Hello Habr.
In the first part of the article about it was told about service stations on long and short waves. Separately, it is worth talking about amateur radio stations. Firstly, this is also interesting, and secondly, anyone can join this process, both for reception and transmission.
As in the first parts, the emphasis will be on the “digit” and how signal processing works. We will also use a Dutch online receiver to receive and decode signals. and the MultiPSK program.
For those who are interested in how it works, the continuation is under the cut.
After it became known more than 100 years ago that it was possible to communicate with the whole world on short waves using a transmitter of literally two lamps, not only corporations, but also enthusiasts became interested in the process. At the time it looked like , well, ham radio is still quite an interesting technical hobby. Let's try to figure out what types of communications are available to modern radio amateurs.
Frequency ranges
The radio air is very actively used by service and broadcasting stations, therefore certain frequency ranges are allocated to radio amateurs so that they do not interfere with others. There are quite a lot of these ranges, from ultra-long waves at 137 kHz to microwave at 1.3, 2.4, 5.6 or 10 GHz (for more details, see ). In general, everyone can choose, depending on their interests and technical equipment.
From the point of view of ease of reception, the most accessible are frequencies with wavelengths of 80-20m:
- 3,5 MHz band (80 m): 3500-3800 kHz.
- 7 MHz band (40 m): 7000-7200 kHz.
- 10 MHz band (30 m): 10100-10140 kHz.
- 14 MHz band (20 m): 14000-14350 kHz.
They can be configured using the above , and from his personal, if he can receive in sideband mode (LSB, USB, SSB).
Now that everything is ready, let's see what can be accepted there.
Voice communication and Morse code
If you look at the entire amateur radio band through websdr, it is easy to see Morse code signals. It practically did not remain in service radio communications, but some enthusiastic radio amateurs use it quite actively.
Previously, to get a call sign, you even had to pass an exam in receiving Morse signals, now it seems to be only for the first, highest, category (they differ mainly only in the maximum allowable power). We will decode CW signals using CW Skimmer and Virtual Audio Card.
Radio amateurs use a shortened code to shorten the message length (), in particular, the string CQ DE DF7FF means a general call to all stations from a radio amateur DF7FF. Each radio amateur has his own call sign, the prefix of which is formed from , this is quite convenient. it is immediately clear where the station is broadcasting from. In our case, the call sign DF7FF belongs to a radio amateur from Germany.
As for voice communication, there are no difficulties with it, those who wish can listen on their own on websdr. Once upon a time in the days of the USSR, not all radio amateurs had the right to conduct radio communications with foreigners, now there are no such restrictions, and the range and quality of communication depends only on the quality of the antennas, equipment and the patience of the operator. For those who are interested, you can read more on amateur radio sites and forums (cqham, qrz), but we will move on to digital signals.
Unfortunately, for many radio amateurs, working digitally is just connecting a computer sound card to a decoder program, few people delve into the intricacies of how it works. Even fewer are conducting their own experiments with digital signal processing and various types of communication. Despite this, over the past 10-15 years, a lot of digital protocols have appeared, some of which are interesting to consider.
RTTY
A fairly old type of communication that uses frequency modulation. The method itself is called FSK (Frequency Shift Keying) and consists in the formation of a bit sequence by changing the transmission frequency.
Data is encoded by fast switching between two frequencies F0 and F1. The difference dF = F1 - F0 is called the frequency spacing, and can be equal to, for example, 85, 170, or 452 Hz. The second parameter is the transmission speed, which can also be different, and be, for example, 45, 50 or 75 bits per second. Because we have two frequencies, then we need to decide which will be the “upper” and which will be the “lower”, this parameter is usually called “inversion”. These three values (speed, spacing and inversion) completely determine the parameters of the RTTY transmission. In almost any decoding program, you can find these settings, and even choosing these parameters “by eye”, you can decode most of these signals.
Once upon a time, RTTY communications were more popular, but now, when I went to websdr, I did not hear a single signal, so it is difficult to give an example of decoding. Those who wish can listen on their own at 7.045 or 14.080 MHz, more details about the teletype were written in Article.
PSK31/63
Another type of communication is phase modulation, . Here it is not the frequency that changes, but the phase, on the graph it looks something like this:
The bit encoding of the signal consists in changing the phase by 180 degrees, and the signal itself is actually a pure sine - this provides a good transmission range with a minimum transmitted power. The phase shift is hard to see in the screenshot, you can see it if you zoom in and overlay one fragment on another.
The encoding itself is relatively simple - in BPSK31, signals are transmitted at a rate of 31.25 baud, a phase change encodes "0", no phase change "1". Character encoding can be found on Wikipedia.
Visually on the spectrum, the BPSK signal is seen as a narrow line, and audibly heard as a fairly pure tone (which it basically is). You can hear BPSK signals, for example, at 7080 or 14070 MHz, you can decode them in MultiPSK.
It is interesting to note that in both BPSK and RTTY, the “brightness” of the line can be used to judge the strength of the signal and the quality of reception - if some part of the message disappears, then there will be “garbage” in this place of the message, but the general meaning of the message often remains the same understandable. The operator himself can choose which signal to aim at in order to decode it. The search for new and weak signals from distant correspondents is quite interesting in itself, and when communicating (as you can see in the picture above), you can use arbitrary text, conduct a “live” dialogue. In contrast, the following protocols are much more automated and require little or no human intervention. Whether it's good or bad is a philosophical question, but one can definitely say that some part of the ham radio spirit in such modes is definitely lost.
FT8/FT4
To decode the following type of signals, you need to install the program . Signals are transmitted using frequency modulation of 8 frequencies with a shift of only 6.25Hz, so that the signal occupies a bandwidth of only 50Hz. Data in the FT8 is transmitted in "bursts" of about 14 seconds, so accurate computer time synchronization is quite important. Reception is almost completely automated - the program decodes the callsign, signal strength.
In the new version of the protocol , which appeared recently the other day, the packet duration is reduced to 5s, 4-tone modulation is used at a transmission rate of 23 baud. The occupied signal bandwidth is approximately 90 Hz.
WSPR
WSPR is a protocol specifically focused on receiving and transmitting weak signals. This is a signal that travels at only 1.4648 baud (yes, just over 1 bit per second). The transmission uses frequency modulation (4-FSK) with a frequency spacing of 1.4648Hz, so the signal bandwidth is only 6Hz. The transmitted data packet has a size of 50 bits, error correction bits are also added to it (non-recursive convolutional code, constraint length K=32, rate=1/2), as a result, the total packet size is 162 bits. These 162bits are transferred in about 2 minutes (anyone else will complain about slow internet? :).
All this allows you to transmit data actually below the noise level, with almost fantastic results - for example, a 100mW signal from a microprocessor leg, using a room loop antenna, it was possible to transmit a signal for 1000km.
WSPR works fully automatically and does not require any operator intervention. It is enough to leave the program running, and after a while you can see the work log. Data can also be sent to the site , which is convenient for assessing the transmission or quality of the antenna - you can transmit a signal and immediately see online where it was received.
By the way, anyone can join WSPR reception, even without an amateur radio call sign (it is not required for reception) - a receiver and a WSPR program are enough, and all this can even work autonomously on a Raspberry Pi (of course, you need a real receiver, send data from strangers online - receivers make no sense). The system is interesting both from a scientific point of view and for experiments with equipment and antennas. Unfortunately, as you can see from the picture below, in terms of the density of receiving stations, Russia has left not far from Sudan, Egypt or Nigeria, so new participants are always useful - there is an opportunity to be the first, and with one receiver you can “cover” a territory of a thousand km.
Very interesting and rather complicated is the transmission of WSPR at frequencies above 1 GHz - the frequency stability of the receiver and transmitter is critical here.
On this I will end the review, although, of course, far from everything is listed, only the most popular.
Conclusion
If someone also wanted to try their hand, then it is not so difficult. To receive signals, you can use either a classic (Tecsun PL-880, Sangean ATS909X, etc.) or an SDR receiver (SDRPlay RSP2, SDR Elad). Further, it is enough to install the programs, as shown above, and you can study the radio broadcast on your own. The issue price is $100-200 depending on the receiver model. You can also use online receivers and not buy anything at all, although this is still not so interesting.
For those who want to work also on transmission, you will have to purchase a transceiver with an antenna and issue an amateur radio license. The price of the transceiver is approximately equal to the price of an iPhone, so if you want it is quite affordable. You will also need to pass a simple exam, and in about a month you can fully work on the air. Of course, all this is not easy - you will have to study the types of antennas, come up with an installation method, deal with frequencies and types of radiation. Although the word “have to” is probably inappropriate here, because that’s what it is and a hobby, which is done for interest and not under duress.
By the way, anyone can try digital communications right now. To do this, it is enough to install the MultiPSK program, and you can connect directly “over the air” from one computer to another using any type of connection of interest through a sound card and microphone.
All successful experiments. Maybe one of the readers will create a new digital form of communication, and I will be happy to include his review in this text 😉
Source: habr.com