2.4 GHz—A Blessing for Model Aviation?

The problem of mutual interference due to channel overlap—as was possible at 35 and 40 MHz—has been eliminated by the transmission methods used at 2.4 GHz. Such protocols have long been proven in the industry. Telemetry capability, as offered by the latest generation of remote controls, provides additional safety regarding the range of the radio link, since the pilot is warned when the radio signal becomes too weak. The transmission response time is significantly shorter than was previously the case with pulse-width modulation of servo pulses.

Temporary signal errors are detected, corrected, or eliminated by the digital encoding of the control commands in the receiver, ensuring that no malfunctions occur. Unwanted transmitters are detected and ignored by the receiver.

Microwave signals behave almost like light. This means you can only fly within visual line of sight. If you fly—intentionally or not—behind buildings, hills, or other obstacles, the signal strength is greatly reduced or may drop out entirely (reflections can sometimes help). The 2.4-GHz band is a so-called ISM band. We share this frequency band—which we utilize to its fullest extent using frequency hopping and spread spectrum—with other services in industry, science, and medicine. This means that, in principle, the transmission channel may be occupied by other users. Frequency bands may be subject to interference (e.g., a leaky microwave oven emitting up to 1 kilowatt).

Even powerful commercial transmitters that do not operate on the ISM band can interfere with remote control receivers due to the presence of strong electromagnetic fields. This applies in principle to all radio transmissions.

• Defects in your own transmitter and receiver or errors in handling
• Third-party users of the 2.4 GHz ISM band
• Presence of strong electromagnetic fields
• Interference from different frequencies in the receiver

Any potential interference with the remote control radio link must be taken seriously, as it can cause our model to crash.

The front part of the receiving antennas is the active part of the antenna. The gray or black part is the shielding (coaxial cable) from which the antenna is made. The inner conductor (which we can see for about 3 cm) consists of 7 fine wires with a thickness of only 0.06 mm—roughly the thickness of a hair. Repeated kinking inside the polyethylene sheath can cause a wire break, which is difficult to see. In this case, this antenna will no longer receive a signal; if there are two antennas, only one would remain active. This could cause gaps in reception. Inside the receiver, the cable is connected to a miniature—

connected via plugs. These plugs have a limited tensile strength for the cable. If the antenna cable is pulled too hard, it can be torn out of the connector, even if this is not visually apparent.

To avoid the risk of antenna attenuation, the antennas should protrude from the model’s fuselage. Carbon fiber, metal, and certain color pigments absorb high-frequency signals. Fiberglass epoxy, however, is permeable to high frequencies.

For optimal reception quality, follow the manufacturers’ recommendations to place the two antennas at a 90° angle and as far apart from each other as possible. The exact angle isn’t critical. However, two antennas running parallel to each other will not serve their purpose. Failsafe should be enabled on fixed-wing models; for safety reasons, it should definitely be set to the throttle.

Incorrect antenna positioning on the transmitter

All rod antennas are Hertzian dipoles, which radiate radially at a 90° angle to the rod’s axis. Along the rod’s axis, radiation is minimal. This can reduce the range by several factors. This also illustrates why the two receiving antennas should be positioned at a 90° angle to each other.

The transmitter rod antenna should therefore never point toward the model. (Modules that do not have a visible antenna use a patch antenna, which has different radiation characteristics.)

Who doesn't have a cell phone in their jacket these days, even when flying? Since many airfields are located away from urban areas, cell phone reception there is often very weak. The cell phone tries to connect

to maintain the connection (even when no calls are being made) and sends short but very strong signals every few minutes. Under certain circumstances, these signals may interfere with our transmitter.

Bluetooth and Wi-Fi use the same frequency band as our 2.4-GHz systems. Wi-Fi also has higher power output. If there are many Wi-Fi networks operating simultaneously—as can be the case in industrial environments—they can completely occupy the available transmission band and interfere with our radio connection to the model. The farther the receiver is from us and the closer it gets to the external transmitter, the greater the risk of interference. Such third-party devices should be turned off at the airfield to avoid the risk of self-interference (even though frequency hopping and spread spectrum could cope with such interfering signals). In our own best interest, we shouldn’t tempt fate.

External transmitters with very high power can interfere with our receivers due to the presence of strong electromagnetic radiation. Radio transmitters operate below 1 GHz, but with power levels ranging from a few tens of watts to over 100 kilowatts. If our model comes into proximity with such transmitters, the receiver’s input stage can become “clogged,” preventing the remote control signal from being received and decoded.

Cell phone antenna systems also have varying transmission powers. These range from 10 to over 1,000 watts. The frequencies range from 900 to 2,200 MHz.

the same effect may occur.

Mixed signals from multiple transmission signals may form at the receiver input, which then fall within the ISM band.

At two model airfields where interference occurred, no interfering signals were detected at 2.4 GHz. However, cell phone towers and broadcast transmitters were present in the vicinity. A spectrum analysis revealed which signals were present and at what field strength at the time of measurement. In the laboratory, the measured signals were reproduced individually. However, no interference was detected at the receiver. Only when two external signals were applied simultaneously did the receiver trigger a fail-safe.

There are two main types of transmission towers.

The Federal Office of Communications (BAKOM) makes maps and data available for viewing on the Internet.

• http://map.funksender.admin.ch/bakom.php

The map shows the locations of transmission facilities and their characteristics. For each facility, there is a table listing the services provided and their transmitted power.

To ensure that reception of our model is not disrupted, the distance from transmission stations should be as great as possible.

• Distance from a digital TV transmitter, e.g., Rigi Kulm (112 kW), at antenna height: > 8 km

• Distance from repeater (10 kW): > 2 km
• Distance from GSM stations (1 kW): > 500 m

This does not mean that flying within these distances is prohibited. However, it is advisable to detect any potential interference as early as possible and to fly out of the danger zone.

Interference signals outside the 2.4-GHz band can be suppressed using appropriate filters. One possible filter designed specifically for 2.4

The 2.4-GHz device measures $5 \times 3 \times 1.5$ mm and can be integrated into the antenna line without causing significant transmission losses. It could be implemented by manufacturers of 2.4-GHz receivers.

For an analysis, it is important to know:

• Time of the disturbance (as precisely as possible)
• Direction, distance, and altitude of the interference
• Which transmitters and receivers were in use? →

The new technology for controlling 2.4 GHz models has become established and is certainly reliable. However, we must be aware that even the best technology has its limits, which cannot be exceeded without consequences. Anyone who pushes past physical limits takes on increased risks.

So those who know the limits will also heed them. ■

*SMV Infrastructure Department,
Paul Rudolf*