The viewing, in this manner, is made possible by a queer phenomenon in which the radio waves or signals from the transmitting station going into space get reflected from the higher layers of the atmosphere to points on the ground that are well outside the normal range of the transmitter.
These reflectors in the atmosphere are transient, forming under certain weather conditions to propagate the waves that reach it, so as to make the anomalous propagation an esoteric form for study by scientists, while providing the Dx-er a rare opportunity for long distance TV viewing.
TV Dx-er Kumar strives to receive signals from remote TV stations by using the mobile reflectors. The first of these reflectors is the troposphere, the lower part of the earth’s atmosphere, going up to a height of 25 km and in which temperature decreases with height. Here clouds form, especially in summer, and remain virtually stationary at night to reflect die waves reaching it from a TV transmitter. Even as it moves away when the sun rises and heats the troposphere, the waves or signals are ducted along the edge of the cloud. The higher the reflector, the greater is the distance between the points on ground between which the signals are transmitted and received.
Similarly from the E and F layers of the ionosphere, the upper atmosphere, 90 km and more above the earth, the signals are bounced back to the ground when patches of the layer get ionised sporadically, forming slow drifting clouds.
Kumar says when these sporadic E and F layer clouds move, sometimes as fast as 400 km per hour, the points between which they can reflect signals vary. Says he: “If one is tuned in and moves with the E and F layer cloud, a large number of stations can be picked up.”
The length of the signal hop is continuously changing, with the signals being received from transmitters closer or further away, depending on the direction in which the cloud moves. Signals can and have been known to hop from a transmitter to a viewer more than 6,000 km away.
Undampened Enthusiasm: A problem with these sporadic E and F layer reflectors is the forming of multiple images or ghosting. This happens because the reflections are from more than one part of the layer and the image appears smeared or with the characteristic flutter seen when tuning in to any station.
Kumar says: “Sometimes it is worse. Either sound or vision only can be picked up while the other fades out.” So, TV Dx-ers use a high gain video amplifier and low noise booster for better images.
“These are only the means,” says Kumar, “for the thrills of viewing life in far away places. Identifying stations is like solving a puzzle, for the test card put out by the TV station with definite patterns has to be matched with that in the International TV Handbook. For this the TV pictures are photographed. And if one picks up a station while a programme is being beamed, the captions, clocks or announcers help in easier identification of the station.”
The advent of communication satellites which make long distance TV viewing easier does not dampen the enthusiasm of Dx-ers. When the Applications Technology Satellite-6 was beaming programmes for the Indian Satellite Instructional Television Experiment in 1975-76, some Dx-ers in Europe rigged dish antennae and received the programmes.
Kumar, however, does not use such sophisticated techniques. He uses the work horse among directional antennae – Yagi. This summer he plans to set up a rhombic antenna which is useful over a wide band of frequencies.