Vakarel radio transmitter

The Vakarel Transmitter is a large broadcasting facility for long- and medium wave near Vakarel, Bulgaria. The Vakarel Transmitter was inaugurated in 1937. It has one directional antenna consisting of three guyed masts and another consisting of two masts.

The Vakarel Blaw-Knox tower and two of the other masts

The most remarkable mast of the Vakarel Transmitter is the 215 metre tall [1][2] Blaw-Knox Tower, built in 1937 by the company Telefunken. Nowadays, along with Lakihegy Tower, Hungary, Riga LVRTC Transmitter, Latvia and Lisnagarvey Radio Mast, Northern Ireland is one of the few Blaw-Knox Towers in Europe.

The transmitter was shut down at 22:00 UTC on December 31, 2014.

Transmitter internal structure

The modulation metod used by the transmitter in Vakarel is called a tube voltage modulation and was successfully used in all powerful AM transmitters at that time. The Vakarel transmitter is supplied with electricity from a substation in Samokov via a medium voltage transmission line. The transmitter itself uses 6 stages of amplification. The first stage contains a single radio tube, which generates alternating current at frequency of 850 Khz (carrier frequency). The electrical oscillations of the anode circuit in the tube are coupled in series to the second and third stage. The signals in these three stages are only amplified, without any other changes.

in the special fourth modulation stage, the form of signals is modulated with speech or music. The audio recordings are send to the transmitter with an underground communication cable from the main radio studio in Sofia. Due to the large distance of almost 36 km, the audio signal is amplified at both ends by separate block of amplifiers.

The fifth stage consists of six transmitting tubes, two of which are in reserve, and four other can be switched on if necessary. All of them are water cooled.

The last 6th stage consists of 4 high power transmitting tubes amplifying the final otput up to 100Kw. The energy is filtered by a high power tuned circuit and send to the feeder of the antenna, which is mounted on big insulator on the ground and suspended with 4 tension ropes with strain insulators.[3]

See also

References
  1. Pictures and description in Bulgarian (Predavatel.com)
  2. 70 years of Bulgarian national radio
  3. https://www.sandacite.bg (2019-02-07). "1937 – вижте тайните на радиопредавателя във Вакарел!". ==> SANDACITE BG (in Bulgarian). Retrieved 2020-06-30.


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