beyond the fringe
p. 59 · Read in the full issue
Stray images from beyond the fringe
All over the world, optimistic viewers and enthusiastic ama teurs are regularly obtaining television signals from sources far beyond the recognised limit of individual transmitters.
To understand these phenomena one needs a basic comprehension of television propagation and reception. This is an extremely technical subject which requires more space than is available. However, we can suggest a few ways in which South Pacific viewers, normally regarded as ‘beyond the fringe’ of television viewing, can take advantage of these quirks of nature which break the general rule.
Though there are a number of television stations in the South Pacific in American Samoa, New Caledonia, Tahiti and Micronesia because they are generally served by low powered units, for practical purposes the stations likely to prove useful to island viewers are those transmitting from Aus tralia’s east coast and from New Zealand’s North Island.
Both countries have the 625-line PAL system and high pow ered transmitters and both transmit at VHF (very high fre quency) within a range of 45-225 MHz (megahertz).
The secret to television reception ‘beyond the fringe’ lies in the way in which the signals in the VHF frequency spectrum strike various layers of the atmosphere. Long distance recep tion becomes possible when the signals are reflected back to earth off certain layers.
The layers which reflect can be likened to a cloud which in some cases are only tiny in area while others can be hun dreds of square kilom-etres.
These ‘clouds’ can drift with prevailing winds and when they are about depending on the climatic conditions - signals are reflected over distances up to 2 500 kilometres.
The reflective layers are in the ionosphere which starts at about 760 mefres. Below this is the troposphere which, accord ing to climatic conditions, can also affect long distance trans mission.
Some viewers in the South Pacific may have noticed that during very settled, warm, anti-cyclonic weather, normally weak signals from distant transmitters improve, providing very viewable pictures. In both cases, viewing is better at night.
Having skipped the details of ‘why’, it is more valuable for
viewers to know ‘how’ to take advantage of these phenom ena.
Since lower frequencies are invariably most successful in long distance penetration, these sources may be useful to people in the South Pacific region: New Zealand: Auckland Ch 2 (55.25/60.75 MHz); Welling ton Ch I (45.25/50.75); Christchurch Ch 3 (62.75/67.75).
Australia: Brisbane Ch 0 (46.25/51.75); Brisbane Ch 2 (64.25/69.75); Sydney Ch 2 (64.25/69.75); Coffs Harbour Ch 2 (64.25/69.75); Stanthorpe (Qld) Ch I (57.25/62.75); Rockhampton Ch 3 (86.25/91.75); Townsville Ch 3 (86.25/91.75).
One problem which may well be encountered, since two pro grammes on the same channel may be roughly in the same direction, is that of cross-viewing one channel adversely affecting the other at the receiver.
Anyone seeking regularly viewable pictures should fit low band yagi type antennae on a mast which can be oriented by a ‘rotator’ unit. The antenna could be pointed in one direction most of the time but variations, due to climatic conditions, can necessitate periodic realignment of the antenna for
optimum effect.
As ‘over the horizon’ signals are hardly affected by antenna height, the use of very high towers would not produce startling effects. However, the antenna should be well clear of ground clutter. A mast of approximately 12 to 15 metres would serve you best. Use of a rotator permits one to ‘search’ over a wide area for the best possible pictures.
For the real enthusiast with some technical knowledge, con siderable room exists for experimentation with various types of receiving antennae. Certainly it is worthwhile fitting more than one type of antennae. For example a broadband antenna or phased array type of antenna would be useful to seek out the periodic Band 111 signals. Whatever is used, it is recom mended that suitable mast head amplifiers are incorpor ated.
People who wish to go into this matter should contact, via their local radio and television suppliers, one of the reputable antennae equipment manufacturers.