For the RC link, most people are using 2.4GHz technology, but we can still encounter some “old” 35MHz and 72MHz systems. Regarding some circumstances, these can be back in the game easily, when we talk about long range fpv.
The RC link basically consists of a transmitter/radio (TX), a receiver (RX) and the antennas.
- TX module frequency
- transmitter MODE
- number of channels
- programming (mixes)
- modularity (exchangeable transmitter modules)
- noise filtering
- CPPM, S-BUS capability (receivers)
- different receiver types, sizes
- telemetry support
All the radios are basically built the same way, there is a circuit inside that generates the PPM signal, they have a HF module for transmitting, sticks, switches, pots. Some are relatively easy to MOD, e.g. to add an LCD backlight, or a them to our needs.
Many rc systems nowadays use telemetry to be able to get information from the airborne device, like battery capacity, RC signal strength and so on.
The rc receiver is installed on a multicopter. In case of a regular receiver it connects to the FC via many cables, in case of a CPPM receiver i cable is enough.
TX module frequency
Vast majority of today’s modern radios use 2.4GHz transmitter modules. This frequency had been proven as good enough for penetrating object, but still available for wide public, because of using the range where WIFI devices transmit. This way, the power limit of the TX modules can be the handy 50mW, that does not disturb anybody. The danger of using 2.4G radios in urban area is the possibility to get a blackout any moment, in despite of the various frequency hopping algorithms.
The old frequencies (35MHz, 72MHz) used by our fathers are not so lucrative anymore, despite the fact that by using them, we would achieve quite a few advantages over the higher frequencies.
There are special control frequencies, that are used exclusively for long range flights, namely the 433MHz and 868MHz ones. While the first one is in the same range where the radio amateurs transmit with much higher RF output levels, the second is barely used, except for special burglar systems and similar. Many says that if you obtain a HAM license for 433MHz you can use your gear legally. Do not be fooled by those saying that, as the HAM license is given for a specific frequency only and all the 433MHz LRS systems are using the frequency hopping technique, and in that case it is hard to legitimate the usage of them.
The range is determined by the chain of: TX->antenna->antenna->RX. By increasing the effectivity of any of these components we gain more range.
Today’s 2.4G based systems range is in 700m-3km stock, depending on the manufacturer, receiver, antennas. The RF output power is generally in the 40mW-100mW range, that is completely enough for basic flying. However, for FPV flying we may need to cover higher distances.
There are some possibilities to extend the factory range:
- using directivity antenna on TX/RX side
- increase the RF output power
- using an LRS (long range system) solution/lower the frequency (to increase penetration)
The radio sets are usually sold with factory antennas. These have many times the quality of a rubbish, except for some vendors (e.g. Frsky, Futaba). Exclusively, the antennas are omnidirectional monopole or dipole antennas. We can gain more range by changing these for patch antennas with higher dBi values.
Other possibility is to increase the RF output power. The easiest way this can be achieved is by using so-called “boosters”. These are small electronic units that are positioned between the HF (high frequency) module and the antenna, and amplify the outgoing RF signal by a specified number of dBs. Before buying any of these, make sure that they are able to switch the incoming/outgoing signal, otherwise the telemetry functions will not work.
Another option to achieve better range is to obtain an LRS system. They are usually sold as an TX+RX combo. Majority of these work on the frequencies around 433MHz (openLRS, Rangelink, Dragonlink, Rlink), however, the frequency range can be adjusted. Some systems even work on 868MHz (Thomas Scherrer). By using such low frequencies, the signal spreads much easier, the penetration gets much better.
With these systems, distances around 50km can easily be reached. of course we do not fly that far with multicopters, but from the penetration perspective, LRS systems can be very useful when flying behind walls, in forest and such. We even get an advantage of using ANY video gear we want, because the harmonic frequencies of the RC signal will not harm the video.
These systems usually radiate at relatively high power levels, in the 100mW-2000mW range, while using powers around 400-600mW is common. Note that the 433MHz is allowed for radio amateurs in many countries, so by using an LRS system we may disturb some radio communication, despite the fact that most guys with HAM license emits at much higher power levels. In any case, obtaining a HAM license is recommended when using LRS systems. On the other side, as all LRS systems work with frequency hopping algorithms, we will still not be on the legal side when using them with a license.
An important parameter when we choose a radio is the so-called “mode”. It defines where the different sticks are positioned on the transmitter. Possible modes are MODE1 through MODE4, the most used ones are MODE1 and MODE2.
* number of channels
Basically, one channel controls one servo in the plane, or controls an esc in the car or multicopter. One channel usually handles 2 directions, for example “forward” and “backwards”.
For controlling a multirotor, we need minimum 4 channels. If we want to change the modes during the flight, we need 1 additional channel for that. The typical MODE2 channel assignment looks like this:
- right stick forward – backwards (elevator -ELE / pitch)
- right stick right – left (aileron – AIL / roll)
- left stick left – right (rudder – RUD / yaw)
- left stick up – down (throttle – THR)
- any switch/pot auxiliary channel for some additional function, e.g. autolevel (auxiliary – AUX)
We can use mixes for mixing of different channels to each other. For example we would like the quadcopter to add more throttle while banking, then we can simply achieve that by mixing the THR and AIL channels. For multirotors, we sometimes use mixing for additional control functions.
The transmitter/radio itself can be modular or comes with an inbuilt module. A modular radio can host different modules, but in case we change the module, we need to change the receivers as well. In case of a modular radio, it is a good practice to choose a transmitter module that can use many different types of receivers. Other important aspect is the price of the receivers, as we always need more of them.
CPPM, S-BUS capability
Modern receivers support the newest technology, like CPPM (PPM, PPM SUM) or S-BUS.
The CPPM protocol allows the connection of the receiver to the FC by 1 wire only, instead of a bunch of them. The S-BUS is a similar protocol, originally developed by Futaba, that allows control of many channels by 1 signal wire as well.
Many modern radios allow using of telemetry, by building the protocols into the receivers or by using additional electronics. By using telemetry, we can get status information from the receiver, sent back to the transmitter when we are able to display and evaluate them.
Note that when flying FPV we are mostly able to evaluate only audio information, so if we need such additional info we may get a radio that is able to transform the incoming info into audio warnings. One of the favorite radios that has this functionality is the Frsky Taranis.