The British Army's Wireless Set, Number 10, was the world's first multi-channel microwave relay telephone system. It transmitted eight full-duplex (two-way) telephone channels between two stations limited only by the line-of-sight, often on the order of 25 to 50 miles (40 to 80 km). The stations were mounted in highly mobile trailers and were set up simply by aiming the two parabolic antennas on the roof at the next station. The system could be extended into a relay by connecting trailers together, or using existing landlines to connect separated trailers.

The basic concept became possible with the introduction of two key technologies in 1940: the cavity magnetron, which produced microwave signals with reasonable efficiency; and pulse-width modulation (PWM), which offered a simple way to encode the signals on a magnetron. As the available bandwidth was high, eight channels were combined into a single link using time-division multiplexing.

Early experiments with single-duplex (one-way) systems were carried out in 1941 and 1942 which demonstrated the basic concept. By that point, improvements in electronics allowed for a full-duplex system. Testing of a long-range system began in 1942 and overwater tests followed. The system was ready for service in 1944, and military-quality sets were available for D-Day operations. The range was enough that it was used to provide secure communications from the D-Day beaches back to England across the English Channel, and the network was eventually extended into Germany. Field Marshal Bernard Montgomery would later note:

By using a chain of No. 10 Set Stations, I was able to maintain my tactical HQ as far forward as I did and still have contact with London. The value of being able to retain personal contact over my Armies in these circumstances cannot be overestimated.

There had been many systems for transmitting telephone conversations over radio before World War II, but they all suffered from a series of similar problems.

The first was that in order to gain long-range transmission, these systems had to work at relatively low frequencies in the kilohertz range or somewhat higher longwave frequencies that could take advantage of the ionosphere to "skip" their signals. A radio antenna has to be within about an order of magnitude of the wavelength in order to be efficient, and in practice, is often sized to exactly 1⁄2 the wavelength to form a half-wave dipole. Thus, these systems used very large antennas.

Another related radio physics effect is the directivity of the antenna, its ability to form the signal into a beam. This is related to optical resolution, which is improved with increasing antenna sizes, and decreased with increasing wavelength. The relatively long wavelengths of the signals made focussing difficult without resorting to enormous antenna arrays, and in many cases such signals were broadcast omni- or semi-directionally instead. This meant the signals could be received by ground stations other than the intended one, sometimes thousands of miles away, leading to interference. For secure military communications, such a system had obvious drawbacks.

Finally, the amount of information that can be carried by a radio signal is a function of its bandwidth. A telephone conversation might make do with a bandwidth as small as 4 kHz, but at 150 kHz this represents a fairly large fractional bandwidth. Depending on the antenna and receiver design, the spread of frequencies that can be efficiently received may limit the link to one or two conversations.