History of HF communications technology: From forgotten medium to modern communications

The history of communications technology has seen remarkable transformations, from the early days of radio to modern digital high frequency radio networks. High Frequency technology was almost forgotten, drowned by the popularity of satellite technology and the internet, but it is making a comeback due to its unique advantages and modern communications enhancements.

The historical roots and rise of HF communications technology

The radio developed independently by Guglielmo Marconi and Nikola Tesla at the beginning of the 20th century was to the early hacker what the microcomputer and the internet would be a hundred years later: new, unexplored technology. It was a way to connect with fellow enthusiasts hundreds, even thousands of kilometres away, a step towards the future electronic society. For many, it represented a step towards a new, better, borderless world.

The enthusiasm was especially great on the new continent, where by the 1910s, thousands of amateur radio operators were transmitting in Morse code, alongside shipping companies and government entities. The technology was what it was: the commonly used spark transmitters shattered the signal across the radio network, interfering with everyone else’s transmissions.

The United States government had had enough of this Wild West activity after the Titanic sank in 1912. After the disaster, it wanted to ensure that radio communications with ships would work without interference in all conditions. Radio amateurs were evicted from emergency frequencies and, for safety’s sake, from all frequencies below 1.5 megahertz, which were taken over by the military and commercial radio stations.

They were left with the higher frequencies (HF), shortwaves, which were considered useless. According to the knowledge of the time, they could only be used to communicate over distances of tens, at most a few hundred kilometres.

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The discovery of new technology and commercial adoptions of HF

Shortly before the First World War, atmospheric research revealed that there is a strange layer in the upper atmosphere where atoms remain electrically charged as ions due to solar radiation. Theories suggested that this ionosphere could bend radio waves in the same way that a light beam bends at the interface between air and water, and reflect them thousands of kilometres away. Radio amateurs demonstrated after the war that the theory works in practice, and specifically with HF.

Commercial operators at that time were sending radio messages across the Atlantic using long, low-frequency waves. The technology worked but required antenna towers hundreds of meters high and transmission power of hundreds of kilowatts. It was generally thought that there was no other way: low frequencies travelled the farthest.

Yet, in 1921, American and British amateur radio operators discovered a way to establish a connection across the Atlantic with their homemade HF devices, using a fraction of the transmission power and cost of professional stations.

Read also: Basics of HF Technology

Layers of the ionosphere

The effect of the uppermost region of the atmosphere, the ionosphere, on radio waves varies with altitude. The sun’s gamma, X-ray, and ultraviolet radiation ionise atmospheric molecules in different ways at different altitudes. The composition of ionospheric layers that significantly affect radio waves varies between day and night.

• The low-lying D layer attenuates the longest HF waves below 10 MHz. The D layer disappears at night when the sun stops ionising it. Therefore, low HF frequencies are better heard at night.
• The E layer reflects the longest HF waves best. The layer is lower during the day than at night. Thus, lower HF frequencies generally travel shorter distances during the day than at night.
• The highest F layer reflects high frequency waves the farthest. It splits into two parts during the day and reunites at night. The F layers float lower in winter than in summer, shortening the range of high frequency radio network communications.

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Propagation of radio waves

1. VHF, UHF propagation (30-3000 MHZ)

   Very high frequency radio waves travel straight and do not curve along the Earth’s surface. Transmissions escape through the atmosphere into space. Mobile phones, WLAN networks, and radio and TV broadcasts operate in these frequency bands.

2. LF, MF propagation (0.03-3 MHZ)

   Low frequency radio waves interact electrically with the Earth’s and sea’s surface. Thus, they can curve along the Earth’s surface and travel beyond the horizon as a ground radio wave. A ground wave can travel over 2000 kilometres before it fades out of audibility.

3. HF propagation (3-30 MHZ)

   High frequency radio waves travel both directly and as ground waves. They can also bend in the atmosphere so that the signal bounces back to the Earth’s surface thousands of kilometres away. This space radio wave can also bounce again from the Earth’s surface to the atmosphere and back, allowing the transmission to travel even to the other side of the globe.

The advantages of HF communications and its rise in popularity

The difference of low-frequency (LF) waves and HF is that the LF waves do not bounce off the atmosphere, but only curve along the Earth’s surface. They can travel far beyond the horizon but inevitably weaken as they go. In contrast, an advantage of high frequency communications is that, under favourable radio network conditions, high frequency waves can travel from one side of the globe to the other with less than a watt of transmission power by bouncing off the atmosphere, also known as space waves.

No wonder HF began to attract interest. Conveniently sized HF transmitters quickly rose in popularity alongside and even surpassing massive longwave stations. HF devices were particularly favoured in maritime traffic due to their size advantage. Broadcast stations also quickly began their HF transmissions. One of the most famous was the Voice of America in the decades following the Second World War, which broadcast Western propaganda to the nations behind the Iron Curtain.

HF communications has an advantage serving this purpose: since the HF signal reaches its destination at various frequencies as a ground wave or a space wave, depending on the time of day and the transmission location, it is difficult to silence completely.

Basics of HF technology

Complexities of HF propagation

The biggest advantage of HF communications, the space wave, is difficult to use due to its peculiar propagation. The ionosphere’s ability to reflect HF waves around the globe varies with the season, time of day, frequency, and geographical location.

Even the often humorously used phrase “it’s because of sunspots” is entirely factual concerning HF propagation: the number of sunspots correlates with solar activity. A more active sun emits more ionising radiation, causing higher frequencies to reflect from the ionosphere better than lower ones.

There is also rapid variation in the atmosphere: the signal strength can decrease due to various disturbances to a hundredth in a few seconds and strengthen back just as quickly. Finding a working HF frequency is sometimes pure black magic.

Read also: Tackling high-noise challenges in HF communications systems

In addition to channel fading, HF is also plagued by the fact that the frequency band is narrow. The entire HF band fits into a 27-megahertz wide strip of the electromagnetic spectrum. It may sound like a lot, as for example modern 4G LTE networks use 20-megahertz or narrower blocks for data transmission.

However, radio waves behave differently depending on their length. On the UHF frequencies used by mobile phones, it does not matter whether the signal is transmitted at 1920 or 1925 megahertz. The radio wave propagates the same way at both frequencies. However, on HF, a 3-megahertz frequency often propagates differently than 8 MHz.

One may be heard from Oulu only as a ground wave in Haukipudas, while the other may be heard through the atmosphere in Honolulu. One frequency may be blocked by interference from power lines and ventilation systems, which do not affect the other at all.

Challenges in HF frequency allocation and usage

The position of HF is not helped by the fact that part of the HF band is reserved for special purposes. Radio amateurs, aircraft emergency communications, guerrilla radio operators, and over-the-horizon radar systems all operate in the HF band. Overlapping transmissions interfere with each other. Therefore, national communications authorities grant HF licenses conservatively and for narrow frequency bands at a time.

Thus, the standard has become to use 3-kilohertz wide channels in the HF band. This is sufficient for voice, but less so for data transmission. Typical data transfer speeds on 3-kilohertz channels are 1200-9600 bits per second. Similar bit rates were achieved with modems over the telephone network four decades ago.

Above all, due to the unpredictability of the radio channel, HF began to lose popularity on ships in the 1980s with the rise of satellite communications. The development of the internet has, in turn, reduced HF broadcasts. Why broadcast with special equipment when every house has a computer suitable for listening to internet radio?

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The comeback of high frequency in radio communications

Another advantage of HF communications is that it does not require infrastructure. No expensive base station network, no telephone cables, no satellites.

As modern military communications demands continue to surge, satellite networks are becoming increasingly congested, with limited capacity and the high costs associated with launching new satellites. This has driven a pressing search for alternative communications solutions.

HF is a promising candidate, and many of its limitations can now be overcome. The troublesome search for a working channel has been accelerated by automation. With a technique called 3G ALE developed in the 1990s, a connection can be established in five seconds, but it requires high frequency radio networks to remain time-synchronised, for example, using GPS positioning.

Read also: Automatic link establishment in 0.5 seconds – discover why KNL’s ALE HF radio outperforms others

The HF communications evolution

The real HF communications evolution has occurred in the last fifteen years. With the advancement of signal processing technology, it has become possible to have the radio monitor the entire HF band in real time.

In such wideband HF technology, long call chirps and GPS synchronisation of 3G ALE are not needed. It is enough for the sender to send a transmission request lasting a tenth of a second somewhere in the HF band. If the radio waves physically reach the receiver, it responds just as quickly, and data transfer can begin.

By monitoring the frequency band in real time, the radio can be made cognitive: the radio always knows where in the spectrum there are interference spikes caused by others and automatically tries to transmit elsewhere than on the interfering frequencies. This minimises the harm caused to other users.

With the increase in processing power, radio modems have also been improved. This means that the bandwidth for data transmission can be increased from three to up to 48 kilohertz and more efficient modulations can be used. These allow more bits to be packed into the band more densely, increasing transmission speed even further.

The highly advanced HF equipment, such KNL’s CNHF Manpack radios, can achieve data transfer speeds of up to 300 kbit/s for various media formats.

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Read also: Innovative communications systems: KNL radios deliver on promise of HF renaissance 

What is a good frequency?

• The signal propagates well: the measured signal-to-noise ratio (SNR) is high, as in this case at 10 MHz.
• The SNR does not vary much: the ground wave is usually better than the space wave, represented by 12 MHz in the figure.
• There is the least interference and low background noise at the receiver.

Handshakes and acknowledgements – Cognitive HF

• Radios are on standby to receive.
• Some systems hop on frequencies synchronously, requiring, for example, GPS timing.
• Some systems hop asynchronously, requiring long call messages.
• Some systems listen to the entire HF band simultaneously, requiring a lot of processing power.

• X transmits a transmission request (RTS) on a frequency it deems good.
• Y listens to the HF band and notices that something is coming on this frequency.
• Y locks onto the frequency and receives the RTS. It can indicate, among other things, what kind of data is coming and how much.
• Y responds with a response message (CTS), indicating the settings it can receive the data with. In a poor radio channel, data must be sent more slowly than in a good one.

• X sends the data in small packets. In the data transfer protocol, the throughput is generally confirmed with an acknowledgment message (ACK). It indicates which packets were received and which need to be resent. With the ACK message, the transmission settings can be adjusted based on how well the data has flowed through. If the channel becomes completely unusable, better frequencies can be agreed upon with a new RTS-CTS handshake.

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