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HWg-CS58: Monitoring FM VHF transmitters using Poseidon2

Czech version

English version

The case study demonstrates remote monitoring of several FM VHF transmitters used by several radio stations around Cheb, Czech Republic. A Poseidon2 4002 unit is used for long-term remote monitoring of broadcasting quality along all signal propagation paths.

The RTV Cheb, k. s. company operates several FM VHF transmitters at several frequencies and at various locations in and around the town of Cheb for various commercial and public radio broadcasting stations. Failures of a single transmitter were often detected by the technical support only after several hours.

A Poseidon2 4002 device is used for long-term monitoring of broadcasting quality along all signal propagation paths. VHF signal outages are detected with special FM measuring receivers whose outputs are connected to digital inputs (DI) of the Poseidon2. As a result, even short signal outages are detected and technical support can quickly react.

What is monitored
  • Broadcasting failure at any FM transmitter
  • Signal quality measured at several locations
  • More monitoring locations can be added in the future
  • For each transmitter, carrier presence and modulation presence (silence detection) are monitored
  • Alarm call generated over a VoIP interface
  • Central monitoring of broadcasting status in PRTG software
  • Data can be sent to an online portal (e.g.
    • Any third-party software can be connected
      SNMP, Modbus/TCP, XML (HTTP), E-mail reporting


Broadcasting outages were mainly caused by technical problems at individual transmitters. Most frequent causes were electricity outages, frost damage to antennae, or human errors (wrong connector disconnected). Biggest problems were, however, caused by short (10 to 30 s) connectivity failures.

Thanks to the antenna system in this particular case, all four frequencies can be monitored at a single location. There, the receivers are placed. Each of the four VHF FM measuring receivers monitors one transmitter. Each measuring receiver is equipped with two relay outputs.
One relay output is controlled by squelch and detects carrier presence. If there is no carrier in the input signal, the relay is activated.
The other relay output detects modulation presence. If there is no modulation present for about 5 seconds (i.e. silence is detected), the second relay output is activated.

Technical description

In total, 4 frequencies are monitored for carrier presence and for modulation (audio signal) presence. Thus, the detection system has a total of 8 relay outputs. The states of these 8 relay outputs are monitored by a Poseidon2 4002 device. The measuring receivers are located approximately 20 km from the transmitters, in a valley, so that a drop in FM transmit power can be also detected.

A dipole antenna, located outside of the rack in the photo, is used. Everything is set up so that an alarm is signaled with a 2-second delay.

Whenever there is a problem with any of the 4 transmitters, the Poseidon2 monitoring unit e-mails two support technicians. At the same time, a SNMP Trap is sent to the PRTG monitoring system.

A special feature is an alarm phone call. Poseidon2 can dial a configured phone number using SMS GW; however, this solution did not suit the client because it was necessary to call different numbers for different transmitters.  Poseidon2 currently doesn't support SIP.

Therefore, whenever a failure is detected, Poseidon2 4002 also activates a relay output, to which a Linksys PAP2T SIP telephone adapter is connected. The adapter then automatically dials a pre-configured number over VoIP. Linksys PAP2T is registered with a SIP telephone service provider.

On the other hand, HWg-SMS-GW would enable text message (SMS) alerts, and phone calls would not depend on Internet connectivity.

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Another advantage is that the Poseidon2 4002 unit is connected to the portal. Thanks to the online portal, transmitter states and alarms can be checked at any time using a mobile phone app or over the Internet. This was especially useful at the initial implementation phase before the extensive PRTG monitoring system was fully set up.

The entire system, including the measuring receivers, is connected to an uninterruptible power supply, and the Poseidon2 unit together with a 230V relay also monitors the power presence for the UPS. In addition, the Poseidon2 4002 monitors the temperature in the rack and in the room.

In the future, we expect to extend the system in order to monitor transmitters at other remote sites. Thanks to the described IP structure, it won't be a problem, we will simply add more receivers and Poseidon2 units at other locations.


Products used




FM transmitter, FM transmitter monitoring, carrier frequency monitoring, silence detection monitoring, VoIP signaling, Poseidon2 monitoring


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