The transmitter site

Connecting ODRmmbTools to the transmitter

Whereas with an FM and an AM transmitter,  one can transmit one radio station per frequency, a DAB+ transmitter will broadcast multiple radio stations simultaneously. When choosing the connection between the location where the ODR-DabMux will reside and the transmitter site(s), one should remember that the full MUX can send up to 1971 kbps of data. One should provide sufficient bandwidth to make it work.

An ETI stream transports ODR-mmbTools DabMux (software) to the transmitter. It is not necessary for the Linux system running ODR-DabMux and the DAB transmitter to be in the same place.  

However, when using only one DAB+ transmitter, it is perfectly possible to place the Linux PC with ODR-mmbtools on the transmitter site. Depending on the number of radio broadcasters that will be listened to via the MUX, one has to consider how much bandwidth the network or internet connection needs. After all, each radio broadcaster must send its programme to the ODR-DabMux.

When using more than one transmitter, whether in “Single Frequency Network (SFN)” or not, and one wants to place the Linux PC on 1 of the transmitter sites, one will have to provide a network connection with sufficient bandwidth for the link to the other transmitter sites.

Running the ODR-DabMux module on a Telecom provider’s server is another solution. Perhaps this will not be the cheapest solution, but all telecom providers will be able to provide the most stable connection with maximum uptime. In case of technical problems with the connection, the provider will also solve the problems.

Even if a particular manufacturer uses software different from ODR, sufficient bandwidth provision is essential.

Where can I find a DAB+ transmitter?

All well-known transmitter manufacturers have had DAB+ transmitters in their range for some time. With such transmitters, it is a matter of installing them, connecting the audio encoder with the transmitter and getting the settings right. Of course, such transmitters cost some money, from +/- €5000 for a 15-watt transmitter to +/- €11000 for a 600-watt transmitter.

Fortunately, cheaper solutions also exist for small-scale projects. Among others, Hfprints and pcs-electronics have affordable transmitters in their range.

The spectrum mask filter

Because the DAB(+) broadcast signal delivered by the OFDM modulator contains huge peaks at the transmitter’s input, these peaks can seriously overload the transmitter amplifier. A “spectrum mask filter” will always have to be placed between the transmitter and the antenna(s). One can find Info on the filter here.

Keep in mind that the filter will cause a slight attenuation of the transmitter power.

The coaxial cable

Because the “spectrum mask filter” will already attenuate part of the transmitter power, choosing coax with the lowest possible attenuation is very important. At 174 MHz, 30 metres of half-inch coax have an average of 0.8 dB attenuation (16.8% loss). With 7/8 coax, it is 0.4 dB (8.8 % loss) (Source: rfelektronik).

The antenna

Simulations and practical tests show that it is best to place the antenna(s) at least 50 to 60 metres above ground level (AGL). The lower the antennas are mounted, the smaller the coverage becomes. The higher the antennas, the greater the range will be.

It is very important to find the best location to obtain the best possible coverage. When choosing the location, not only is the height above sea level (ASL) important, a good geographical location of the transmitter site is all the more important, and the height relative to the surroundings is even more important. For example, the altitude of a particular area may be 60 metres above sea level. Still, if there is a 90-metre ASL hill a little further away, behind that hill, the transmitter will be barely or even impossible to receive.