6.
Emmerson, Andrew, Micro-Rays Now Span The Channel, Radio Bygones, No106, April/May
2007, p16.
7.
McPhersom, W.L. and Ulrilrich, E.H. Micro-Ray Communication, Read before the
I.E.E.January 1936, p 642.
8.
Ibid, pp 640, 641.
9.
Emmerson, Andrew, Micro-Rays Now Span The Channel, Radio Bygones, No106, April/May
2007.
Acknowledgements.
Figure 2: McPherson, W.L. and Ullrich, E.H.Micro-Ray Communication, Read before the I.E.E,
January 1936.
Figure 3: Clavier, A.G. Production and Utilisation of Micro-Rays, Les Laboratoires, Le Matériel
Téléphonique.
Figure 4: McPherson, W.L. and Ullrich, E.H. Micro-Ray Communication, Read before the I.E.E.
January 1936.
Figure 5: Ibid.
Figure 6: Ibid.
Figure 7: Clavier, A.G. and Gallant L.C. The Anglo-French Micro-Ray Link Between Lympne and
St. Inglevert. Les Laboratoires, Le Matériel Téléphonique.
Figure 8: Emmerson, Andrew, Micro-Rays Now Span The Channel, Radio Bygones, No 106,
April/May 2007, p 16.
Figure 9: Ibid p 17.
Figure 10: Emmerson, Andrew, Radio Yesterday; Cross-channel microwave 1934 style, Ham Radio
Today, January 1983.
Figure 11: Emmerson, Andrew, Micro-Rays Now Span The Channel, Radio Bygones, No 106,
April/May 2007, p 16.