| ||||
| ||||
Sunspots Mar TV Reception | ||||
 local communications blackouts, and some are significant enough to cause entire power grids to fault and shut down. Frequency and intensity of the CMEs and flares is correlated with the well-established 11-year (approximately) cycle between solar maximums** and solar minimums. This article discusses some of the ramifications of solar disturbances using terms familiar to DX (long distance) Ham radio operators. I wonder how many televisions were taken to the repair shop because of these solar effects?* The term "coronal mass ejection" (CME) is relatively new, have been first used in 1982, so it is not mentioned in this 1957 article even though CMEs certainly would have been occurring at the time. ** We are currently experiencing one of the weakest solar maximums in the last century. Sunspots Mar TV Reception By Sidney C. Silver, Service Editor, Radio & TV News
A viewer who lives about 50 miles from the metropolitan area in which his favorite TV stations are located, and who normally gets pretty good reception, is reclining in his living-room chair one afternoon, completely relaxed, enjoying his favorite program on, say, channel 4. He becomes aware of fine horizontal lines faintly visible across the picture. Since he has been enjoying reception on this channel for a number of years with the same receiver and the same antenna, this entirely new phenomenon puzzles him somewhat, but it is not sufficiently prominent to be really annoying: he remains in his chair in the hope that the symptom will go away of itself. As he watches, the lines become somewhat heavier and eventually mar his enjoyment. A somewhat darker vertical bar is now noticeable, swinging back and forth across the screen. Now he can barely make out his program at all; the lines are practically dominating the screen. Then the picture goes completely out of sync. Although he is bewildered, many a technician would state at this point confidently - and correctly, to a degree - that a serious case of adjacent - or co-channel interference is causing all the fuss. The harried viewer has left his chair and is heading toward the set to apply the only remedial technique he knows. He is preparing to twist every knob with which the receiver manufacturer has supplied him in an attempt to exorcise the crazy quilt on the screen. Before he can do this the receiver, as though acting in self defense, suddenly permits an intelligible picture to fill the screen again. As our viewer gets ready to relax again, he realizes that the characters on the screen are completely unfamiliar. The show itself is completely unfamiliar. It has nothing to do with the program he was watching a short while ago and which should still be on the air. While he is trying to make some sense of this odd development, the program ends and a station break comes along. A completely unheard-of station with call letters he never knew existed identifies itself as "his" channel 4. Its location is given as some metropolis in another part of the country, hundreds of miles away. Before reaching into his pocket for a tranquillizing pill, the victim just barely manages to reach the telephone and pour out a garbled account of what has happened to his incredulous service technician. While awaiting the technician's arrival, he stalks to his window and stares out, puzzled, at his antenna, which is just visible in one corner of his field of vision. He has to squint uncomfortably because he is partially blinded by the bright light beyond his antenna on this fine, clear day. The light comes from the sun. the unperturbed culprit in our little drama. Admittedly, the account just given of interference resulting from so-called sunspot activity is of a severe case; but it is based on an authenticated experience. Nor will it be the last of its kind before we have drifted past the current sunspot cycle maximum. Often, the effect does not become as severe as in the unfortunate drama we have just presented; that is, the interfering, distant transmission working on the same frequency does not always become strong enough to ride over the desired local program. In these less startling cases, the symptom will take the form only of enough co-channel interference to ruin the program being viewed or to mar it considerably, usually by introducing instability or complete loss of sync, as well as by making a hash of picture content. Uppermost in the minds of affected technicians and set owners will be the question, "What can we do about it?" Before we can start to supply answers - and there aren't many - we have to have some picture of what is going on. As we rise above the earth, penetrating its surrounding atmosphere, we reach a region beginning about 50 or 60 miles up known as the ionosphere. This consists of several layers in which free ions and electrons occur with far greater frequency than they do in the more immediate atmosphere that hugs the earth intimately. The highest of these layers is about 200 miles straight up - quite a trip on the elevator.
When a pencil is put in a glass of water, it appears to be bent to the viewer standing away from the glass. What has happened is this: the normally straight-beamed light rays (very super high-frequency radiation) from that part of the pencil which has been submerged, in travelling to our eyes, have been bent in going through the water and glass, because they have been slowed up by the denser medium. In like manner, radio signals are bent or refracted as they pass through - or try to pass through - a "thicker" medium, like the ionosphere. This phenomenon gives us our long-range or DX short-wave transmission. As shown in Fig. 1, ordinary radio waves, essentially unbent, travel line-of-sight and cannot be picked up by receivers beyond the horizon. Other waves are refracted so severely that they finally reflect downward and return to the earth at some distant point beyond the horizon (receiver 2). The higher the frequency of either sound or electromagnetic waves, the more resistant they are to refraction and reflection. The bass end of the audio range, for example, seems to spread around the room from a loudspeaker. The treble end of the range is more narrowly beamed in front of the speaker and is not heard as clearly off the speaker axis. With electromagnetic waves, the signals can bounce around the world, between ionosphere and earth in the shortwave bands; however, when we go up in frequency into the TV bands, the signals tend to resist the bending effect of the ionosphere and transmissions manage to fight their way through this medium without being hurled back to earth. Thus, we ordinarily think of TV reception as not being practical beyond the horizon from the transmission point. The highest frequency that can be bounced back to earth depends on the degree of ionization in the upper layers. This m.u.f. (maximum usable frequency) seldom moves up as high as the TV frequencies under ordinary conditions. However, along comes our sun to shed a new, if somewhat confusing, light on the situation. Alone in space, millions of miles from its nearest neighbor, the solar orb gets bored now and then - about every eleven years or so - and begins to amuse itself with what we have come to know as sunspot activity. There is much speculation and less actual knowledge about the whys and wherefores of this sunspot cycle. As to effects, however, we do know that, during the period when the sun is riding the peak of a sunspot cycle, disturbances also occur in the ionosphere. Along with marked changes in the degree of ionization, the m.u.f. soars upward, and may get well into the lower v.h.f. band. When it does, TV transmissions at or below the m.u.f. can be thrown back to earth hundreds and even more than a thousand miles from the point of origin. The lensing action of the ionosphere may concentrate the refracted energy sent back down into the distant area to the degree that the returned signal will be strong enough to force its way over local transmissions on the same channel, and take over the screen completely. We are going through a period of heavy sunspot activity right now, and this condition is likely to persist for half a year, or for more than a year; it is never easy to predict its exact termination. This type of disturbance is a new problem in the TV era: during the last sunspot peak, which occurred in 1947, there were neither enough receivers nor enough operating stations in the country to create much difficulty. Although the disturbing effects already described may occur anywhere, areas of primary reception will be less susceptible than others. The particular instance with which this article begins occurred in a near-fringe sector about 50 or 60 miles west of an eastern metropolis. Since the locality is on high ground, many favored set owners are able to get acceptable reception from the big city with nothing more than indoor rabbit ears. The indoor antenna was beamed east, of course, but antennas of this type are equally sensitive in the opposite direction. The interfering station was identified as one from the midwest. Most reports of DX TV reception at this time come ,from fringe areas, where the inherently weaker signals available locally can put up less of a battle against intruders. Nevertheless, the author, who resides in a near suburb of New York City where there is signal strength to throwaway, has suffered some mild, occasional co-channel effects - horizontal lines, windshield-wiper effect, infrequent sync instability - on channel 2. This has occurred three or four times over the last half year, and has lasted for two or three hours on each occasion. To the DX fan, these random pick-ups are gifts from heaven - or from the sky, in any case - especially when they fall on channels that are normally vacant in the local area. To most viewers, these invading signals are unwelcome obstacles to TV enjoyment, and these people can't understand what is wrong with the idiotic technician who shrugs his shoulders helplessly when he is asked to "fix the set." The situation is a tough one, because a sure, universal cure does not exist. In areas where the victim has been getting by with an antenna that is largely nondirectional, a narrowly beamed unit, aimed in the direction from which transmission is desired, will cut down hobo signals that drop in uninvited from random angles. However, the refracted intelligence may also swoop down from the angle of optimum orientation. Even in these cases, the fact that normal TV transmissions travel in the horizontal direction gives us something to work on. The angle of incidence of radiation bounced back from the ionosphere will be oblique (see Fig. 1). There are many good antennas that not only discriminate against signals arriving at the rear and sides, but also reject signals that do not come in horizontally. A check of the vertical radiation patterns supplied by most manufacturers of good antennas will be useful in making a choice. Recommending the expense of a new antenna installation to a victim of the sun is a delicate problem, at best. There is no assurance as to how effective it will be, and the unpredictable sunspot cycle may come to an end before the cost of a new antenna can be justified in terms of whatever relief it will provide from the difficulty. The technician would do just as well to use the opportunity for stressing the need for a better, newer antenna on general principles, with possible reduction of sunspot interference as an added inducement. Overstressing the possible protection against interference from DX TV transmissions, even where this symptom has been a fairly regular nuisance, leaves the technician open to recrimination by the set owner where the results will not justify the expenditure involved. Few technicians will want to take such a risk. In any case - and especially in those where the condition exists despite a good antenna installation - an important public-relations problem confronts the TV service worker. Unless it is properly handled, he may suffer loss of confidence with some customers. His best bet is to make a rough sketch like the one shown in Fig. 1 and try to explain what is going on. The simplified explanation given here has been tried out on several nontechnical people with good success. The technician is less likely to be looked upon as an idiot if he can do this successfully; he is also giving his customer an honest picture of the situation and expectations. In effect, the customer, not the technician, is responsible for the decision as to whether a gamble on a new antenna should be taken. Besides, while the explanation is being given, the symptoms may very well disappear altogether. Posted November 25, 2013 | ||||
 |
Fig. 1. Normally propagated TV transmissions travel in straight lines, and cannot be picked up beyond the horizon. When the sun acts up, they may bounce for great distance.
 |
 |
 |
 |
| About RF Cafe | |
 Copyright: 1996 - 2024 Webmaster: Kirt Blattenberger, BSEE - KB3UON |
|
|
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ... All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged. My Hobby Website: AirplanesAndRockets.com
|
|
