Radio on the Moon

... Radio Signaling on the Moon Differs Vastly From That on Earth ...

"After the first rocket from the earth has landed on the moon, supplies are unloaded feverishly and our exploration parties are organized. We arrived at the beginning of the two weeks' (earthtime) daylight that makes the lunar day. Six men stayed near the rocket base, the other four in their tank-mooncar were to explore a radius of about 250 miles of lunar terrain. The little tank-car was radio-equipped, while the four explorers all had walkie-talkies. Thus the tank and men could always be in constant touch with the moon-base headquarters, via radio."

Rubbish and nonsense — as far as the last sentence goes! Recently we have all read accounts similar to the above written by amateur scientists, who should know better. The facts are that one cannot signal by radio on the moon as one does on earth.*

The moon has no true atmosphere, hence no Heaviside- layer reflecting surface.† That means that if one tried to communicate, let us say, at broadcast frequencies, signals could not be transmitted to points much beyond the visible horizon.

It is true that on the moon we have many mountains, some up to 30,000 feet high. Couldn't the mountains be used to reflect the signals? Yes and no. If you draw a circle of the moon to scale, you will see that a signal bounced against a mountainside will be reflected out into space. The signals will be received only in the vicinity of the transmitter, as long as the receiver and transmitter can see each other, but not beyond the horizon. And that goes for all frequencies.

What about the so-called obstacle gain, or knife-edge effect, whereby a sharp object like a mountain peak will scatter the radio waves on the other side of the mountain, diffracting the wave, i.e., bending it? In this way, TV signals today can leap over an interfering mountain, yet be re- ceived well in a valley on the other side, if the transmitting angle in reference to the mountain peak is suitable.

The obstacle effect, as far as has been ascertained, is a tropospheric condition. As there is no troposphere on the moon, we cannot depend on such an effect.

Unfortunately, too, the moon (2,160 miles in diameter), a much smaller body than the earth (7,926 miles in diameter), has an exceedingly short horizon. On earth, the average person's eyes are, let us say, 5 feet above the sea or lake level when standing up. The horizon for him is 2 1/2 miles away; actually, due to the refraction of the air, the line of sight is curved, hence he sees the horizon further away, or 3 1/4 miles.

On the moon, however, conditions are vastly different. There being no atmosphere, the same man sees the horizon much closer — only 1.4 miles away. Even with his walkie- talkie antenna extended to 10 feet, the horizon still is only 2 miles away. True, the explorer could climb up a mountain to extend his range, but mountain climbing on the moon for the first lunar explorers is much too time-consuming, hence not very practical.

Thus, under normal conditions, an exploring party could, under no circumstances, travel 250 miles from its lunar base and hope to be in direct radio contact with it in case of emergency, or even to give hourly reports.

One solution would be to station personnel on a number of high mountain tops. These then could relay messages over fairly long distances. This, again, is impractical because of the restricted horizon. Far too many people would be needed just for relaying purposes — and no expedition in the initial stages of moon exploration could even think of transporting scores of people.

Later on, automatic relays could of course be built, and probably will, although the cost of installing such a relay network, running in all directions on an extremely hostile world, will be astronomical. There is also the difficulty of reaching isolated plains, with which the moon abounds. Many of these plains are far from mountains, hence would always be isolated unless, of course, one were to build relay towers to stretch across these flat lunar surfaces.

But we need not despair. There is a very simple, safe and practical radio communication means for the moon's first explorers. It will enable the first men on the moon to roam from pole to pole over thousands of miles and be constantly in touch with their lunar base, as long as they are on the side of the moon facing the earth.

The signaling simply will be by relay to the earth. §

For best operation, the explorers' walkie-talkies, as well as their tank-car, will be equipped with directional dishpan radar type antennas. No extra power is necessary. The antennas will be directed toward the home land, if it is in sight. If, because of the earth's rotation, it is not, there will be listeners in other lands waiting to relay the message back to the moon. The total time of such relay transmittals — coming and going — is about 2 1/2 second.

Before the expedition to the moon, certain radio relay stations are selected on earth to handle the lunar traffic on frequencies specifically assigned for just such a purpose. This is obviously necessary as there cannot be too many earth-moon transmitters on the same frequencies, lest jamming and confusion occur. Most transmissions will probably be by code, many by phone.

Such signaling is also quite feasible from within the moon's thousands of craters. The latter measure from 1/4 to 150 miles in diameter. Usually they are surrounded by a chain of high mountains, which often makes it impossible to signal by radio directly through the mountains, even over short distances. It is not even possible to radio within one crater over 10 miles in diameter, because of the sharp curvature of the moon and its short horizon. Hence, relay to earth is the best and surest means. There is one signaling means that we cannot test until explorers have actually tried it out on the moon.

It was Nikola Tesla who theorized that it should be possible to signal with alternating ground currents only. No antenna was to be used. It was he, too, who, in his famed Colorado experiments, lighted electric light bulbs several miles from the transmitter. There were no conducting wires — ground connections only at the receiver.

Time will tell if long-distance signaling solely via the ground will prove practical on the moon.

—H.G.

* Excerpts of this editorial were given in a lecture by the author before the Radio Club of America in New York, April 28, 1959.

† The moon may have some vestigial gas pockets originating from lunar volcanos, but these, because of the extremely low gravity, are dissipated quickly into space.

§ The author was the first to point out earth-moon signaling in his article "Can We Radio the Planets?" in Radio News for February, 1927. Nineteen years later, the US Signal Corps made its first moon contact on 111.6 megacycles. The author's predicted time in 1927 was 2.5 seconds; the actual time 2.4 seconds.