Your studio is already set up and it’s now time to go LIVE! Right? Well, there is the issue of how to get that audio to your audience. In Kenya, FM radio is the most common, and profitable, form of broadcast. This is why most of the infrastructure to support it has been set up across the country. Installation might be a grueling task for a small team (such as ours about a month ago), but the outcome becomes far more rewarding than I expected. In this post I go over the technologies that make radio broadcast transmission possible.
With a new studio seeking to have their first broadcast transmission or an existing one seeking to expand to other regions, the requirements to be installed remain largely the same as they have been for decades, only this time, we have IP on our side, making things much easier.
Audio Link
Now starting from your studio, that fresh audio has been captured by your state of the art audio console and you need the people to hear. Transmitting audio over IP is as simple as installing a PtP (point-to-point) microwave link to your transmitting site. Usually the studio site and transmitting site are not in the same location as the transmitting sites are commonly in isolated highland regions. The audio over IP network can be achieved using audio IP-STL (Internet Protocol – Studio Transmitter Link). In our use case, Sigmacom’s Digital IP-STL encoder/decoder system. We set up them using their first recommended configuration as in Figure 1.
In our previous installation, the IP link was of about 35 kilometers from the studio to the transmission site. The link was achieved through a PtP microwave link using Ubiquiti’s Rocket M5 radios (TX and RX) attached to RocketDish antennas that we mounted on masts at the TX and RX sites. After the signal has been captured and decoded, it is fed to the exciter. In our example the EuroCaster DS2000 FM Transmitter can power an RF signal up to 2kW of power. This transmitter power (TPO) is adjustable as is the frequency range of transmission, in Kenya it ranges from 87.5 MHz to 108.0 MHz. From this point, the signal can be sent to the broadcast antenna system.
Filters, Feeders and Splitters
Try saying that three times. The signal from the transmitter has to be propagated to the antenna system somehow. In between comes in a connection of transmission line components. The filter is a factory calibrated FM band-pass filter, which is set to the exact desired frequency, example 98.00MHz. The filter is connected to the antenna system through a coaxial feeder cable. A large feeder cable, like in Figure 2 that can go up to 170mm diameter, carries the large amount of energy from the filter with minimum attenuation to a splitter which splits the signal, in our example 8 times, to the low power antenna elements. The low power antennas may be fed with smaller feeder cables of 20mm.
At every connection point with the feeder cable, for example, connection with the filter and splitter, a waveguide rotary joint is used in connecting the RF waveguides. The feeder cable is also grounded to the mast tower. The signal has now reached the antenna system.
Transmission Antenna
For this application, a circularly polarized antenna was used to achieve omni-directional signal transmission. Figure 3 shows one bay of the antenna. The antenna achieves circular polarization by having the radiating components perpendicular to one another, thereby propagation of horizontal and vertical components occur simultaneously.
A singular antenna as in Figure 3 has the following features:
- Impedance of 50 Ohm
- Omni-directional pattern, approx. +/- 3dB
- Band start 87.5MHz, band stop 108.0Mhz
- Lightning protected, all metal parts DC grounded
The antenna is combined with 7 other antennas producing an 8-bay antenna as in Figure 4. Increasing the antenna bays improves several performance parameters such as the directionality and gain. How? Circular polarization splits the effective radiated power (ERP) between the horizontal and vertical components, adding antenna bays on top of each other leads to addition in gain per bay. ERP is used to calculate the range of the signal, mathematically defined as: ERP = [Antenna Power] X [Antenna Power Gain]
A 2-bay system has a gain of approximately 4.1dBd/6.25dBi while an 8-bay system has a gain of 10.1dBd/12.25dBi. Since the gain is higher and losses minimized, the system requires less TPO. Additional bays, however, increase overall system weight as the main trade-off.
These are the basic components one would require to achieve broadcast, in our example, a radiating diameter of 100km could be achieved with the above discussed configuration. When setting up such a system, it is important to capture the necessary requirements so as not to have conflicting components during set up. Hopefully this guide will bring you closer to understanding what goes on before you tune in to your favorite FM channel.