The radio receiver or RX is what receives the signal from the drone transmitter or TX and tells the flight controller what to do. They are generally small and can come in a few different shapes and sizes with several different features. Having a strong and reliable Radio signal is of utmost importance. Let's explore what's behind it.
Radio signals are what helps the pilot's control and fly the quads. Most Radio Transmitters are able of transmitting Radio signals effectively up to a mile, but having a receiver that is capable of receiving the transmitted signal efficiently is equally important. As the quad flies farther and farther, the radio signals received by the receivers also diminish exponentially, hence making it harder to fly the quad.
Types of Radio Receivers
Cased receiver RX usually comes in plastic shell or cover. They are bigger and often have multiple connections for servos and ESC. These are mostly used on fixed-wing aircraft.
A bare RX has no cover case. They are smaller and covered in plastic film or heat shrink. Bare RX are lightweight and easy to mount in tight spaces on your drone. They normally have fewer connections and are geared more towards drone use.
Radio Receiver Range
Receivers operate on 2.4gHz frequencies, this normally gets you plenty of range when flying in open areas. Though range can vary greatly depending on flying location and equipment. Normally you will be able to get up to 1000 feet or more of range out of a 2.4gHz system. There are TXs and RXs that operate on lower frequencies that will get you miles worth of range, we will be going over these in a long-range guide.
Transmitter and Receiver setups are one thing, the protocol used to communicate between the transmitter and receiver also matters. Some of the older protocols like PWM are slow and protocols like IBUS and SBUS are some of the faster modern protocols used. Let's take a look at the receiver protocols used by different manufacturers.
Radio Receiver Protocols
Receiver protocols define the process in which the receiver signals are processed and sent to the flight controller. Transmitter protocols are a whole another topic that we do not wish to stress on much. For this article, we are going to stick with receiver protocols.
Analog receivers- PPM and PWM
PPM and PWM were one of the first receiver protocols used in the drone industry. Both PPM and PWM are analog communication standards. Back when the days RC was limited to fixed wings and airplanes, the receivers didn't need to go faster because fixed wings did not require the high refresh rates, refresh rates define the number of times a signal from the receiver is sent to the flight controller.
PWM receivers use an individual signal wire for every signal wire, whereas all the signals are sent through one signal wire in a PPM receiver. PPM receivers work by sending the position of all the channel positions in a sequence, a 6-channel receiver will send the values of all 6 channels sequentially one after the other, and the process repeats. PWM on the other hand sends the value of each channel through independent wires to the respective channel ports.
Digital Receivers- SBUS, IBUS, XBUS, CRSF, DSMX
But when the quads were adopted into the RC community, manufacturers realized faster refresh rates by the receivers resulted in a more accurate signal processing by the Flight controller. As the analog protocols like PPM and PWM were analog standards that were susceptible to noise and have an even delayed error data handling, PPM and PWM protocols to avoid errors in data take the average of 3 consecutive signals adding up to the already delayed signal, Digital receiver protocols such as SBUS, IBUS and CRSF.
Digital receivers like PPM receivers use a single receiver to communicate with the Flight controllers. They have a very low latency of under 10ms, the rate at which the digital receivers send the data to Flight controllers is several hundred times higher than Analog receivers, 9ms as compared to 27ms of an analog receiver. Digital receivers instead of sending the raw analog signals, digital receivers send the data in the form of 0s and 1s. This digital data is less susceptible to noise and have in built error checking and corrections resulting in a faster response time.
Digital receivers also paved the way for 2-way communication where the data like signal strength and telemetry can be sent back to the transmitter. Digital receivers connect through the many UARTS present on the Flight controllers. SBUS protocol is used by Futaba and Frsky radios, IBUS is used by Flysky radios, XBUX is used by JR radios, CRSF by Crossfire modules and DSMX by Spektrum radios.
Radio Receiver Options
There are dozens of offerings from $10 inexpensive receivers to the most expensive offering from Futaba costing $70. Each manufacturer has their own range of products and different manufacturer warranting different types of receivers. Read below to know about the popular receivers for each brand but not limited to just this list.
The Frsky R-XSR receiver succeeded the X4B-SB receiver. The R-XSR is a diversity full range SBUS receiver that comes in a compact package weighing 1/3rd the weight of the X4B and still retaining all the features. The receiver supports up to 16 channels and can do smart port telemetry. The R-XSR receiver supports the newest ACCESS protocols and the older ACCST protocol from Frsky.
The R-XSR has IPEX antenna connector that can be replaced if damaged. But the R-XSR is not perfect and has its flaws. The antenna element is quite smaller than that of the other offerings from Frsky and this has caused some range issues for some users. The reasoning by Frsky behind this is to avoid prop strikes, but isn’t the receiver marketed as a full range receiver?
The Frsky XM+ is the 2nd generation receiver of the much popular XM receiver. The upgrades are quite obvious, the XM+ is a diversity receiver whereas the XM is a single antenna receiver. The lack of pin headers making the XM+ lighter and cheaper than the R-XSR receiver. The XM+ is also a good $5 cheaper than the R-XSR. The XM+ is also an SBUS receiver best suited for micro sized builds. To achieve this compact size, the XM+ loses telemetry but still retains RSSI.
The XM+ is one of the easiest to work with, with no room to get confused working with only 3 pins. The antennas are also user replaceable. One of the common problems of the XM+ is getting the receiver to the communicate with the flight controller. This can be solved by making sure the transmitter is in D16 mode and try setting SBUS inversion ON and OFF.
TBS Crossfire Receivers
Team Black sheep manufacturers the TBS Crossfire RC modules which has blazing fast refresh rates and a solid RF link. The Crossfire or CRSF protocol is a wholly owned by TBS and as a result no aftermarket receivers are available. The Crossfire nano RX is the single best receiver for FPV. The nano RX comes in 2 types- a $25 receiver normal antenna and a $30 receiver that comes with a much sturdier immortal T antenna.
The $5 spent is justified because the Immortal T antenna is well built, can handle a bunch of crashes before needing replacement. Unfortunately, the nano RX is not a diversity receiver, but with the reliability being the single most sales pitch that TBS uses, diversity is unnecessary for Crossfire atleast.
Spektrum is often a name that pops up from time to time when RC planes and involved. There are people, including myself, enjoy flying RC planes just as much as we enjoy flying FPV. Spektrum is a really popular brand when it comes to RC planes and some people swear by the reliability and the functionality it offers. DSMX is the protocol used by Spektrum transmitters and Spektrum has a receiver dedicated to FPV fliers, Spektrum SPM 4650 receiver.
The 4650 is a full range diversity receiver, telemetry and a binding button. The SPM 4650 is not the only option on the market for Spektrum receivers, no it is not. But buying a $400 radio for the reliability it offers and saving $10 by buying an aftermarket receiver is not a wise choice.
When choosing your Drone Radio Receiver it's important they are compatible with your Drone Transmitter Tx. Also confirm RX has the necessary size and features for your specific needs. It is recommended to get a high quality Tx Radio Transmitter when starting out to fly FPV as it is one of the components that will last you the longest in this hobby.