Television... It's a Cinch |
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E. Aisberg wrote a series of columns for Radio-Electronics magazine in the middle 1950s titled "Television... It's a Cinch," informing the reading public about television in general and the newly emerging color TV more specifically. He chose to use a unique dialog format where two people were in a back-and-forth discussion about the technology. This July installment is the first half of the seventeenth conversation. Antennas, tuned circuits, component reliability, transmission lines, installation, and many other aspects of successful television viewing are covered. The series began with the February 1953 issue, and ran through October 1955. I will post the others as time permits. Seventeenth conversation, first half: Capturing the TV signals; the half-wave antenna; problem of the pass-band; the lead-in and the apartment stairs.
From the original "la Télévision? ... Mais c'est très simple!" Translated from the French by Fred Shunaman. All North American rights reserved. No extract may be printed without the permission of Radio-Electronics and the author. Will - Well, we seem to be pretty well away! Now that we've learned to supply our televiser with low, high and very high voltage ... Ken - Do you really think you've got the job done? Do you think the set will be satisfied with your menu? Will - Why, is it going to want a special dessert of some kind?
Will - I didn't forget! It just isn't a problem. All I have to do is hook a piece of wire on the TV set, call it an antenna, and I'm off! Ken - Further than you think, Will, further than you think! Unless you're close enough to the transmitter to be in a very strong signal field, your piece of wire will be a pretty sorry collector of waves at television frequencies! Will - I can't see why TV should be so different from radio. After all, they're radio waves! Ken - Don't forget that the lowest TV frequencies are far higher than the broadcast band and even most of the short-wave radio you've been receiving. The TV waves are shorter - are absorbed by conducting objects in their way and can't get around obstacles like the longer broadcast waves. That's one reason the receiving range of TV is so much shorter, by the way. Will - So we've got to take a lot more trouble with our TV antennas? Ken - Will, the antenna is the most important part of the whole receiving installation. Well designed and well installed, your antenna is as good as one or two extra r.f. stages. In television the waves are short enough so that you can make your antenna length as long as a TV wavelength, unlike broadcast reception where the antenna must be a small fraction of the wavelength being received. You can make quite a profit out of that fundamental difference - because you can tune your TV antenna to the frequencies being received. Will - Wait a minute, Ken! Do you mean to tell me you can make a tuned circuit out of a piece of wire - no coil, no capacitor, no nothin"? And it will have a resonant frequency and a resonance curve and everything? Ken - Exactly, Will. And the resonance curve of a television antenna has to be pretty wide at times. Even an antenna intended for only one channel has to receive a band 6 mc wide, without attenuating either end. That's particularly important in color TV, for the color information is out near one end of the band received. Will - Well, if I didn't see TV antennas every day, I'd say they must be very complex, with tuning capacitors and damping resistors to widen the passband ... Ken - But you know they aren't. The facts are infinitely more simple, and even you can figure them out if you do a little reasoning. What are these radio-TV waves? Will - Well, they're electromagnetic fields, created at the transmitting antenna and moving through space at about 186,000 miles a second. Ken - You have the idea O.K., if not the exact terms. You understand that these waves produce electromotive forces - currents - in any conductors they find in their way? Can you tell me the minimum distance in a conductor in which a given wave can produce a maximum voltage? Will - The greatest voltage a wave could produce would be between the crest of a positive alternation and the trough of the following negative one. That would be a half-wave-length. Ken - So, if I want an antenna that will get the most voltage possible from a signal of a given wavelength, I'll use a wire, strip or rod a half-wavelength long. Such a conductor is a half-wave antenna. Will - But how long a piece do you pick out for one of these all-wave antennas I've seen advertised? Ken - We'll come to that. Meanwhile, let's stick to a single channel till we understand it. A tuned rod
Ken - And that - of course - takes place in exactly one cycle of the transmitting frequency. Will - That's why you use a tuned antenna, I suppose. Ken - Exactly. Even if they were disturbed by a single pulse, the electrons in our antenna would oscillate back and forth at its natural frequency. So if a wave of that frequency comes along, it's easy to build up big currents or voltages. But remember all this is a little theoretical. It would be exactly correct for a very fine wire suspended far from the earth or any other conductor. In real life, the mast, the ground or roof and any nearby conductors create capacitances that increase the wavelength of the antenna. We have to shorten it a little to compensate for this end effect - cut it down about 6%. Will - So if I were going to receive channel 4 pictures, at 67.25 mc, which is just about 4 1/2 meters, I wouldn't cut my antenna 2 1/4 meters long, but a little less than 2 1/8? Ken - Better get a chart where it's all figured out in inches for you, Will. But if you want to get the channel 4 sound at 71.75 mc equally well, you'll have to cut your antenna a little shorter. And if you want to get channel 6-up beyond 80 mc - as well as channel 4 - you'll have to go still shorter. Will - That's going to call for quite some passband! How are we going to get it? Ken - One of the most effective ways to broaden the response curve of the antenna is to increase the diameter - or more precisely, the ratio of diameter to length. That's why TV antennas are usually made of tubing. Will - I suppose that if you wanted a really wide passband, you could go in for a cage, made of several wires in the form of a cylinder, like some old commercial and ship antennas? Ken - Believe it or not, that's exactly what's done, though you can't recognize the cage. You know that they bring the wires of the cage together at a point to take off the lead-in. And that's at the center of a TV antenna. So your cage becomes a sort of double cone. Then it was found possible to leave out some of the wires. So a "conical" antenna now consists of two pairs of three rods each, radiating in opposite directions from the lead-in. In some cases there are only two rods, and it's then often called an "X" antenna. Will - But how about the channels from 7 to 13? They're up around 200 mc. Ken - You'll notice that the whole upper section of the v.h.f. TV spectrum is about three times the frequency of the lower part. That means that a half-wave antenna for channels 3 and 4 will be approximately three half-waves on channels 8 to 12. Will - And a three-half-wave antenna works like a half-wave one? Ken - Much the same, though not quite so well. But a few tricks are used and compromises made that give us antennas that work fairly well over the whole v.h.f. band. For example, the way the ends of a conical are tipped ahead is to help it on the upper part of the band. Will - And these double-V's are dipoles with a lot of tip-ahead? Ken - No, they belong to a different group of antennas, the so-called "long-wire" type, and are even better on the upper than the lower section of the band. In fact, they can be used on u.h.f. too, as well as the rhombic, another member of the long-wire family. But if you have to receive both u.h.f. and v.h.f. stations, it's usually best to have separate antennas. The most common are simple dipoles, fanned out into bowties to make their response curve broader. Will - Another thing. Why do the English stand their antennas up on end, while we lay ours flat? Ken - That's because of the polarization of the electro-magnetic field. The English use vertical transmitting antennas, which send out vertically polarized waves, and you have to use a vertical antenna to receive them. In the early days of TV it was thought there were certain advantages in vertical polarization. For instance, a vertical transmitting antenna sends out equally strong signals in all directions, but a horizontal one sends best in a direction broadside to the antenna and transmits very little in the direction of its ends. The pickup of receiving antennas follows the same pattern. But our horizontal polarization is - among other things - less susceptible to certain types of interference, and I believe the latest European systems - for instance, the French 819-line setup - also use horizontal polarization. Atom-age apartment house Will - Tell me, why does the lead-in come from the middle of a TV antenna? You have the greatest voltage at the ends. That's where you should be able to get the most signal. Ken - You tell me - where is the stair carpet in your house worn the most? Will - Along the middle, naturally, but a stair carpet isn't a TV antenna! How does it get in on this? Ken - Simply because it may give us the clearest explanation of the way currents and voltages are distributed in our antenna. But let's go a little further. Let's imagine an atomic-age apartment house built with the thought of hydrogen-bomb attacks in mind. It has eight stories above ground and eight below. Each floor has approximately the same number of tenants. The place is a walkup - we can't depend on elevators in emergencies, or at least so the landlord says. Now, do you imagine all the stairs would be equally worn? Will - Of course not. You'll have only a few fresh-air lovers and safety-first enthusiasts on the very top and bottom floors. Their part of the stairs will get very little wear. But the first flight up and down from the entrance would be used by everyone in that half of the building. Ken - Now you see the analogy between the tenants in our 16-story walkup and the electrons in our antenna? Will - I get it! There can be very little motion of electrons at the ends of the antenna - they have nowhere to go. But as you approach the middle, the number of electrons increases and the very maximum is right at the center. Ken - So you see the example was useful, if it did seem a little far-fetched at first. Now you know where the current is greatest, you can tell what point the lead-in should come from. Will - I do know there are two wires in a piece of flat lead-in, and they are attached to the inner ends of a pair of straight aluminum tubes - in the simple antenna you've been talking about, that is. But how can these be half-wave antennas? They are really two antennas in line. Surely the currents can't jump the space between two antennas - especially if we are going to have the most current there. How do you go about attaching the lead-in to a real half-wave antenna? Ken - Don't worry, Will. The job you're talking about is a real half-wave antenna and is the most commonly used of all antennas, in some form or another. It is the dipole, made of two quarter-wave sections. The current has no problem, for as it flows toward the center of the antenna, the lead-in offers it the same impedance that the rod itself would if you had a straight dipole with no lead attached to it. So if you make a gap in the center just long enough that the impedance at the ends of it is about 72 ohms and attach a 72-ohm lead-in, the current acts just as if it were flowing on a straight rod. Of course, you have to remember to cut your antenna short enough to account for the end effect due to the mast and other things, as I explained a few minutes ago. To Be Continued...
Posted April 20, 2022 |
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