Getting High

Ultra High Frequency (UHF) radio, despite being over eight decades old, is still indispensable for military communications. How did this vital technology evolve and how does it work?

Calling it Ultra High Frequency is a misnomer, compared to other frequencies in the radio part of the electromagnetic spectrum. Judged by today’s standard, UHF is not all that ‘high’, for instance X-band (8-12 GHz) and Ka-band (27-40 GHz) frequencies are much further up the spectrum. This was not the case back in the early 1940s when UHF was pioneered. Long- and Medium- Wave (LW/MW) transmission techniques were perfected in the early 20th Century. Longwave was so-called because it was precisely that. Still in use today, LW signals exceed lengths of 1,000 m (3,281 ft), corresponding to frequencies of 300 kHz and above. MW uses frequencies of 520 kHz to 1.611 MHz (or up to 1.710 MHz when using North American standards). This produces wavelengths of between 576.5 m (1,891 ft) and 186 m (610.2ft) in length (or 175 m (574.1ft) when using North American standards).

The problem with LW and MW signals is that they need large antennas. A rule of thumb says antenna length must be one half or one quarter of the wavelength it transmits. An LW antenna transmitting 300 kHz signals must be between 500 m (1,640 ft) and 250 m (820 ft) long. An MW antenna handling signals of 520 kHz must be between 288.3 m (945.9 ft) and 144.1 m (472.8 ft) long. Such antennas need very tall towers capable of mounting these antennas vertically, or alternatively, large arrays comprising several towers with the antennas mounted horizontally can be used.

High Frequency

An important breakthrough occurred in June 1923, when radio pioneer Guglielmo Marconi, together with his assistant Charles Franklin, showed short wave radio could be used for long-distance transmissions. Short wave comprises signals in the 3- 30 MHz frequency range, with 99.9 m (327.8ft) and 9.99 m (32.8ft) wavelengths. Using an antenna built at Poldhu Wireless Station in Cornwall, southwest England they showed that signals could be transmitted to Mr. Marconi’s steam yacht Elettra. The vessel was located 4,142 km (2,236 NM) away in the Cape Verde islands off the West African coast.

High Frequency (HF) voice communications had been proven. HF and shortwave monikers were used for years, although HF is the preferred term today. Like LW and MW, HF transmits across thousands of kilometres. It does this by aiming transmissions at an angle towards the ionosphere. This is an ionised part of the atmosphere 48,000 m (157,480 ft) and 965,000 m (3.1 million ft) above Earth. As HF signals cannot penetrate the ionosphere, they are bounced back to the surface. This lets them ‘skip’ over the curvature of the Earth, a process known as Skywave transmission. HF signals can also be used for point-to-point transmissions provided the transmitting (Tx) and receiving (Rx) antennas have an unobstructed Line-of-Sight (LoS) between each other.

HF brought some advantages. Antennas were smaller, between 49.95 m (163.9 ft) and 2.4 m (7.9 ft) in length depending on the operating frequency. Thus, HF antennas were more practical than LW and MW for installation on aircraft, ships, and land vehicles. Nonetheless, HF had disadvantages. Although smaller than LW and MW antennas, some HF antennas remained large. Secondly, Skywave HF signal quality was at the mercy of the ionosphere which was at the mercy of the sun. Sunspots and solar flares can greatly affect the ionosphere, which in turn affects HF signal propagation. This made using an HF radio as much an art as a science, demanding skilled operators. Today, much of this work is done using software.

HF Shortcomings

By WWII, wartime exigencies demanded radio become more practical. Help was at hand from the cavity magnetron, itself a spin-off from radar engineering. Radar had previously been successfully demonstrated as means of detecting and tracking aircraft in February 1935, and would become vitally important for all sides during the Second World War. The Royal Air Force’s (RAF) Type-1 Chain Home radar network along Britain’s coastline helped detect and track Luftwaffe (German Air Force) aircraft so they could be engaged by RAF fighters. This helped the RAF win the Battle of Britain in the late summer of 1940.

These Type-1 radars used frequencies of between 20 MHz and 50 MHz to indicate enemy aircraft locations so fighters could be vectored towards them. Nonetheless, the Type-1 had shortcomings. The radar needed towers 110 m (360.9 ft) high to hold the transmitting antenna strung between them, while two towers 73 m (239.5 ft) high supported the receiving antenna. These heights afforded the radar its 190 km (118 NM) range. Yet these large installations were easy to find, and the Luftwaffe attacked several Type-1 radars at the start of the Battle of Britain.

As such, the trend moved toward smaller radars operating at higher frequencies, which were more difficult to locate and easier to transport. Additionally, operating at frequencies beyond 50 MHz also promised sharper radar beams, which meant more accurate location of enemy aircraft, improving the efficacy of RAF air defence.

Cavity Magnetron

A technology to generate these microwave frequencies was found in the cavity magnetron, which was pioneered in 1940 at the University of Birmingham, by physicists John Randall and Harry Boot.

A cavity magnetron consists of a cathode (negatively-charged electrode) attached to a filament which heats the cathode, mounted at the centre of the magnetron. The cathode and filament is surrounded by a large hollow cylindrical anode (positively-charged electrode), with several cavities cut at regular intervals along its inner walls, known as ‘resonating cavities’. There is a vacuum gap between the anode and the cathode, and permanent magnets are mounted above and below the anode, opposite poles facing one another, providing a magnetic field along the longitudinal axis of the anode.

As the cathode heats up, electrons ‘boil’ off it in a process known as thermionic emission. The magnetic field curves the travel path of the electrons during their journey through the hollow between anode and cathode. As the electrons coming off the cathode zoom past the inner grooves of the cavities, they cause electrons inside the inner grooves to oscillate, or move back and forth, to and from neighbouring grooves along the outer ring of the cathode. This causes the grooves to alternate between positively charged (caused by losing electrons) and negatively charged (caused by gaining electrons) states, forming an alternating electric field between the two sides of the cavity. In addition to inducing oscillation, the electrons zooming past the cavities also impart some of their energy onto the electric field in the cavities.

As this oscillation process occurs, the resonating cavity generates electromagnetic waves, with the frequency determined by the cavity dimensions. This electromagnetic wave can then be collected by a ‘tap’ – either an antenna or waveguide, and emitted. This process can be likened to blowing across the top of a bottle to produce a sound, however the end result here is the generation of electromagnetic waves rather than sound waves.

Microwaves have frequencies upwards of 300MHz and wavelengths downwards of 1 m (3.3 ft). These wavelengths and frequencies provided the desired improvements in radar accuracy. They resulted in smaller radars and radios, as smaller antennas below 500 mm could now be exploited.

UHF

Cavity magnetrons led to the birth of Very High Frequency (VHF) and Ultra High Frequency (UHF) radar and radio. The latter uses frequencies of 30 MHz up to 3 GHz. VHF and UHF is routinely used for military communications, particularly tactical radios for land forces and for airborne communications.

Today’s UHF radios commonly use crystal oscillators to generate their signals, and have an interesting means of generating an Alternating Current (AC) signal from a Direct Current (DC) input.  In DC, the electrons flow in one direction, maintaining a constant voltage, while in AC, the electrons alternate their direction of flow back and forth along the circuit, between an alternating positively- or negatively-charged region which respectively work to attract or repel electrons.

A crystal oscillator consists of a suitably shaped/dimensioned crystal made out of a suitable material such as quartz, sandwiched between two layers of conductive metal on either side, each of which is connected to an electrode. It should be noted that the final frequencies generated by the crystal oscillator are determined by the size and dimensions of the crystal, so once it has been cut, it has a fixed operating frequency range. The frequency can also be affected by factors such as temperature.

Applying a DC voltage to the crystal causes it to deform sharply, in a process known as the Inverse Piezoelectric effect. This rapid deformation causes the crystal to resonate briefly, not unlike a bell will resonate when struck, creating a rapidly-diminishing audio signal. In much the same way, the application of DC voltage to the crystal causes it to briefly resonate. This mechanical resonance causes the crystal to emit a brief AC electrical signal whose frequency is determined by the shape and size of the crystal. This signal can then be sampled and amplified, and reapplied to the crystal in the same phase in a process of positive feedback. This causes the crystal to continue to resonate, generating a highly-stable AC signal. This AC signal provides the basis of a carrier wave, and can then be sent to an antenna to generate the electromagnetic wave for transmission.

Compared to cavity magnetrons, crystal oscillators can be smaller helping reduce the size and weight of equipment like radios. Unlike HF signals, VHF/UHF signals achieve impressive data speeds. The latest HF radios carry data at speeds of up to 120 Kbps, compared to rates measured in Mbps for VHF/UHF transmissions. This is because VHF/UHF radios have wider channel bandwidths available compared to HF. To use an analogy, you can get more cars moving faster down a six-lane highway than you can with the same number of cars using a narrow country road.

Mobile phones use VHF/UHF and a standard fourth-generation mobile phone achieves data speeds of 100 Mbps with a 20 MHz channel bandwidth. The sheer amount of data that can be moved with VHF/UHF makes it attractive to militaries. This makes it possible to not only send and receive comparatively clear voice communications, but also data-heavy traffic like photos and video.

UHF suffers less electrical interference from sources such as high voltage power lines compared to other frequencies. VHF signals of 30-300 MHz struggle to penetrate obstructions like walls. These are less of a problem for UHF, which performs better in urban areas. Nonetheless, neither VHF nor UHF can achieve the intercontinental ranges of HF. Although UHF can penetrate walls, like VHF it is still largely used for LoS voice and data traffic. LoS restrictions force tactical VHF/UHF radios to use Mobile Ad Hoc Networking (MANET). MANET alleviates problems when the path between two radios is blocked by the horizon or a similar large obstacle. One radio will transmit its traffic to another in range, this radio transmitting the traffic to another, and so on, until the traffic reaches its recipient.

UHF has been widely used by militaries since the Second World War and will continue to be for the foreseeable future. The major pressure on UHF military use comes from the commercial world. As noted above, cellular communications also use UHF. Parts of the UHF spectrum reserved by governments around the world for military use may be auctioned off to private cellular network operators, which could put pressure on the size of the UHF wavebands available to militaries. One solution could be to migrate some military radio communications to higher frequencies like terahertz (300 GHz to 3 THz). That, dear readers, is a discussion best left to a future article.

Thomas Withington