Recently, airports in major cities such as Sao Paulo (Brazil) had serious problems with interference from clandestine FM broadcasters, which affected the operation of navigation systems, causing major aircraft landing and maneuvering to be aborted. Drastic action against these broadcasters is being planned for the authorities, as air traffic safety is at stake. It is better to prevent a serious accident now than to only take the action steps after something happens.


The problem

With the fake "Community Broadcasters" label, many people and entities have put on the air FM broadcasters of powers far beyond what the initial legislation that existed on the subject allowed. Thus, the 5 W of maximum power are not respected, as well as the maximum height of the towers. We have seen transmitters up to 500 W (!) With towers that reach 40 meters on the aircraft approaching route at Guarulhos Airport! But it is not only the power that causes problems. The used transmitters do not have any quality control, they are usually precarious home assemblies that end up having their signal scattering across a spectrum that extends well beyond the FM range.

It is precisely here that we have the critical point. The range for communications between tower and aircraft and the airport beacon system operates in the range of 108 MHz to 136 MHz, that is, just above the FM range. Thus, a not correctly adjusted FM transmitter, and need not be very powerful, may have spurious signals in the aviation band, interfering with aircraft approach and communication equipment, as shown in figure 1.


Figure 1 - FM spectrum and range used for aircraft communications.
Figure 1 - FM spectrum and range used for aircraft communications.



And, for a transmitter to emit spurious and harmonic signals, it does not take much. It is enough that the oscillator and amplifier stages circuits are not well adjusted for this to occur. The absence of output filters that eliminate harmonics and spurious also causes problems and the antenna system itself and poorly dimensioned cable also causes problems.


Mounting Transmitters

We often publish FM transmitters, some even with high powers. Always, however, we alert readers to the legal restrictions that exist regarding their use, and especially the care that should occur with their installation and assembly. In short, mounting and operating transmitters is not a toy. It is justified to use a transmitter with some power in an isolated location, such as a village or small town where the service does not exist. The installation of a small transmitter in a club or school is justified, as long as this does not affect any type of communication service that may exist in the place.

In fact, this is the basic philosophy of the Community Broadcaster, which would be to provide a service where it is not available. However, at a time when such equipment can cause problems and especially endanger human lives, their presence has no justification.

Installing and operating FM transmitters improperly in densely populated areas may also cause interference to TV and other services near airports where they may interfere with aircraft equipment and should be strictly counteracted.




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