May 1963 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Although you wouldn't know it from the title, this is actually another of John T. Frye's "Mac's Radio Service Shop" stories. MPATI stands for "Midwest Program on Airborne Television Instruction," and was a pre-satellite-era system for broadcasting educational programming to areas that otherwise did not experience good quality over-the-air reception. DC-6 airplanes were outfitted with a transmitter and a hydraulically stabilized antenna, and would fly for many hours at a time to provide rural areas with classroom instruction via TV. Purdue University, in Indiana, played a key role in the program. MPATI is a obvious spin-off of the Stratovision system experimented with by Westinghouse Electric Corporation and The Glenn L. Martin Company in the mid 1940s.
Also addressed in the article is the reason for using 400 Hz alternating current (AC) for supplying power to airborne systems - it's all about reducing the sizes of transformers, inductors, and capacitors in the power supplies and noise filtering circuits.
Incidentally, back in the early 1990s, I took a couple Master's Degree courses in Microwave Engineering from National Technological University (NTU) via satellite, the next step in broadcasting college courses over the air.
MPATI - Its Problems & Solutions
By John Frye
Mac has lunch with Colonel Schatzel, Director of Engineering of the Midwest Program on Airborne Television Instruction.
Barney, the youthful technician at Mac's Electronics Service Shop, was not hurrying back to work after lunch. The feel of the warm sun beating down on his shoulders, the summer-scented breeze wafting past his nostrils, the sight of girls tripping along in their pretty spring dresses, and the gay "churlikking" of robins sound-searching the greening lawns for hapless worms - all these things dragged at his feet and slowed his steps to a saunter.
But Mac, Barney's employer, was obviously in a good mood; for when the spring-fevered youth ambled into the service department, the older man didn't even glance disapprovingly at the clock.
"Ah, there you are," he said. "I just had a most interesting lunch with Colonel Schatzel, Director of Engineering of the Midwest Program on Airborne Television Instruction, and I'm busting to recite some of the things I've learned to sort of fix them in my mind."
"Be my guest," Barney invited, smothering a yawn. "You've got the best listener in seven states standing - or rather sitting - in front of you," his voice trailed off as he slumped down on a stool.
"Those MPATI boys have been doing some real pioneering," Mac said enthusiastically. "When you load two complete v.h.f. transmitters and their accompanying video tape recorders into an airplane, translate their outputs to u.h.f. channels 72 and 76, amplify these outputs with klystron amplifiers, feed both of these u.h.f. signals into a single slotted antenna jutting down from the belly of the plane, and finally radiate an effective 50 kw. apiece from the transmitters carrying entirely different programs while the plane is flying at 23,000 feet in a skinny figure-8 pattern inside a ten-mile-radius circle centered over a point on the earth's surface, you're doing something no one else has ever done before."
"And I'll bet you're going to run into some dandy new headachy problems," Barney offered.
"True, but actually the problems were not as many nor as bad as you might imagine. One trouble in the early days was with the hydraulic system of the plane. As you probably know, the antenna is hydraulically and automatically stabilized so that it never varies more than one degree from the vertical while the plane is flying a normal course. The hydraulic system for doing this was married to the plane's hydraulic system. Now ordinarily the only time the plane's hydraulic system is required to do much work is when the plane is landing or taking off. Then it controls the flaps, raises and lowers the wheels, does the braking, etc. Once the plane is in the air, the system is placed in a 'bypass' condition and has little to do. This is not true, however, when the antenna stabilizer is connected to the hydraulic system. The system is kept working almost constantly, and severe problems were encountered with heat that broke seals, destroyed pumps, and impaired valves.
"To solve the problem, elaborate instrumentation was installed that continuously monitored the heat and pressure at almost every point in the system. Once the trouble points were spotted, the solution was simply a matter of rearranging the plumbing so that pressures were reduced, cavitation of the pumps was prevented, valve leakage was stopped, and so forth."
"When the plane first started transmitting, I noticed dark bands shifting back and forth across the picture," Barney said. "I was told this was caused by vibration and that it was cured by better shock-mounting of the tubes and by going to transistors wherever possible. Is that true?"
"Only partially. Vibration was a problem. The CAA insists all equipment be solidly and rigidly mounted, and this doesn't help much when you're trying to keep vibration from reaching sensitive electronic components, such as amplifier tubes in the tape recorders. But a great deal of this 'banding' was caused by the 400-cycle voltage from the power supply getting into the video circuits. Careful filtering and shielding was necessary to reduce this, and it is contemplated shifting the power supply frequency to 420 cycles so that it will be an integral multiple of the 60-cycle frequency to which the vertical and horizontal oscillators of the TV receiver are geared. This would stop the movement of any banding that cannot be entirely eliminated and so render it less noticeable."
"Wouldn't it be a heck of a lot simpler just to use 50-cycle a.c. in the first place?" Barney asked. "I know the transmitter power is produced by an alternator driven by a gasoline engine back in the tail of the plane; so why not use a 60-cycle alternator instead of a 400-cycle job?"
"I'm comforted to see you're as dumb about aviation problems as I was," Mac answered with a grin. "I asked the same question. What we both forgot is that every pound of unnecessary weight must be eliminated from equipment you intend to fly. Right now the transmitting equipment in a single plane weighs about 10,000 pounds. If 60-cycle instead of 400-cycle current were used, the extra iron necessary in the alternator, transformers, and similar equipment would add hundreds and hundreds of pounds."
"Makes sense," Barney admitted grudgingly. "Say, I understand an antenna was wrecked. What happened? Did the pilot try to land with the antenna still pointing straight down?"
"MPATI people are pretty weary of that joke," Mac said.
"It's true, though, one antenna was ruined. It was burned out. There is a directional coupler in each feedline, and when the s.w.r. indication from one of these goes too high that transmitter is automatically cut off. In the beginning, though, the power was automatically restored almost immediately; and since the trouble was not cleared, the resulting arc-overs ruined the antenna. This has been changed to an arrangement in which the power has to be restored manually after a high s.w.r. has caused it to be disconnected.
"You have to keep in mind that MPATI is a complete system involving both transmission and reception of the educational programs, and the reception of u.h.f. TV signals from a constantly moving airplane at distances up to and beyond 200 miles presents some unique problems, too. MPATI people start with the reasonable premise that dependable u.h.f. reception has to be line-of-sight. Since the signal is attenuated only 6 db each time the distance from the transmitter is doubled, and since a good TV antenna can contribute 15 db of gain, this line-of-sight signal is adequate for good reception out to better than 200 miles - without complications.
"Unfortunately fading introduces a complication, fading that is caused by the ground-reflected signal combining with the direct signal. The reflected signal necessarily travels a longer path than the direct signal; so it arrives at the antenna a fraction of a microsecond later than the direct wave. Depending on how much later it arrives, its cycles will either reinforce the cycles of the direct wave or oppose them. If the cycles are completely out of step, or phase, and if the reflected signal is exactly as strong as the direct signal, complete cancellation can result. On the other hand, if both signals are precisely in phase, the combined reception strength can be twice that from the direct signal alone.
"Prevention of fading, then, has two possible solutions: either we can try to prevent the reflected wave from reaching the antenna at all, or we can attempt to arrange matters so that the reflected wave always helps instead of hinders our direct-wave reception. Close to the plane the use of a reflected-wave screen placed in front of and below the antenna can be quite effective, because the direct wave comes in at a fairly steep angle above the horizon and the reflected wave comes in to the antenna at about the same angle below the horizon; but when you get out to 100 miles where the elevation of the plane above the horizon is less than 20° any barrier high enough to block the reflected wave is likely to interfere with reception of the direct wave.
"When we start trying to make the reflected wave work for us, several things must be kept in mind. At ground level the direct and reflected waves are 180° out of phase and cancel completely. As the antenna is raised, signal strength increases until it reaches a maximum at a certain height above ground. This height is inversely related to both the frequency of the signal and the angle the line-of-sight path makes with the horizontal. MPATI telecast frequencies are fixed at 800 megacycles plus, but the size of the angle is a function of the distance from the high-flying plane.
"You can draw a little diagram and use high school trig to convince yourself the farther the receiving antenna is from the plane, the higher it must be raised to maintain this optimum relationship between direct and reflected path distances. Directly beneath the plane at Montpelier, Indiana, the height is less than 4 inches; at 60 miles it is 4.6 feet; at 100 miles it is 9 feet. As the antenna is raised beyond this critical point, signals arriving along the two paths combine to produce alternate nulls and peaks of strength. Peaks are found at odd multiples of the first height, and nulls are midway between the peaks. At 100 miles, for example, peaks will be found at 9', 27', 45', 63', etc. Nulls will appear at 18', 36', 54', etc. For reasons having to do with the vertical lobing characteristics of the receiving antenna, less fading will be encountered if the antenna is placed at the lowest of these maximum signal points."
"But the plane doesn't hold still. It flies that figure-8 pattern so that turns are always made into the wind; so its pattern-axis is as variable as the wind. On some days the plane will be twenty miles closer to a particular receiving location than it will be ten or twelve minutes later."
"Right you are, and that's what causes trouble at locations fairly near Montpelier where this distance variation makes a substantial change in the line-of-sight angle and consequently in the optimum antenna height. A fixed antenna location may be bringing in maximum signal at one time and be in a near-null a few minutes later. Colonel Schatzel proved this to himself by erecting two antennas separated vertically by the peak-null separation distance at his location at Purdue University. He could select either antenna with a relay, and he found that when snow began to appear in the picture he could always get a much stronger picture by switching to the other antenna. A few minutes later he would have to switch back to maintain a snow-free picture.
"Being an engineer, he started thinking about doing the switching automatically, and he came up with an interesting little dual-diversity arrangement. He showed me the diagram, and the gadget works essentially like this: a gas-tube relaxation oscillator fires every four or five seconds. Each time it fires it operates an impulse relay that changes the receiver from one antenna to the other. I should say this oscillator fires unless something happens. That 'something' is the development of another a.g.c. voltage by the receiver to bias the oscillator and prevent its firing. Only when the strength of the received signal falls below a predetermined level will the lowered a.g.c. voltage permit the oscillator to fire and change antennas. Then that antenna will stay in use until the signal strength again goes down, whereupon the oscillator fires once more and returns the receiver to the original antenna. Colonel Schatzel has worked out an all-electronic version in which the firing of the oscillator causes a bi-stable multi-vibrator to flip. The "B+" of the multi-vibrator is at ground with a negative voltage on the cathodes. A negative voltage is thus available at either conducting plate that can be used to cut off the appropriate one of two mast-mounted amplifiers and permit the antenna in the favored position to feed a signal through its amplifier to the receiver."
"Is such an elaborate installation usually necessary to receive MPATI programs?" Barney asked.
"By no means. The receiver's a.g.c. system is usually more than adequate to maintain a steady picture with fading ordinarily encountered, especially if the antenna installer has taken the precaution to mount the antenna at a location that produces the best average signal as observed over a considerable length of time. An installer used to working with reception from a fixed transmitter is prone to move the antenna around until he gets the best signal strength reading at that particular moment and mount it there.
"You know," Barney mused, "what these MPATI people have learned can be applied to other fields. For example, I'd like to try this diversity reception bit on some of our u.h.f. ham bands where signals arrive by more than one path, too. Also, when we start receiving programs from satellites, the problems involved will be similar in many ways to the ones you have just been discussing."
"Knowledge is usually a kind of monkey wrench that will fit a wide variety of nuts," Mac offered as he stood up and stretched. "And now that I've delivered my wise observation for today, let's get to work."
Posted December 21, 2016