FT8 The digital evolution in amateur radio A look into the future

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Digital revolution
in amateur radio

Digital signal processing makes it much easier to use noisy signals in propagation modes such as meteor scatter and Earth-Moon-Earth radio operation. If analog radio signals are converted into digital data for processing in a computer, the calculation rule in the PC determines the performance of the radio link. of the radio link. The importance of antenna gain and radio equipment decreases.

Minimalism in perfection

The two callsigns, both large locator fields, the report in dB over noise, and finally the QSO confirmation with RRR, RR73 or 73 are transmitted.

FT8: The developers and their mathematical tricks for ultra-quiet signals

FT8 is named after the first letters of the surnames of the two developers Steven Franke, K9AN, and Joe Taylor, K1JT, whereby the number 8 describes the number of transmit symbols (frequencies) used in the of transmit symbols (frequencies) for the type of modulation used, Gaussian Minimum Shift Keying (GMSK) with 8 symbols. However, Fourier transformation would probably also be appropriate, as the “Fast Fourier Transformation” FFT is the mathematical trick for calculating extremely quiet signals from the noise.

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Dr. Joe Taylor:
Master of silent radio signals and Nobel Prize winner for gravitational waves

Extremely quiet radio signals are the specialty of Dr. Joe Taylor, who has developed methods for evaluating signals in radio astronomy. Hours of recordings of the noise received by the Arecibo space telescope in Puerto Rico with a mirror diameter of 305 meters were subsequently processed and evaluated in the laboratory at Princeton University using mainframe computers. Joe Taylor was awarded the Nobel Prize in Physics for the detection of gravitational waves in the radio spectrum of quasars, calculated using his methods for integrating the noise.

From noise to clarity: the magic of the Fourier transform

The digitized sound recordings are converted from time to frequency space using the mathematics of the Fourier transformation and then added up in narrow frequency ranges of just a few Hertz. This averages the noise over a longer period of time and you suddenly see differences in the amplitudes of the frequency ranges.

The home PC at the radio station works like this, it converts the tones or just the noise from the radio into a WAVE file with a length of about 12 seconds, in less than 2 seconds the signals of the stations are then calculated and decoded from the spectrum using the Fast Fourier Transform method. You can imagine that a signal on the oscilloscope, e.g. a sine tone, is converted into a line on the spectrum analyzer. First you have the course of the signal over time, then the spectrum and the amplitude. And if you integrate these two pieces of information over time, these extremely weak signals can still be calculated out of the noise.

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WSJT: The Evolution of PC Software in Amateur Radio by Joe Taylor

With WSJT, Joe Taylor developed the first PC software in 2005, initially for operating modes for meteor scatter and EME, WSJT-X in 2010 had further operating modes implemented. FT8 was introduced in 2017, transmitting 13 characters in a transmission period of 15 seconds with a signal bandwidth of 50Hz. In 2019 came FT4 with a cycle time of 7.5 seconds at a bandwidth of 90Hz. FT8 uses 8 tones, which corresponds to 3 binary bits. The software is open source, other programs for the PC are JTDX and MSHV.

FT8: Transmission, synchronization and the magic of symbols

FT8 transmissions have 77 information bits, 14 CRC bits and 83 parity bits, i.e. a total of 174 bits. These are converted into Gray code and assembled in groups of three to obtain 58 symbols or tones. Adding the three Costas arrays of seven bits each (21 bits in total) for start and stop synchronization results in an FT8 message comprising 79 symbols/tones or the equivalent of 237 binary bits. The time difference between the transmitter and receiver stations must not exceed +/- 1 second, otherwise the receiver can no longer phase in correctly to the transmitter's signal.

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Why FT8 is the champion in noise radio

In SSB mode, the signal from the other station must be about +10dB louder than the noise on the band in order to be intelligible. In Morse telegraphy, the signal may even be quieter than the noise, as a good CW radio operator can still hear a change in the noise. At -15dB below noise, however, that's the end of it. Let's convert this into the required transmission power. We won't go into the math (the logarithm of a voltage or power ratio), but you should have a few values in mind: +3dB is the doubling of the transmitting power, +6dB = 3dB + 3dB is therefore the quadrupling of the power. +10dB is a tenfold increase. If there is a minus sign in front of the value, this number is in the denominator, i.e. -10dB corresponds to one tenth of the power. In the world of decibels, you only ever add or subtract.

If the SSB signal must be 10dB above noise and the CW signal may be -15dB below noise, that is a difference of 25dB. If we assume 26dB, then we can calculate: 10dB + 10dB + 6dB, or as the power of the remote station: 10 * 10 * 4.

This means: If the SSB station transmits with 400 watts to be just intelligible, a CW station with 1 watt generates a signal at the receiver that can also just be read. Now you understand why CW is still practiced. And FT8? There the signal may be -26 dB below noise in order to be just decodable. So instead of the 400W SSB signal, a transmission power reduced by 36dB in FT8, namely 100mW, is sufficient. This is why FT8 is also known as noise radio.

The sunspot maxima and their effects

As we are currently experiencing the sunspot maximum, which only occurs every 11 years, and therefore sometimes all continents can be heard at the same time during the day, a directional antenna is not so advantageous. Omnidirectional antennas such as a groundplane or the horizontally polarized Cobweb make it possible to hear stations from Africa, South America, Asia and Oceania in the same 15-second interval and then make QSOs directly one after the other. The whole world in 10 minutes! In such a situation, the beam would simply block out a lot.

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The art of the connection between radio and PC

In principle, any radio that is suitable for SSB can also be used for FT8. The microphone input and loudspeaker output of the radio must then be connected to the sound card of the PC. Ideally via AF transformers to prevent ground loops. A more elegant solution is to use modern shortwave transceivers that have a CAT interface and are remotely controlled by WSJT-X. The transceiver should also have a built-in sound card, then CAT commands and the data from the TRX sound card can be transmitted between the radio and the PC via a single USB cable. The PC program can then select the FT8 frequencies stored in it and start decoding immediately.

Radios with integrated sound card

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FT8 on the rise

The operators of the Clublog website keep daily updated statistics on how many QSOs were reported the previous day in the operating modes CW, SSB, RTTY, FT8, FT4 and others.

The proportion of FT8 QSOs was up to 80 % of all connections in the years of sunspot minimum from 2018 onwards, as FT8 can score points with its incredible sensitivity in poor conditions on the short wave. In the 2024 sunspot maximum, the share is still around 75 %, with CW and SSB each accounting for 10 % to 12 %.

FT8 performance under control

As with all other continuous wave modes, the rule here is that the maximum power of the transmitter or the power amplifier must not exceed one fifth of the PEP power for which the tuner, the coaxial cable or the antenna are designed in SSB. The transmitter should be driven at a level at which the ALC of the transmitter just starts to cut out. You will soon realize that a transmission power of 10 to 25 watts is completely sufficient for most connections.

Etiquette: Why less is sometimes more when sending

If you are used to being heard quite quickly in worldwide radio traffic due to a powerful antenna system and a suitable power amplifier, this high field strength at the receiving location can be a hindrance with FT8. The WSJT-X software can simply ignore stations that use significantly more transmitting power than necessary, i.e. that generate levels far above +5 or +10dB, using a selectable parameter. In this case, no amount of calling will help and you will only get your turn after the transmission power has been reduced.