Your RF Cafe
April 1960 Popular Electronics[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from Popular Electronics.
Early masers (microwave amplification by simulated emission of radiation), like the lasers (light amplification...), began life with the requirement of a rare earth-based mineral at its core - in this case a ruby. Early applications of the maser, as reported in this 1960 Popular Electronics article, were centered on radars. High amplification and high power was beyond the capability of common semiconductors like Si, GaAs, or GaN. Required substrate impurities, gate widths, and thermal control was well beyond the state of the art of the day. As always, the early pioneers like Dr. Charles H. Townes, inventor of the maser, accomplished incredible feats with rudimentary tools, including the venerable slide rule.
Also see The Maser / Receiver Signals from Space in the November 1960 Electronics World.
This strange amplifier, the heart of which is synthetic ruby, harnesses the energy of spinning electrons to increase the sensitivity of receiving equipment up to 100 times
By Miles Dillard
On February 10th of last year, eight scientists gathered at MIT's famous Lincoln Laboratory near Boston. They carefully checked over the powerful research radar installed there, then angled its huge dish antenna sharply into the afternoon sky. At exactly 2:21 p.m., a pulse shot out from the antenna in the direction of the planet Venus. Just under five minutes later, an echo so faint as to be hardly recognizable came back to earth. Man had made his first direct contact with the planets.
Searching Space. Columbia University's Dr. Charles H. Townes, who invented the maser, and scientists at the Naval Research Laboratory near Washington have built a maser-operated 50-foot radio telescope which has a range three and one half times as great as the best light telescopes - over seven billion light years! This has opened up for exploration a total volume of space about 40 times greater than that seen by the 200-inch Mount Palomar telescope and earlier radio telescopes.
The great sensitivity of the new 50-foot "space eye" is already enabling astronomers to learn something about the surface of Venus for the first time. This has been perhaps the most mysterious of all the planets because it is eternally obscured by thick clouds. But the maser telescope can pick up its feeble surface radiation easily, and astronomers now know far more about it than ever before. They have recently learned, for example, that the surface of Venus is a sizzling 585°F - far too hot for the existence of life as we know it.
But this is just the beginning. Dr. Frank D. Drake of the National Astronomy Observatory at Green Bank, West Virginia, predicts that within about a year a radio telescope with three times the range of the 50-foot unit at the Naval Research Laboratory and ten times the range of earlier radio telescopes will allow scientists to "see" the actual surface of Venus and to determine for the first time its speed of rotation - that is, the length of its days.
This new instrument is already under construction at the West Virginia observatory site. Its gigantic dish antenna will be 600 feet in diameter; two football fields would fit end to end across its span. Not only will it "see" nearby planets more clearly, but its tremendous sensitivity will enable it to probe 30 to 40 times as much space as can now be explored with the 50-foot telescope. Its range may extend as far as 20 billion light years! Astronomers think they will even be able to observe the "edge of space," where radiation emitted at the time of the formation of the universe may perhaps be detected or where space may actually be seen to curve.
"Such observations," says Dr. Townes, "will quite possibly indicate whether present ideas of an expanding universe are correct, as well as providing a means of checking other cosmological theories."
One of the maser's most obvious applications is in the field of satellite communications. Since tons of fuel must be burned for every pound of satellite put into orbit, scientists use every conceivable trick to design the lightest possible equipment for space probes. With maser amplifiers one hundred times as sensitive as older types in ground listening posts, smaller and lighter transmitters can be installed in satellites. Smaller transmitters also use lighter batteries, saving more weight.
Thermal Noise. The maser - this weird piece of frigid hardware - is able to perform its many tricks because its unique principle of operation virtually eliminates that old bug-a-boo, thermal noise.
Why is this so important? Let's take a look at the maser's use in radar and find out. Figure 1 shows a radarscope with an echo from a nearby airplane. The echo is strong and clear. But notice the wiggly lines along the bottom of the scope trace. Radar operators call this irregular pattern "grass." Engineers call it thermal noise.
If there were no thermal noise, the scope trace with the
same echo would look like Fig. 2. Here, the transmitter pulse
and the echo is unchanged; the only difference is that there
is now no "grass," or thermal noise. It makes little difference
whether or not the thermal noise is there, as long as we get
a strong echo from a nearby target.
But what about that echo from Venus? By the time a signal travels 55 million miles, there isn't much of it left. On a maser radarscope, the trace would look like Fig. 3. Without the maser, it would look like Fig. 4. Where is the echo? Completely blanked out by thermal noise.
How does the maser do away with thermal noise? To answer this question, let's quickly review the cause of thermal noise.
If you could look inside the tubes and wires of your hi-fi set, for example, you would see streams of electrons rushing along in orderly groups. This flow of electrons is the "signal" that eventually comes out of the speaker as music or speech. But here and there a few electrons, stirred up by the heat present in any circuit, scamper around aimlessly. This random movement generates a small but measurable current of its own, which comes out as noise. It is called thermal noise, or thermal agitation, since it is caused by heat: the more heat, the more noise.
You can actually hear thermal noise on your hi-fi amplifier,
just as you can see it on a radarscope. With no signal applied
to your hi-fi set, turn up the volume control and put your ear
next to the speaker .. The hissing sound you hear is thermal
noise greatly amplified. Although such noise is rarely objectionable
in hi-fi amplifiers, it seriously limits the range of radar,
as we have seen.
Since the maser does not depend on electron flow, there is little random noise created. And even the few stray electrons that would normally wander about are much less likely to do so when the maser is dipped in a chilling bath of liquid helium. At temperatures. close to absolute zero (-473°F), random electron movement becomes virtually non-existent. A radar echo from Venus, minute signals from a star six billion light years away, or a feeble message from a satellite with a small, light-weight transmitter can come right in without competing with amplifier noise.
How the Maser Works
Scientists tell us that materials such as synthetic ruby contain electrons spinning at different rates, or to be more accurate, at different "energy levels." Under normal conditions, most electrons are at the lowest energy level, which is called. "Energy Level 1." Fewer electrons are at Energy Lever 2, and still fewer at Energy Level 3. When an electron "falls" from a high level to a lower one, it gets rid of its excess energy by radiating that energy in the form of microwave signals.
Although at this time we can only guess where future developments may lead, we can be sure that the maser and its applications will grow increasingly valuable - both here on earth and in the empty vastness or space when man leaves his planet to explore the stars.
Posted July 24, 2012