What is heard on the radio? Part 3, ham radio

What is heard on the radio? Part 3, ham radio


Hello, Habr.

The first part of the article about the what is heard on the air was told about service stations on long and short waves. We should also talk about radio amateur stations. Firstly, it is also interesting, and secondly, anyone can join this process, both at the reception and at the transfer.



As in the first parts, emphasis will be placed on the “figure” and on how the signal processing is arranged. We will also use the Dutch online receiver websdr and the MultiPSK program to receive and decode signals.

For those who are interested in how it works, continued under the cut.

After more than 100 years ago it became known that on short waves one could contact the whole world using a transmitter from literally two lamps, not only corporations but also enthusiasts became interested in the process. In those years, it looked like like this , and so far ham radio remains quite an interesting technical hobby. What types of connections are available to modern radio amateurs, try to figure it out.

Frequency Ranges


Radio broadcasts are very widely used by service and broadcasting stations, therefore, radio amateurs are allocated certain frequency ranges so that they do not interfere with others. There are quite a few of these ranges, from superlong waves at 137 KHz to microwave at 1.3, 2.4, 5.6 or 10 GHz (for more details, here ). In general, everyone can choose, depending on the 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 (80 m): 3500-3800 kHz.
  • 7 MHz (40 m): 7000-7200 kHz.
  • 10 MHz (30 m) range: 10100-10140 kHz.
  • 14 MHz (20 m): 14000-14350 kHz.

You can tune in to them using the above online receiver , or from your personal receiver if it can be received in sidebar mode. lanes (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. She practically did not remain in office radio communications, but some enthusiastic amateur radio operators are quite actively using it.



Previously, in order to receive a call sign, it was even necessary to pass an exam on receiving Morse signals, now it seems to be left only for the first, highest, category (they differ mainly in the maximum allowable power). We will decode CW signals with the CW Skimmer and Virtual Audio Card.



Hams, to reduce the length of the message, use a short code ( Q-code ), in particular, the string CQ DE DF7FF means a common call to all stations from the DF7FF radio amateur. Each radio amateur has its own call sign, the prefix of which is formed from the country code , which is quite convenient because Immediately clear where the station is broadcasting. In our case, the call sign DF7FF belongs to a radio amateur from Germany.

As for voice communication, there are no difficulties with it, anyone can listen on their own to websdr.Sometime in Soviet times, not all radio amateurs had the right to make radio communications with foreigners, now there are no such restrictions, and the range and quality of communication depends only on the quality of 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), we will move on to digital signals.

Unfortunately, for many radio amateurs the work “digitally” is simply the connection of a computer sound card with a decoder program, few people get into the subtleties of how it works. Even fewer conduct their own experiments with digital signal processing and various types of communication. Despite this, over the past 10-15 years, quite a lot of digital protocols have appeared, some of which are interesting to consider.

RTTY


A rather old type of communication using frequency modulation. The method itself is called FSK (Frequency Shift Keying) and consists in shaping the bit sequence by changing the transmission frequency.

image

The data is encoded by fast switching between the two frequencies F0 and F1. The difference dF = F1 - F0 is called the frequency spacing, and can be, for example, 85, 170, or 452 Hz. The second parameter is the transmission rate, which can also be different, and can be, for example, 45, 50 or 75 bits per second. Because we have two frequencies, then we need to decide which one will be “upper”, which will be “lower”, this parameter is usually called “inversion”. These three values ​​(speed, separation and inversion) fully determine the parameters of the RTTY transmission. In almost any decoding program, you can find these settings, and having even selected these parameters “by eye”, you can decode most of these signals.



More information about teletype and signals of this type was written in the part one of the article.

PSK31/63


Another type of communication is phase modulation, Phase Shift Keying . Here it is not the frequency that changes, but the phase, on the graph it looks like this:


Bit-coding 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 distance with a minimum transmitted power. The phase shift is difficult to see in the screenshot, it can be seen if you enlarge and apply one fragment to another.



The coding itself is relatively simple - in BPSK31, signals are transmitted at a speed of 31.25 baud, the phase change encodes "0", the absence of a phase change "1". Character encoding can be viewed on Wikipedia.



Visually, on the spectrum, the BPSK signal is seen as a narrow line, and by ear it is heard as a rather pure tone (which it is in principle). BPSK signals can be heard, for example, at 7080 or 14070 MHz, they can be decoded in MultiPSK.



It is interesting to note that both in BPSK and RTTY, by the "brightness" of the line, one can judge the strength of the signal and the reception quality - if some part of the message disappears, there will be "garbage" in this place of the message, but the general meaning of the message often remains understandable. The operator himself can choose which signal to aim at to decode it. The search for new and weak signals from distant correspondents is in itself quite interesting, also when communicating (as can be seen in the picture above), you can use arbitrary text, conduct a “live” dialogue.In contrast, the following protocols are much more automated, and practically do not require human participation. Good or bad, this is a philosophical question, but one can definitely say that some part of the ham radio spirit has definitely been lost in such regimes.

FT8/FT4


To decode the following type of signals, you need to install the WSJT program. Signals FT8 are transmitted using frequency modulation from 8 frequencies with a shift of only 6.25Hz, so that the signal occupies a band of only 50Hz . The data in FT8 is transmitted in “packets” with a duration of about 14 seconds, so that the exact time synchronization of the computer is quite relevant. Reception is almost completely automated - the program decodes the call sign, signal strength.



In the new version of the protocol FT4 , which appeared recently, the packet duration is reduced to 5c, 4-tone is used modulation at a transmission rate of 23 baud. The width of the occupied signal band is approximately 90 Hz.

WSPR


WSPR is a protocol specifically targeted to the reception and transmission of weak signals. This is a signal transmitted at a rate of only 1.4648 baud (yes, only slightly more than 1 bit per second). Frequency modulation (4-FSK) with a frequency separation of 1.4648 Hz is used for transmission, so the signal bandwidth is only 6 Hz. 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 transmitted in about 2 minutes (someone else will complain about slow internet? :).



All this allows you to transfer 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 at 1000km.

WSPR works fully automatically and does not require operator participation. Enough to leave the program to work, and after some time you can see the log of work. The data can also be sent to the site wsprnet.org , which is convenient for assessing the passage or quality of the antenna - you can send a signal, and immediately watch online, where he was taken. It is convenient to use the program for testing receiving antennas - you can leave the receiver, for example, for a day, and see how many signals were received, their power and direction.

image

By the way, anyone can join the WSPR reception, even without an amateur call sign (it is not required for reception) - the receiver and the WSPR program are enough, and all this can even work autonomously on the Raspberry Pi (of course, you need a real receiver, send data from someone else’s 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 can be seen from the picture below, according to the density of receiving stations, Russia left not far from Sudan, Egypt or Nigeria, so the new participants are always useful - there is an opportunity to be the first, and you can cover one thousand kilometers with one receiver.



Very interesting and rather complicated, is the transmission of WSPR at frequencies above 1 GHz - the frequency stability of the receiver and transmitter here is critical.

At this point I’ll finish the review, although not all of them are listed, of course, only the most popular.There are many other standards, both digital and analog - Olivia , Hellschriber , SSTV , JT65 , etc.

Conclusion


If someone wanted to try their hand too, 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 yourself. Issue price is $ 100-200, depending on the model of the receiver. You can also use online receivers and not buy anything at all, although it is still not so interesting.

For those who want to work also on the transmission, you will have to purchase a transceiver with an antenna, and issue a radio amateur license. The price of the transceiver is approximately equal to the price of the iPhone, so if you wish, it is quite affordable. It will also be necessary to pass a simple exam, and already somewhere in a month you can fully work on the air. Of course, this is not easy - you will have to study the types of antennas, think up a way to install, deal with frequencies and types of radiation. Although the word "will" is probably inappropriate here, because it is a hobby, what is done for interest and not by coercion.

By the way, anyone can try digital communications right now. To do this, it is enough to install the MultiPSK program, and it is possible through a sound card and microphone directly “through the air” to communicate from one computer to another with any type of communication of interest.

All successful experiments. Maybe one of the readers will create a new digital form of communication, and I will gladly include his review in this text;)

Source text: What is heard on the radio? Part 3, ham radio