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The range of ultra-short-wave signals, and especially the dependable range - in other words, the service area - is still a matter for controversy. This article, written after making a series of observations on television signals from the Alexandra Palace, will help to clear up many misconceptions on the subject.
The Alexandra Palace television transmissions at he Radio Exhibition made it possible to carry out extended tests which may shed some light on the question of the range at which the station may be expected to give a reliable service.
For this purpose it was necessary to have a receiver that was easily portable, did not take long to lash up, and that enabled the strength of the signal to be judged. The loudness of reception is generally a very uncertain guide as the modulation is constantly changing (the sound programme was used for the tests, being less irritating to listen to for prolonged periods than the vision. The super-regenerative type of receiver might at first thought be considered to be particularly unsuitable because of its almost perfect AVC characteristics, whereby all except very weak signals are brought to a uniform level. But actually it is ideal for the purpose, because the background noise, which is initially very loud, decreases as the carrier wave strength increases and gives a useful indication of field strength over a wide range of intensity.
The receiver used in the tests.
The tests to be described were made with a simple super-regenerative receiver having wire ends reaching to floor and non-metallic roof of the car. These were later replaced by a telescopic dipole made from tubular camera tripod legs.
The ground covered lay in the direction that presumably is least favourable for reception, having the whole of London lying between it and the Alexandra Palace transmitter. Even the National transmission from Brookmans Park, admittedly some miles farther away, but very much higher in power and wavelength is known to be absorbed very seriously by this path and diminished to a barely satisfactory field strength within a mere twenty miles. Absorption in built-up areas is agreed to be still more pronounced as the wavelength is shortened. But whether this is serious when the whole of the transmitting aerial is raised well above the level of the city is open to question.
A section of the ground covered during the tests described. Note the difference between horizontal and vertical scales.
Starting at the south-eastern boundary of London, at Lewisham, the main road was followed as far as Tonbridge. To explain why this route was considered likely to be particularly interesting, a section of the ground between the Alexandra Palace and Tonbridge, 33 miles long, is shown. The vertical scale is, of course, exaggerated; actually about 25 times. But it will be seen to include two hills, or, rather, ridges, of even greater height than the transmitting tower itself. There is also appreciable curvature of the earth, which is shown. The section is along a straight line, but the road happens to follow it fairly closely all the way, and the high ground stretches for considerable distances at right angles to the section.
The field strength was very good at the start, the background noise being negligible; but just before actually joining the main road the car had to pass through a deep cutting at right angles to the direction of the transmitter and bordered by large trees. As was expected, reception fell off very badly here, but was still fairly good, except that at more or less uniform intervals there were curious bursts of background noise, corresponding to almost complete extinction of the signal. At first it was supposed that these might occur at the same intervals as the lamp standards, but this was soon found to be wrong. The phenomenon was to be noticed quite often later on.
Local Absorption
South East England - courtesy Ordnance Survey.
Passing through busy streets the reception fluctuated a good deal, and local absorptions were noted, but the severest effects of these were generally confined within limits of a foot or two. Some interference from the ignition systems of passing cars could occasionally be heard, and of course the continuous noise of the car in which the receiver was installed (not fitted with suppressors), but this was not bad enough to render speech hard to follow or to mask the background noise generated by the receiver itself.
An open level stretch of road conformed to expectations in giving excellent reception, but even here a periodical extinction or fluctuation was sometimes observed. Mental calculation based on the speed of the car showed that whenever this effect was observed it was usually at intervals of the order of half a wavelength. This is rather surprising, as the 'interference' (in the optical sense) between sets of waves converging from directions not widely separate, such as the direct ray and that reflected from the ground, would lead to minima at intervals of a good many wavelengths.
This effect was examined later with improved apparatus; and the intervals between minima estimated to be slightly less than half a wavelength. The idea that standing waves on overhead wires might have some connection with it was disproved when similar results were obtained along roads devoid of overhead wires or metal fencing. Though there seems no doubt that standing-wave patterns due to reflections from various objects were responsible, it was not possible to identify these objects in any given situation. The same phenomenon can be demonstrated in acoustics by moving the ear into different positions when listening to a high-pitched note.
A somewhat similar effect takes place when the receiver is stationary and turned so as to obtain a minimum of reception. A vehicle moving past upsets the balance-out of the signal and gives rise to a succession of maxima and minima. It was found possible to detect even a bicycle at 40 feet and an aeroplane at several thousand feet. The moving object reflects a certain amount of the signal (an aeroplane is in a strong field and contains many wires and other parts, some of which may quite possibly resonate) and, when the direct signal is balanced out by the angle at which the receiving aerial is placed, the receiver is at its maximum sensitivity for picking up such stray reflections. As the object moves, the phase of the reflected signal changes periodically, causing fluctuations in reception.
Easily Explained Fluctuations
Circuit diagram of the super-regenerative portable receiver. The aerial circuit capacitors C and C1 are of about 100 pF. Quenching is carried out by a separate valve. Valves: XP & XL.
The screening effect of woods or thick borders of trees close to the road was noticeable and also that due to buildings or high walls, but perhaps less so than was expected. The contour of the road likewise had its effects; in fact, most, but not all, of the fluctuations in signal strength could be related to the visible surroundings according to known principles.
The descent of the first steep slope carrying the road well below the direct line of the transmitter was awaited with interest. The upper part of the ascent gave excellent signal strength, but rather unexpectedly it fell off very noticeably before the descent of Polhill actually commenced. The fact that trees became fairly numerous at this point seemed hardly to be enough to account for it, but it is possible that the effect was supplemented by a less favourable angle between the radiation and the receiver, and by the cancellation of the direct ray by a reflected ray. Since making this observation it has been found to be in agreement with those of other experimenters.
As the hill was descended the signal declined steadily, and about half-way down, surrounded by trees, was so weak as to enable only occasional words of speech to be distinguished even with the engine stopped. But it must be remembered that the aerial and receiving conditions generally were far from ideal, and even in this exceptionally unfavourable location, 24 miles from the transmitter, a more ambitious receiving system with a raised aerial might achieve fair results. Incidentally, the vision signal on 6.7 metres seemed rather stronger, in so far as such different types of transmission could be compared at all.
Further descent of the hill, but at an easier gradient, restored a large part of the lost signal strength and at Dunton Green, 500 feet below the horizon of the transmitting aerial, reception was as good as in the more urban localities at half the range.
The next big hill, beyond Sevenoaks, again showed a rapid declension of signal very little beyond the crest. Although five miles farther on, and steeper in gradient, the loss of strength was less than at Polhill, and again there was a marked recovery on the lower slopes.
On the very low ground of the Medway Valley, around Tonbridge, reception was undoubtedly a lot weaker than on the rising country before Polhill, but hardly more so than would have been expected in view of the increased distance, let alone the intervention of two ranges of the North Downs.
Some other roads were surveyed without adding materially to the findings already described, except to dispel a faint suspicion that the overhead wires along the main road might have been helping reception.
A justifiable conclusion seems to be that 33 miles need not by any means be regarded as the maximum reliable range, even when the receiver is below the transmitter horizon. In the experiments described there was good reception in places where the radiation would have to turn through an angle of as much as 4 degrees to clear the high ground.
Incidentally, the configuration of the ground in the opposite direction is curiously similar; Aylesbury taking the place of Tonbridge, and the Chilterns corresponding to the North Downs. Elsewhere, except perhaps towards the south-west, the topographical conditions for long range are more favourable. With the development of improved methods of reception, such as directional aerial arrays, considerable ranges are likely to be established. Such aerials are valuable not only for the increased signal reception, but also for reduction in interference when they are properly designed and located. One may also expect a diminution of interference at the source as suppressors become normal practice. It seems likely that the most troublesome factor at the fringe of ultra-short wave-range will be fading due to variation in the standing wave patterns set up by objects that divert the radiation, for these are not necessarily stationary.
Since the results already described were obtained, an investigation was made along the Great North Road for a distance of 36 miles from the Alexandra Palace, using the improved telescopic dipole. A photograph and circuit diagram of this receiver are shown. Although the conditions of use while in motion did not permit of more than half of the full 6-foot extension being brought into faction, quite strong reception was obtained up to the maximum distance, some miles north of Biggleswade, where the signal was actually received with the aerial completely telescoped and lying horizontally in the bottom of the car - the worst possible conditions. There was appreciable screening just beyond the hill (440 feet) near Stevenage, but otherwise little of interest. In general, the strength of reception over the greater part of the distance was such as to render even the car ignition inaudible.
The relationship between strength of reception and orientation of dipole was subject to local variations, but most usually there was a maximum with the aerial vertical, and there was always a very large increase when the apparatus was raised even as little as a foot from the normal position with one end resting on the floor of the car.
Going to still greater distances, the sound programme was heard quite strongly and clearly, though with a fair background of super-regenerative noise, indoors about 100 feet above sea-level at Eastbourne. This is sixty-two miles from the transmitter, and no fewer than 3,000 feet below the horizon. Not only is there considerable earth curvature intervening, but a direct line passes right over Tatsfield, selected by the BBC for a receiving station by virtue of its altitude. Similar results were obtained farther along the South coast at Rye, while out-of-doors, on the top of Beachy Head, the signal was really strong good enough to suppress all background noise even when unmodulated.
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