Which channel to broadcast FM radio on: selecting a frequency

The question of what frequency or "channel" to broadcast on is a fundamental one for anyone planning to launch a radio station. Unlike digital platforms or Internet broadcasting, the FM band is strictly regulated by the physical laws of radio wave propagation and strict government regulations. It is impossible to simply “turn on” the transmitter on any number you like, since the airwaves are crowded, and chaotic use of frequencies will lead to jamming of legal stations and interference with the operation of critical infrastructure.

Choosing a specific broadcast channel is always a compromise between the desired coverage area, the availability of free niches on the air and the technical capabilities of your equipment. Frequency plan in most CIS countries and Europe it is built on a grid with a step of 100 kHz (0.1 MHz), which means that broadcasting is only possible at values that are multiples of this step, for example, 101.2, 101.3 or 105.5 MHz. Understanding this grid is the first step to working legally.

Before considering the technical details of transmitter setup, it is necessary to realize that airspace is a limited resource. Range 87.5–108.0 MHz, which we used to call FM, is divided into hundreds of channels, each of which in large cities is already occupied by powerful commercial broadcasters. Trying to wedge between them requires a professional approach and precise calculations so as not to create interference.

⚠️ Attention: Using the radio frequency spectrum without a license is an administrative and, in some cases, a criminal offense. Homemade transmitters operating at powers above 0.1 W can interfere with aviation navigation and emergency response services.

There is a common misconception that a "channel" in FM radio is just a random setting, like a TV channel number. In fact, behind each number there is complex engineering logic. If you plan to broadcast locally, such as within a single building or small yard, your options are wider, but the technical limitations remain the same. For large-scale projects, the choice of frequency becomes a matter of business survival, since the potential audience depends on it.

FM band structure and frequency grid

The standard FM band used in Europe, Russia and most CIS countries extends from 87.5 MHz to 108.0 MHz. Within this segment, frequencies are distributed with a certain step, which is called raster grid. In the post-Soviet space, a step of 100 kHz (0.1 MHz) has historically developed, while in some countries in Europe and the USA a step of 50 kHz or even 200 kHz is used. This means that your radio must transmit a signal strictly at a frequency that is a multiple of 0.1, for example, 99.1, 99.2, 99.3 MHz and so on.

Why can't we broadcast on an intermediate frequency, say 100.15 MHz? The fact is that radio signal receivers have a certain selectivity. If the signal is spread out between channels or falls into a guard interval, the reception quality will drop and neighboring stations will receive severe interference. Bandwidth One stereo channel is approximately 200 kHz (0.2 MHz), of which the useful signal occupies the main part, and the side bands are used to transmit stereo information and additional RDS service data.

When planning broadcasts, it is important to take into account so-called “mirror channels” and harmonics. A high-power transmitter operating at 100.0 MHz may cause interference at multiples of the fundamental frequency or at adjacent frequencies if the signal is poorly filtered. Therefore, regulators often leave buffer zones between powerful transmitters in neighboring cities.

Below is a table showing the distribution of frequencies in a typical range section and their occupancy status in a metropolitan area:

Frequency (MHz)	Typical status	Features of use	Recommendation
87.5 – 88.0	Partially free	Low priority, often used for weak transmitters	Suitable for local broadcasting
88.0 – 100.0	High density	Occupied by federal and large commercial chains	Requires approval from Roskomnadzor
100.0 – 104.0	Critical employment	The most popular frequencies, maximum traffic	Almost inaccessible to new players
104.0 – 108.0	Average density	Often used by regional and niche stations	Optimal for finding a free niche

The specific frequency you choose also depends on how far away other transmitters are from you. If you are in a point where you can hear 5-6 stations on the same frequency from different cities, it will be technically impossible to get in there without creating interference. In such cases, engineers are looking for “windows” - frequencies that are free in your location, even if someone is broadcasting on them in a neighboring region.

Legal aspects and frequency licensing

In the Russian Federation and most CIS countries, the use of the radio frequency spectrum is regulated by government agencies (in the Russian Federation this is Roskomnadzor). Any broadcast beyond micropower devices (usually up to 10 mW) requires a license. This process is complex, expensive and lengthy. You will need to not only submit an application, but also carry out frequency assignments, obtain permission to use radio frequencies (RUFC) and coordinate the work with the GRFC (Main Radio Frequency Center).

There is a concept of “micropower transmitters” that do not require a license. However, their power is limited so much (often to 10-50 mW) that the range is from 10 to 50 meters in line of sight. Such devices are suitable for broadcasting music in a fitness room, a small store or at an exhibition. But once you increase your antenna or power to penetrate walls or cover a parking lot, you automatically become a violation of the communications law.

What happens if you broadcast without a license?

If an illegal transmitter is discovered by security services, the equipment will be confiscated and a large fine will be imposed on the owner. In the worst case scenario, if your interference affects the operation of emergency services or aviation, you may be subject to criminal liability under the article on violation of the rules for the operation of radio-electronic equipment.

For legal commercial broadcasting it is necessary to participate in competitions for frequencies. The state periodically puts up for auction free frequencies in different cities. The winner of the competition receives the right to use this frequency for a certain period (usually 5-10 years). The cost of such a license can amount to millions of rubles, not counting the cost of equipment and copyright royalties.

⚠️ Attention: Purchasing a “licensed” transmitter on the marketplace does not give you the right to broadcast. It is not the device that is licensed, but the right to use a specific frequency in a specific geographic location.

In addition, there are international agreements regarding the use of radio frequencies. In border areas, the choice of frequency is coordinated with neighboring countries so that signals do not cross the border and interfere with broadcasters of another country. This makes the process of obtaining a frequency in cities like Belgorod, Pskov or Vladivostok even more difficult.

Technical limitations and equipment selection

The choice of broadcast channel directly depends on the characteristics of your transmitter. Cheap Chinese modules, often used by enthusiasts, have a wide frequency spread and low stability. Crystal oscillator in such devices it can “float” with temperature, leading the signal from the center of the channel to its edge, which leads to sound distortion and listener complaints. Professional transmitters use phase-locked loop (PLL) frequency synthesizers that provide a tight grid lock.

The most important parameter is antenna-feeder path. Even if you have selected a free frequency, incorrectly setting the antenna will result in the signal not being radiated, but going back to the transmitter, burning it up. The antenna must be tuned to the exact frequency on which you plan to broadcast. Universal antennas “for all frequencies” have low efficiency.

When choosing equipment, also consider:

— Nonlinear distortion: Cheap transmitters give a dirty signal, capturing adjacent channels.

— Power: Real power often differs from that stated in the specifications.

— RDS encoder: A device for transmitting station name and track to a receiver display, operating on a 57 kHz subcarrier.

If you are using a professional equalizer or broadcast processor, remember that it also affects the spectrum of the signal. Excessive compression and raising of high frequencies can expand the spectrum of radiation beyond the allocated channel, which will be regarded as a violation of the regulations.

Interference and signal quality

The main problem when choosing a frequency is interference. This is a phenomenon in which signals from two transmitters operating at the same or similar frequencies overlap each other. In the FM range, this manifests itself as a characteristic crackling sound, “croaking” or complete loss of sound. The capture effect in FM receivers is designed in such a way that the receiver only receives the stronger signal, completely ignoring the weaker one. But if the powers are comparable, the listener gets a mess of sounds.

There is a concept protective attitude. To reliably receive a useful signal, it must be more powerful than the interfering signal by a certain amount (usually 15-20 dB). If you choose a frequency on which a powerful station is broadcasting 50 km away from you, your signal will simply be “crushed” and your broadcast radius will collapse to zero.

To combat this, directional antennas are used. Instead of radiating the signal in all directions (omnidirectional antenna), you can point the main lobe of the radiation pattern in the desired direction (for example, towards the city center), minimizing radiation towards strong interference. This allows you to legally work on a frequency that is formally occupied, but physically free in the desired direction.

Local broadcasting and micro-power transmitters

For most small businesses (cafes, car services, fitness centers), the issue of global FM broadcasting is not an issue. They only need to cover one room or parking lot with a signal. There are rules here. Transmitters with powers ranging from 1 mW to 50 mW are used. In this case, the frequency is selected using the “poke” method, scanning the air in the immediate vicinity of the object.

The optimal solution for local problems is to search for “dead zones” in the lower or upper part of the range (87.5–88.5 or 106–108 MHz), where the probability of collision with powerful urban stations is minimal. Even if someone is broadcasting on the selected frequency 10 km away, inside a concrete building with reinforcement, their signal can be weakened so much that your low-power transmitter becomes dominant.

It is important to remember about shielding. If you are broadcasting in a shopping center, your signal should not “go out” onto the street so as not to interfere with others. To do this, they use shielded rooms or carefully calculate the power so that it is extinguished by the walls of the building. Critical: do not connect external antennas to micro-power transmitters unless this is provided for by the design, as this may lead to power beyond the limits permitted for free sale.

Prospects: digital broadcasting (DAB+)

While we are discussing analogue FM, the world is gradually moving to the digital standard DAB+ (Digital Audio Broadcasting). In this format, the concept of “channel” changes. Digital stations are grouped into multiplexes, occupying wider frequency bands (usually 1.5 MHz), but allowing 10-15 radio stations to be transmitted simultaneously in one channel in HD quality.

Although the transition to DAB+ is slow in Russia and the CIS countries and the FM band will remain relevant for many years to come, it is worth taking this trend into account when planning long-term projects. Licenses for digital broadcasting are issued differently, and competition there is still lower. However, for local tasks and live broadcasts “here and now,” FM remains the uncontested king thanks to the ubiquity of receivers.

Is it possible to play FM radio at home without a license?

Technically, you can start the transmitter by purchasing a module for 500 rubles. However, this is legally prohibited if the power exceeds 10 mW (in the Russian Federation). Even low-power devices can cause interference, and if a complaint is received, you will be the target of an audit. For indoor environments, use wired systems or IP radio.

Which frequency is best for long distance transmission?

There are no “magic” frequencies in the FM band. The range depends on the transmitter power, the height of the antenna and the terrain. However, frequencies at the beginning of the range (88-92 MHz) avoid obstacles a little better, and at the end (104-108 MHz) they are a little worse, but have less industrial interference.

What is RDS and why is it needed?

RDS (Radio Data System) is a technology for transmitting digital information (station name, song title, news) along with an audio signal. This is convenient for listeners, and a way of branding for the station. Without an RDS encoder, your station will appear as "No Name" or frequency.

Why is my transmitter jamming Wi-Fi?

The frequencies of FM radio (about 100 MHz) and Wi-Fi (2.4 GHz) are far from each other. However, cheap transmitters generate powerful harmonics. The third or fifth harmonic of the fundamental signal may enter the Wi-Fi or Bluetooth range, causing interference. The solution is to install harmonic filters at the transmitter output.