June 1949 Radio & TV News
[Table of Contents]
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
As one who recently installed an outdoor antenna with a signal booster on it, I definitely considered whether my exercise and investment would be worthwhile because all the preamplification in the world wouldn't help if the signal-to-noise ratio was lousy to begin with. This statement from Mac McGregor sums it up well, "One thing you have to remember is that the booster has to have something to boost. Unless the antenna can deliver some sort of signal to it, it has nothing to work on. The results are about the same as when a small boy reaches the bottom of his soda. He keeps on trying, but about all his straw delivers is noise."
Mac's Radio Service Shop: Barney and the Boosters
By John T. Frye
After having had that young Irishman, Barney, as an assistant in his radio service shop for over a year, Mac thought he was pretty well prepared for anything the boy might do; but he was still somewhat taken aback when one bright June day the lad came dashing and sliding into the service department after his lunch hour, yelling at the top of his voice, "Your-a-kay! Your-a-kay!"
Instantly Mac grabbed up the atomizer-bottle of cleaning fluid used to brighten up dial glasses and squirted a couple of shots at Barney's fiery red hair.
"Calm down! Calm down!" he ordered. "And what's 'this 'your-a-kay' business?"
"You know: 'your-a-kay,' " Barney said impatiently. "It is what Archimedes said when he discovered the law of flotation. It's Greek and means 'I have found it.' "
"I always thought the word was pronounced 'Eureka,' " 'Mac said mildly; "but what particular 'it' have you found this time?"
"I've just solved the problem of how to get good TV reception in the fringe areas," Barney announced importantly.
"I told you not to run around in this hot sun without your hat on," Mac muttered, "but tell me more! I, and about fifty million other Americans, are strangely interested."
"Like all great inventions, it is really quite simple," Barney said modestly. "You know that an airplane flying overhead reflects a signal down to the set; right?"
"My idea is simply to paint the outside of a spherical balloon with metallic paint, so that it will reflect TV waves and then to send it up a thousand feet or so on the side of town farthest from the TV station. Because of the spherical shape, it will reflect the signals passing high overhead down into every part of the town, and there will always be some point on its curved surface that will reflect the signal to a given spot, no matter how much the balloon turns or bobs around."
"Perhaps if you were to lie down there on the bench for a while and if I were to place cool, moist compresses on your head," Mac mused, "or maybe squeeze your head just a little bit in the vise-?"
"All right! All right! They laughed at Marconi, too," Barney said: "but I am going to try it all the same - just as soon as I can get hold of a suitable balloon."
"What brought on this unusual attack of deep thinking?" Mac asked curiously.
"The gang over at the hash-house was arguing about fringe-reception in general and boosters in particular. Some said boosters were next to worthless; others swore by them. How do you feel about boosters?"
"Well, that's a good bit like asking what you think about blondes. In both cases the answer might well be: 'There are lots of different kinds, and even the same kind behaves differently in different situations.' "
"Is that why there is so much disagreement about blondes - I mean boosters ?"
"I think so. One thing you have to remember is that the booster has to have something to boost. Unless the antenna can deliver some sort of signal to it, it has nothing to work on. The results are about the same as when a small boy reaches the bottom of his soda. He keeps on trying, but about all his straw delivers is noise.
"And speaking of noise," Mac went on, "the important thing to a booster is not how much signal you have, but how much signal you have in comparison with the noise level. In a very quiet location, the booster can take a noise-free signal of only a few microvolts and push it up to where it will make a good picture. In another location where a much stronger signal is available but where it is accompanied by an equally-strong noise, the booster is helpless, for it cannot boost the one and not the other. In the latter case, the owner of the TV set could truthfully say that a booster did not help him, for the gain of his unaided receiver would be sufficient to reach down to his high noise level."
"What could he do about it?"
"Well, he would have to make changes in the kind, orientation, or location of his antenna, or in the type or path of his lead-in so as to produce a more favorable signal-to-noise ratio. He could do this by increasing the signal strength, decreasing the noise, or by raising or lowering both, non-uniformly, so that the signal has the better of it. Once he has that signal sticking up out of the noise level, the booster can take hold of it and raise it to picture-making strength."
"In other words, if the signal in your antenna is well above your noise level, the booster can do a good job on it; if not, the thing to do is not to cuss the booster but to work on the antenna."
"Go to the head of the class, Junior."
"What does it take to make a good booster?"
"The perfect booster," Mac said slowly, "would be one that would amplify uniformly the full six megacycle width of each of the twelve channels and nothing else. It would contribute no noise of its own, would be completely stable, would not upset the impedance of the lead-in into which it was inserted, and would have high amplification."
"How close can we come to that?" "Not too close. Tight coupling to the antenna and resistor loading of the tuned circuits flatten out the response curve but only at the expense of gain; and we still cannot achieve the straight-sided flat-topped response curve we'd like. What's more, every booster contributes some noise of its own to that picked up by the antenna, and it is difficult to achieve both high gain and low noise."
"What does too narrow a bandpass do?"
"Shows up as a lack of detail in the picture or, in the case of a weak signal, difficulty in getting both the picture and the sound at the same time. By tuning the narrowband booster, you can peak up one or the other, but not both."
"Is there any good way to test or compare boosters?"
"One way is to run a signal from a signal generator into the booster with a 300-ohm resistor across its output. The r.f. probe of the VTVM can be used to measure the input and output voltages and so arrive at the gain. By shifting the frequency of the signal generator and plotting the consequent changes in output voltages, you can get a rough idea of the bandwidth. A better way to do this is to put in a signal from a TV sweep generator with about a ten-megacycle sweep and to pick off this amplified signal from across the 300-ohm load resistor with a crystal diode probe feeding into your 'scope. That way the response curve can actually be seen, and you can use marker pips to tell exactly at which frequency it peaks, dips, etc."
"Why is it that the same booster wants to oscillate on some receivers and not on others?"
"Very probably it is the antenna that makes the difference. The input circuit of the booster is designed to be loaded with the 300-ohm load of the lead-in. If the lead-in has standing waves on it, the impedance at the booster may be much higher and afford little or no loading, and the booster will want to oscillate. Incidentally, when a booster is on the edge of oscillation, its normal bandwidth is considerably narrowed, for regeneration sharpens up the response."
"Hey, Boss," Barney said suddenly, "see if you can explain this one: The other night I was visiting a guy who owned a TV set, and the joker had three boosters lined up on the floor in front of the receiver. He had the dial reading and the position of each booster written down on a card for every live channel, and he followed this to the letter. For example, when he went from channel 5 to channel 4, he swung the booster nearest the set forty-five degrees out of line with the other two; and the picture was twice as good as it was when the booster was moved back into line."
"That sounds wacky," Mac chuckled, "but I think I understand it. Moving the booster changed the coupling, and hence the regeneration, between it and the other units. This change in regeneration increased the gain when the booster was in the critical position and so improved the picture."
"What do you think of cascaded boosters? Isn't one enough?"
"All I know is what I have seen, and I know cases where two or even three boosters got a picture when one could not do it. The effect is apparently the same as you get with stagger-tuning in the i.f. stages. The over-all gain is not much greater than that of a single sharply-tuned unit, but the response curve of the group is much better-hey!" Mac broke off, "aren't you listening? "
"Sure," Barney told him, "but I was just wondering if I couldn't borrow a balloon off of one of those bubble-dancers down at the Bijou!"
Posted June 24, 2016