In 2014, the Csepel spark telegraph station turned one hundred years old. The equipment transmitted messages from both Béla Kun and Lenin. I decided that for the 105th anniversary, I would build a spark transmitter myself.
Csepel spark telegraph radio station
The construction of the Csepel spark telegraph radio station, which marked the beginning of Hungarian radio, was commissioned by the Hungarian government due to the First World War. At that time, the Monarchy already operated high-power spark transmitters in its ports (Pula, Cattaro, and Trieste), as well as in Vienna. The network primarily served military purposes, as wired telegraphy handled civilian communications within the Monarchy. The experiences of the Balkan War, which ended in 1913, drew military leaders’ attention to the advantages of wireless communication, and so in the summer of 1914, construction began on spark telegraph stations in Lemberg, Kraków, Petrovaradin, and Budapest. The Budapest station’s task was to provide a southern connection toward Sofia and Constantinople for the general staff.
The Transmission Station
Amazingly by today’s standards, the then Royal Hungarian Post (under the supervision of Károly Follért, Director General of Post and Telegraph, with Endre Kolossváry as Technical Director General and József Hollós as Technical Councillor) built the transmission tower, its associated building, and equipment in just three months. The transmitter began operating on October 15, 1914. The first radio message was transmitted in the presence of the then Minister of Commerce: the post office leaders exchanged messages with the Minister of Commerce in Sofia.
This transmission tower should not be confused with the Lakihegy radio tower! The Lakihegy tower stands beyond the M0, on the border of Szigetszentmiklós, north of the Tököl airfield; it is 314 meters high and was built in 1933. The spark telegraph station was built at the northern tip of Csepel Island, on a plot registered as 4557 in Kikötő dűlő.
The antenna was a Telefunken-type so-called umbrella antenna, held by a total of 13 masts. The central, 120-meter-high tower was manufactured by the Rendal company of Berlin. Around the tower, at a distance of about 240 meters, 12 poles, each 45 meters high, supported the umbrella wires. From the top of the central tower, twelve 150-meter-long bronze wires, each 3 mm in diameter, were stretched downward at about a 45° angle. The whole thing looked like the frame of a giant umbrella. All the station’s buildings were placed beneath this antenna: the telegraph room, engine room, water tower, and caretaker’s house. The central tower itself participated in radiation: the entire structure stood on a glass-disc insulator, separated from the guy wires and the ground by insulators. Power was fed into the tower just above the glass-disc insulator at its base. The ground network consisted of 72 radial wires, each 300 meters long, buried about 70 cm deep. The soil quality favored grounding, which was the main reason for choosing the northern part of Csepel Island as the site.
The Csepel Spark Telegraph served Hungary’s wireless communication for 20 years. On November 30, 1917, under orders from the Vienna War Ministry, the Central Powers’ peace offer was transmitted to Russia through it. Messages were exchanged between the Hungarian government and delegates participating in the Treaty of Trianon negotiations, and through this telegraph, Hungarian prisoners of war could send messages with the help of the Maltese Charity Service and the International Red Cross. Legend has it that after the proclamation of the Hungarian Soviet Republic, Béla Kun sent his telegram to Lenin from here, who, in his reply, welcomed the Hungarian Soviet Republic.
On December 1, 1925, the 2 kW Telefunken transmitter of the Hungarian Telephone Herald and Radio Co. was also commissioned here, from which radio broadcasts were transmitted at a wavelength of 546 meters (initially 50 hours a week). The studio where the program was produced was at 22 Rákóczi Street; the signal reached the transmitter via cable. In memory of the start of radio broadcasting, December 1 is now Hungarian Radio Day. In fact, there were musical and program broadcasts even earlier. Experimental broadcasts had begun as early as March 15, 1924, from a studio set up in the famous furniture delivery wagon of Henrik Reiss, using an associated 250-watt Huth transmitter. After two decades, the more modern Lakihegy transmitter took over the role of the station. The tower and the building were demolished in 1934; today, the site houses the Budapest Central Wastewater Treatment Plant of Fővárosi Vízművek Zrt.
Henrik Reiss’s famous furniture delivery wagon, from which Hungary’s first experimental radio broadcasts were transmitted, was still in its original place in 2021, in the courtyard of the Puskás Tivadar Telecommunications and IT Technical School (1097 Budapest, Gyáli út 22). I took this photo in 2016. I remember the wagon was still there in 2021. Recently, I noticed that everywhere it now says “it stood there”—in the past tense. And indeed! According to Google satellite imagery, by 2023 it was gone.
This famous furniture wagon was even immortalized in the song The Radio by the band Locomotiv GT (LGT).
Note: The Puskás Technical School was a prestigious institution with a long history, founded in 1912 next to the Royal Hungarian Post and Telegraph facility on Gyáli Road. Among its students was János Bródy, a Kossuth Prize and Ferenc Liszt Prize-winning singer-songwriter. Until 2022, the school also hosted the renowned “Puskás Fair,” a regular amateur radio meetup and market. In 2010, one of the first actions of the Fidesz government was to merge the school into the Budapest Technical Vocational Training Center. In 2021, a new director took over the institution; from 2022 onward, the leadership no longer supported the monthly weekend fairs, and eventually, the furniture wagon disappeared as well.
Today, the antenna visible at the entrance to Csepel Island, at 1 Kossuth Lajos Street in the 11th district (corner of Kossuth Lajos St. and Corvin St.), is neither the original transmission tower nor even similar to it. It is merely a monument, built by Csepel Works employees as a community effort based on the design of Munkácsy Prize-winning sculptor Tibor Vilt. It was erected in 1980 to commemorate the 61st anniversary of the Hungarian Soviet Republic and in honor of the 12th Congress of the Hungarian Socialist Workers’ Party. The 52-meter-high monument now serves not only as a landmark but also as a mobile phone relay station, and is surrounded by a dilapidated, abandoned car showroom. On its pedestal, the coat of arms of Csepel faces those entering the island. Originally, the following inscription was in place instead of the coat of arms:
The Hungarian
Soviet Republic
calls Lenin
to the Hungarian
proletariat…
It conquered all
state power.
Introduced
the proletarian
dictatorship.
On the other side:
Here Lenin
My sincere greetings
to the Hungarian
Soviet Republic’s
proletarian
government…
I have conveyed
their message
to the Congress
of the RCP(b).
After the regime change, this was, of course, quickly removed…
We older folks still remember that communists had an odd passion for spark telegraphs. There are dozens of stories where Lenin sends messages back and forth via telegraph. They always emphasize it was by spark telegraph—no other device could be considered. Perhaps the revolutionary ring of the word “spark” caused this spark-telegraph fetishism. And this telegraph really did spark!
Of course, the exchange of messages did not go as smoothly as it was later portrayed. Comrade Kun was not standing by the radio chatting with Comrade Lenin in a conversational style, as Lenin later described. In reality, the party leadership drafted a telegram, which was sent to the Csepel station at five o’clock in the afternoon, where it was transmitted. At eight in the evening, Moscow replied that due to atmospheric disturbances they had only been able to receive the message in fragments. Therefore, Csepel retransmitted the missing parts. At nine o’clock, Moscow responded: Lenin sent his greetings. That was all. But let us be proud—the rest of the story, the “conversation” with Comrade Kun, was Lenin’s own invention.
Operation of the Spark Telegraph
The station was equipped with a 7.5 kW Telefunken spark transmitter, an auxiliary Poulsen-type arc transmitter, and a detector receiver. The 30 kW Poulsen-type arc lamp transmitter was manufactured by the Vienna-based Telefon Fabrik AG. In the arc lamp, the arc burned in a space filled with noble gas, under a strong magnetic field, between a carbon and a copper electrode. The carbon rod burned away during operation and therefore had to be replaced frequently. Due to its delicate operation, this device was considered only an auxiliary unit and was rarely used. Since the Telefunken spark transmitter fully met expectations, the arc lamp transmitter was transferred to Timișoara in 1915. Little is known today about this equipment and its later fate.
The Telefunken transmitter was of the so-called “tropical” design. The spark gaps and the high-frequency oscillatory circuits were mounted on a marble panel. The 20 kV transformer supplying the equipment was located next to the marble panel, while the power generator and its switchboard were placed further away from the transmitter. The transmitting equipment operated in the 50 kHz–150 kHz frequency range, while the receiver was capable of operating across the 35 kHz to 300 kHz band without coil changes.
The spark telegraph was unsuitable for broadcasting or telephony; it was essentially the wireless equivalent of Samuel Morse’s wired telegraph. The world honors the Italian Guglielmo Marconi as its inventor, and he even received a Nobel Prize for it. However, Marconi’s primacy was much debated, as the Russian Popov and the Croatian-born American Tesla demonstrated their own similar devices before him (Popov in 1895, Tesla independently in 1896, while Marconi only in 1899). Marconi worked for a time as Tesla’s assistant, and Tesla sued him for stealing his invention. The lawsuit dragged on, but after Tesla’s death, in 1943, the Supreme Court of the United States officially credited Tesla with the invention of radio and annulled Marconi’s patent. The twist is that in 1909, when Marconi accepted the Nobel Prize in Physics, he publicly admitted in his Nobel lecture that he had no real idea how his invention actually worked. So, let’s see how it works!
The schematic of the transmitter is shown in the diagram. The high-voltage transformer was powered by an alternating current generator delivering 220 V at 500 cycles, so the alternating current reached its maximum and minimum a thousand times per second. Accordingly, the sparks were extinguished and reignited a thousand times per second. After pressing the Morse key, the thousand interruptions per second produced a 1 kHz tone at the spark gap and in the headphones. The spark gaps were cooled by a fan that started automatically when the transmitter was switched on and stopped when switched to reception mode, as the telegrapher needed complete silence to receive messages.
The heart of the spark transmitter is the spark gap and the oscillatory circuit. The oscillatory circuit consists of a coil and one (or more) capacitors. The spark gap and the oscillatory circuit together work like a spring toy mouse. If you hang such a toy mouse and let it settle, it dangles quietly. If you then pull its leg and release it, it begins to bounce up and down. The rhythm of the bouncing is well-defined. No matter how you tug its leg, once released, the number of oscillations per minute will always be the same. This is the resonant frequency, which in the case of the toy mouse depends on the stiffness of the spring and the square root of the mouse’s mass, and practically not on how hard you tugged. Of course, the mouse doesn’t bounce forever—it performs damped oscillations. Air resistance and other losses gradually consume its energy, the amplitude decreases, and eventually, it stops.
The oscillatory circuit of the spark transmitter does the same thing as the toy mouse, except instead of mechanical motion, it performs electrical oscillation. The “leg tugging” in electrical terms is done by the sparks. When the high-voltage transformer output reaches sufficient voltage, it begins to charge the capacitor. After some time, the voltage becomes high enough to ionize the air in the spark gap. At this point, an arc discharge occurs, and the capacitor discharges. The spark itself does not generate radio waves; it merely excites the oscillatory circuit. The electrical oscillation is very fast—not one or two swings per minute as with the mouse, but 3.5 million per second in our device, and 50,000 to 150,000 per second in the Csepel transmitter.
From today’s perspective, it may seem surprising that the transmitter’s power supply was far more complex than the transmitter itself. As shown in the diagram, the AC generator was driven by a DC motor (on the far right of the diagram). The motor operated on 220 V DC. Due to its military application, enhanced safety was a priority, and the station had an uninterruptible power supply provided by two 220 V battery banks. Each battery, rated at 413 Ah, was manufactured by the Hungarian Tudor Battery Company and could supply the spark transmitter for 24 hours of operation. The batteries could be charged from the Municipal Electric Works network via a motor-generator. On the Siemens-Schuckert control panel of the station, it was possible to set the system so that while one battery powered the transmitter, the other could be charged by the generator.
Later, when the nearby HÉV (Suburban Railway) line was electrified, a branch connection was made from the railway’s power line and brought into the high-voltage panel. In an emergency, the batteries could be charged from the railway’s network as well. Interestingly, the charging current was regulated manually using a sliding resistor. It required considerable expertise and attention from the technician to avoid applying more voltage to the battery bank than necessary for charging!
The Receiver
Along with the transmitting equipment, the Telefunken company supplied a small E-5 type crystal detector receiver. This was soon replaced with what Tolnai Henrik, in his work The 10-Year History of the Csepel Radio Station, referred to as a “large station receiver.” This was a Telefunken E-90 crystal detector receiver, which can still be seen today at the Radio and Television Museum in Diósd. The detector used a carborundum crystal. To improve reception, the detector receiver was upgraded in 1915 with an audio-frequency amplifier. This included the first vacuum tube ever used at the station. The single-tube audio-frequency amplifier had been originally developed by the Post for use on international telephone lines and was later repurposed for the radio station.
After the First World War and the subsequent revolutions, the station ceased to serve a military role and was used exclusively for postal telecommunications and later for the emerging field of radio broadcasting. From 1921 onward, modern vacuum tube equipment was gradually installed at the Csepel station, and the antenna was modified: the number of wires was doubled, and the edge of the umbrella antenna was lowered. The next era in the station’s history was linked to the beginning of broadcasting. From this period, more photographs have survived. One of them features János Marczal—immortalized in a Locomotiv GT song—working as a transmitter operator. However, the 500 Hz power supply system built for the spark transmitter continued to cause problems. Despite the built-in filter capacitors, it constantly introduced a characteristic “machine noise” into the voice transmissions.
The Last Days of the Csepel Station
The emerging era of Hungarian radio broadcasting excited the public and prompted the Post to commission a dedicated transmitter for this purpose. This new equipment was installed in a separate building in the southern part of the station’s grounds and had its own antenna. At this time (in 1926), the spark transmitter—which had been out of service for years—was dismantled, and its marble-mounted frame was repurposed for the new transmitter.
However, demand continued to grow, and the old station could no longer keep pace with technological progress. Moreover, the Free Port expansion required the land occupied by the station, so it was gradually dismantled and its equipment relocated. On December 20, 1934, the Rendal antenna tower was finally brought down, marking the station’s permanent closure.
The spectacular event was attended by representatives of the Hungarian Film Office, journalists, the chief of police, and technical advisors from the Post. The tower was felled toward the south by cutting its northern guy wires. The fall was recorded by the Filmhíradó newsreel, and the footage still exists today in the MTVA archives.
Our Spark Transmitter
For simplicity, our device will not be tunable; it will operate at a fixed frequency at the lower end of the 3.5 MHz amateur radio band. The frequency can be calculated using the Thomson formula:
f = 1/[2π √(LC)
where LLL is the inductance of the coil, and CCC is the capacitance. An online calculator for this can be found, for example, here.
I built the coil by winding 30 turns of 0.6 mm diameter enamel-coated copper wire loosely over a 40 mm diameter tube, covering a length of 40 mm. A 100 pF / 30 kV capacitor is needed, which can be difficult to obtain (it can be ordered on eBay for about 5–6,000 HUF). As an alternative, a properly sized Leyden jar can also be used.
As a cheaper solution, I connected ten 1,000 pF / 3 kV capacitors in series and installed the entire assembly into the brown plastic housing next to the spark gap, which I then filled with epoxy resin.
At Csepel, they used 40 spark gaps of 0.2 mm each in series, but for our purposes, we will only use a single spark gap. The advantage of using multiple gaps is that the discharge lasts longer and the transmitter operates more efficiently—but for us, that’s not important right now.
The signal from the oscillatory circuit coil can be coupled to the antenna either directly (as was done at the Csepel transmitter) or via a coupling coil. In our small device, we’ll omit this, as well as the antenna-tuning variometer—because we’re not trying to send messages to Lenin.
The stray field of the coil is sufficient for the signals to be picked up by a receiver in the next room, even without an antenna on the transmitter. This way, at least we won’t interfere with anyone else’s radio reception.
The operation of the antenna and crystal detector receiver is discussed in the article The Radio Speaks. Those interested in the antenna can read about it there.
The spark gap must be powered by high voltage. At the Csepel station, this was supplied by a 20 kV transformer, which was driven by a 220 V, 500 Hz alternating-current generator. This generator was turned by a 220 V direct-current motor, powered from the station’s batteries. This part of the equipment, by today’s standards, looked more like an electric locomotive than a radio transmitter. The Morse key operated by the telegraph operator was connected between the generator and the transformer. When the operator pressed the key, the transformer was energized, and sparks began to crackle at the high-voltage side. Since the 500 Hz current reached its peak and zero point a thousand times per second, the sparks ignited and extinguished a thousand times each second. From the spark gap, a clear 1000 Hz tone could be heard. This tone also appeared in the transmitted radio waves, so the operator at the receiving station heard the same tone in his headphones. In essence, this was a very simple form of amplitude modulation. It worked perfectly for the spark transmitter, but later caused major problems when the station was upgraded with vacuum-tube transmitters for broadcasting. Despite all filtering attempts, the hum from the high-voltage transformer leaked into the transmitted signal, creating an unpleasant “machine noise” in the background of broadcasts.
Our spark transmitter also uses a high-voltage transformer: a car ignition coil. Essentially, any older car type (such as Skoda or Wartburg) will do, even a used one. These coils produce 10–20 kV sparks with 12 V supply voltage, which is more than enough for our purposes. The coil is not powered directly through the Morse key but through a relay. For authenticity, I used an old telephone relay from a “boss-secretary” intercom system, but any relay will work. The key is to wire the relay so that when you press the Morse key, it energizes and immediately breaks its own circuit. Thus, with the key pressed, the relay continuously makes and breaks the current to the ignition coil at about 100–150 Hz. This makes our transmitter produce an amplitude-modulated signal. It won’t sound as elegant as the old Csepel transmitter, but if you tune in with a receiver, you can hear its sound. A suitable receiver could be the crystal set mentioned earlier (with some retuning) or even a software-defined radio.
And of course, how else could I end this? Naturally, by showing you what the radio sounds like. Let’s turn it on! And also fire up the Yaesu rig. I set it to AM mode and tuned it to 3.5 MHz. And here’s where I ran into a problem. I haven’t listened to this band for a while, and to my surprise, there’s some persistent interference here where I live. Specifically, on 3.495 MHz, in a fairly narrow band, there’s a buzzing machine-like tone, as if some electric motor were running. I tried at different times of day, but it’s always there. Curse that infernal Bubo cuprumpenis that causes such interference! I built my spark transmitter for 3.5 MHz, and it can’t (or at least not easily) be retuned. The interference completely masks its signal. Fortunately, the spark transmitter radiates across a fairly wide band, so by tuning the receiver a little higher and adjusting the noise blanker, I could still pick up the signal where the interference wasn’t so bad. Reception was choppy and noisier, but still, it was clear enough to declare: the radio speaks! Here it is:
References
S.n. (1919): Lenin elvtárs szikratávírón üdvözli a magyar proletariátust. Jászberényi Újság (Official Bulletin of the Jászberény Socialist Party), March 28, 1919.
Hungaroton Records Ltd. (1939): Lenin Kun Béláról. Report on the Conversation with Béla Kun via Spark Telegraph. Audio record supplement to the book Kun Béla és az ifjúság [Béla Kun and the Youth].
Dr. Szabolcs Ottó et al. (1977): Szemelvények Marx, Engels, Lenin műveiből. Tankönyvkiadó Vállalat, Budapest.
M. KIR. Posta (1925): A csepeli rádióállomás 10 éves története 1914-1924 között. Publication by the station’s staff, Budapest.
Ormos Vilmos (1957): A csepeli szikraadótól a Televízióig. Article series in Rádiótechnika magazine, issues 4–12, 1957.
Balás Dénes (2006): Az 1906-os adriai szikratávíró-kísérlet eszközei. Híradástechnika, Vol. LXI, Special Issue, pp. 22–26.
Dósa György (2004): Száz éve kezdődtek meg Magyarországon az első rádiótávíró kísérletek. Híradástechnika, Vol. LIX, No. 7, pp. 58–59.
Codella, Christopher F. (2016): Spark Radio. Ham Radio History. Available at: http://w2pa.net/HRH/spark-radio/
Kennedy, Hal (1990): How Spark Transmitters Work. The History of QST Vol. 1 – Technology. American Radio Relay League (ARRL). Available at: http://www.arrl.org/files/file/History/History%20of%20QST%20Volume%201%20-%20Technology/Kennedy%20N4GG.pdf