They're Taking the Guesswork out of Scatter Communications
September 1969 Electronics Illustrated

As with many areas of electronics communications, much of both the initial and continued research in atmospheric scattering of electromagnetic signals was/is done by amateur radio operators. The phenomenon is routinely used for accomplishing long distance communications (DX, in Ham terms) by exploiting the reflection property of ionized layers when radio signals impinge at a certain angle. The portion of the signal that returns to the transmitter location, when monitored, can provide information to the sender about the height, distance, and frequency range of the reflecting atmospheric layer. Some of the first indications of backscattering were noticed by radar operators who would receive echo returns from "phantom" targets that were really atmospheric reflections.

They're Taking the Guesswork out of Scatter Communications

By Stanley Leinwoll

International broadcasters located in the United States are developing a system based on the principles of radar that will boost the signals aimed into target areas without requiring increased power or antenna gain. The system, called backscatter sounding, is expected to be operational within a year. If successful it could revolutionize broadcasting techniques on both short-wave and amateur bands.

How It Works

Radio waves striking objects on or above the earth are reflected and scattered in all directions. A small portion of this reflected energy always returns to its source. By measuring the time between the transmission of the energy and its return to the point of origin, accurate determination of the reflecting objects distance from the transmitter can be made.

We know radio waves travel at the speed of light - 186,000 mi. per second. A delay of a microsecond between the time a radio wave is transmitted and the time it's received back at the transmitter corresponds to a round trip (984 ft.) over a path 492 ft. long. All radar measurements are based on this principle. Most radar devices, however, operate at frequencies of 1,000 mc or more because short wavelengths give a more accurate determination of the size and shape of physical objects.

Shortly after World War II it was discovered that short radio waves (between 3 and 30 mc) display radar-like characteristics. These properties help to reveal information about a signal as well as valuable information about the ionosphere.

Long-distance short-wave communication is possible because there exists in the atmosphere a series of electrified layers collectively referred to as the ionosphere. The ionosphere is capable of reflecting radio waves in the high-frequency (3-30 mc) portion of the RF spectrum. However, the ionized gases making up the ionosphere change from day to night, from season to season and over an 11-year cycle dependent on the number of sunspots.

A high-frequency radio wave entering the ionosphere will either be absorbed, be reflected back to earth or be lost in outer space. Reflection back to earth depends on the amount of ionization in the layers, the frequency of the radio signal and the angle at which the wave strikes a particular layer. Most radio energy returning to earth is reflected by the earth back to the ionosphere, where it is again reflected to a distant point. These sky waves make communication over great distances possible.

Backscatter Technique

Because of irregularities on the surface of the earth, a small portion of the energy striking it at some point will be scattered in all directions and even scattered back toward the transmitter. (Hence, the term backscatter.) By setting up a directional antenna and a receiver that feeds its output into an oscilloscope the backscattered signal can be monitored and analyzed right in the vicinity of the transmitter.

If the transmitter should send a series of pulses at different frequencies, say beginning at 3 mc and sweeping up at predetermined intervals to 30 mc, a receiver monitoring these pulses will show just what frequencies are being backscattered. Some of the signals will be absorbed by the ionosphere, some will penetrate it and some will be propagated. Backscatter sounding tells the broadcaster which frequencies are being propagated by the ionosphere and consequently which frequencies can be used for communication to a specific area of the world.

In addition, when the pulses are viewed on a scope, the time delay can be calculated. The time delay between the transmitted pulse and received echo gives the approximate location of the point of reception. This is illustrated in Fig 1. If a scope shows a delay time of 20 milliseconds at a particular frequency, we can arrive at our target's distance by estimating the height of the ionosphere (HL). In general, 300 kilometers is a reasonable assumption and the chart gives us a target distance of about 2,900 kilometers. Since no energy has been returned from a closer location it follows that the target distance was about 2,900 kilometers (or 1828 miles).

Radio Free Europe. The backscatter sounding system currently under test by Radio Free Europe is more sophisticated than the technique described above. It consists of a high-gain curtain antenna for transmitting and a frequency-shift pulse keyer at the input. This device shifts the carrier frequency of a transmitter for short periods of time while pulses of energy are sent out. A receiver and antenna system are tuned in step with the shifted carrier. The antenna consists of nine vertically-stacked cubicle-quad elements mounted on a 108-meter tower. It operates in the 11.8-, 15.3-, and 17.8-mc international short-wave bands and can be adjusted so that the vertical-radiation angle changes to receive maximum energy at different angles (from 3.5 to 22.5 degrees).

Changing the optimum receiving angle is important because it helps determine which receiving mode is producing the strongest signal at a given frequency. International broadcasters such as Radio Free Europe operate on a fixed frequency schedule and cannot utilize backscatter equipment to determine which band is propagating best in order to make an assignment in that band. Instead, they have to determine which angle is producing the strongest signal on a particular frequency and then adjust the transmitting antenna to fire at that angle; this way a better signal is propagated into the target area.

The technique of adjusting your radiation angle is called slewing. RFE has a number of vertically-slewable antennas and is planning to construct others to take maximum advantage of its backscatter equipment. Fig. 2 shows what a typical display looks like. Mode I corresponds to the lowest transmitting angle and Modes 2 to 5 refer to increasing vertical angles. As expected, Fig. 2 shows that as the transmitting angle is decreased, the backscattering distance increases. Modes 1 and 2 show energy returning from two distinct zones; this is probably caused by reflection of the signal from different layers of the ionosphere. This situation frequently occurs during daylight hours.

Amateur Applications

Backscatter sounding promises to lend itself even more to amateur radio than to international broadcasting because the amateur is not limited to a fixed frequency schedule. To understand the great potential for an amateur backscatter network consider the display shown in Fig. 3. Instead of making soundings at different radiation angles on a single frequency, we switch to soundings for the different ham bands (i.e., 40, 20, 15, 10 and 6 meters).

The first pulse is the one received directly from the transmitter. The 40-meter trace shows two other pulses, one relatively close to the transmitter, the second about twice as far away. This is probably due to a second-hop transmission. Because of signal strength. the signal has returned to the ionosphere a second time, been reflected by the earth a second time, and both backscattered signals have been picked up at the receiver.

Succeeding traces show that as frequency is increased, the skip also increases-until at 6 meters we find there is no trace at all. This means that at 6 meters the ionosphere was not propagating and the radio energy was probably penetrating into outer space. Communication on all the other bands was possible, however.

Fig. 3 emphasizes the significance backscatter sounding has for the radio amateur. Here is a method by which instantaneous information on skip in various bands can be obtained. One might envision a network of backscatter sounding stations run by the ARRL and located at various points in the United States. Each such station would have a horizontally-rotatable backscatter array so that soundings could be made in all directions.

Periodic announcements could be made over official ARRL station W1AW, as well as over regional stations, giving the best bands and the approximate skip distances on all bands. By listening to these stations hams would know which bands to operate in. Such forecasts would help CBers and SWLs too.