Questions With Radio Modules

Hi All

I am really happy with my DIY F450 and 1200mm build. Its pretty strong and responsive

I work with a small R&D firm where we test various RF products for various purpose and need help with some questions.
FIY, FC Pixhawk 4 mini, T Motor ESC and U7 motors remains as it is in frame. I keep changing Telemetry, Rx & Tx and VTx for testing purpose.

So far I have tested

Telemetries :-

  • RFD 900x Long Range Modules
  • Holybro SiK Telemetry 915 MHz

Rx Tx

  • FrSky Taranix X7 with X8R Pro Rx, SR10Pro Rx
  • Jumper T-Pro 4 in 1 Multi Protocol Tx with Flysky AFHDS Rx, AFHDS2 Rx, Radiomaster R86, RadioLink CM921, Spectrum AR620
  • Herelink Video Transmitter
  • Bosscam 5.8 Analog Vtx

Now I am procuring more Rx and Tx with different protocols mainly for recording RF signatures from different manufacturers.

The questions I have (Suppose my quad will be strictly within 200 meters from me, nothing more)

  1. Why we use RF modules? Like XJT or FrSky R9M? What changes they introduce which a normal transmitter can not provide?
  2. Can I purchase “Jumper AION ELRS Nano Tx module” to upgrade my “Jumper T-Pro 4-in1” transmitter so it supports ELRS Receivers?
  3. Are ELRS Rx compatible with ELRS Tx irrespective of brands?
  4. What makes RFD900x Telemetry (Supports upto 50km range) different from HolyBro Telemetry (1km range) apart from distance?
  5. I use telemetry to connect my Quad/Octa with Mission Planner GCS. What different Exotic telemetries like DragonLink or XBEE offers apart from mission planner connection?
  6. What are the decision factors in selection of 433 Mhz Telm vs 915 Mhz Telm irrespective of frequency considering distance is 200 meters. Both offers connectivity.
  7. Why there are exotic protocols exists like AFHDS3 and ANT (particularly for Flysky Brand) when AFHDS & AFHDS2 already exists?
  8. Why to use Long Range ELRS modules when the operator will be using Long range teletry to command the quad? (In case of 2km+ flight)
  9. What makes a Receiver and Telemetry different from each other since both offer Data (RSSI or etc) back & forth.

I request you all to please help me clearing my doubts.


I don’t know a lot of those but I’ll help where I can.

6. Strictly for legal reasons. For example in Canada 433Mhz is reserved for something weird like RFID on shipping containers. You can use it but it’s technically illegal. So you use 915Mhz. In Europe it’s the opposite. 915Mhz belongs to something else so you use 433Mhz. It’s strictly because of regulations. There’s no appreciable performance difference.

9. None at all. In fact in some newer radios the air unit and ground unit are identical and interchangeable. Both will have USB and regular-style connections.

4. (I’m less certain about this one). RFD900+ or RFD900x or RFD900x-US are all radio modems, instead of regular radios. They have a higher baud rate, states “Air data rate: 500kbit/s”. This is in addition to their long range.

1. The FrSkyR9M is a long range, low(er) frequency transmitter which requires a similar long range low frequency receiver on the vehicle. It operates at 900~ish Mhz vs. the 2.4Ghz that a regular Tx/Rx pair operate at.

The RFD radios can also handle s-bus RX signals. This way you can send your radio’s command signals, and also have full two-way telemetry. The RFD radios will also run up to 1w whereas the Holybro are 100mw. I have flown with 100mw radios and had telemetry to about 700m, but that was it. Maybe with better antennas, more thoughtful antenna placement, and maybe some parameter tweaks I could get more but really it works. If I’m going further I use the RFD900.

Not familiar with XBEE, but I have used DragonLink in the past. I would compare it to the RFD system. Personally I was more impressed with the RFD system, but to be fair the Dragonlink did work once I got it going. That was a few years ago so things may have changed since.

As already pointed out, it’s mostly where you live in the world. Arguably 433 will give slightly better range for the same power, but you will get less bandwidth of data. In practice I think there are so many other factors involved (antennas, placement, etc) that I don’t think the real world differences are that great.

I don’t know the Flysky system specifically but I’m going to guess it’s like Spektrum DSM, DSMX, etc. Manufactures will update protocols as they find ways to reduce latency and add features like telemetry or more channels. In some cases the new protocols are backwards compatible or the hardware can be updated with the newest firmware. In other cases new protocols ignore the old hardware and create big headaches (FrSky…) It really means buyer beware when buying “new” hardware.

I think these two questions come to the same point. The Receiver/Transmitter is also known as the Command and Control link. This is the system that sends the very basic stick commands from your radio control to the drone. This data link needs to be fast, low latency, reliable, and stable. Some protocols can send back some basic telemetry data (ELRS/CRSF, FrSky, etc) but that is secondary to the command and control.

The telemetry radios are about sending greater volumes of data, often in two directions. They can send commands to the drone. As a result telemetry radios are often slower or have greater latency. We’re talking about milliseconds, but when that comes to stick commands it can be a big deal. I wouldn’t recommend doing a first flight of a new build on a high latency system, but a fully setup drone flying an auto mission may be perfectly adept for flying just off a slower telemetry link.

It becomes confusing when systems like RFD900 can do both. The RFD TXMODV2 system is designed specifically for that. This lowers the parts count in a build, but there is an argument for redundancy by having telemetry and C2 links separate. The best choice is up to you and your needs.


A few add on’s:

Many of the systems listed are intended for longer ranges.
You can save a lot of money and hassle by omitting them.

Yes, you can.

Yes, they are, as long as both are 2.4G or both are 900MHz. The module mentioned above is a 2.4G module. You could run any 2.4G elrs RX with it.

ELRS was created for a very different purpose. It was created by Drone racers for their specific need’s.

The XBEE module was one of the earliest way’s to create a transparent serial link used for bi-directional telemetry.
The SiK-radio’s were invented to have a cheaper and more reliable replacement. Back in the day’s it was common to have bricked XBEE radio’s. Don’t ask how I know.

1 Like

There is some great info in this thread,
I peronally for long range use dsm2 during takeoff and landing line of sight, (purely because the lte telemetry link i use has seconds of latency which is not good enough for landing) then for the middle of the flight i turn off the transmitter (rc link loss is not set to trigger failsafe, only telemetry loss) and connect to gcs through trainer port usb interface as a joystick input to pissionplanner. This is an incredibly simple system for long range fc assisted flights as there are no ground antennas, additional batteries etc, just a laptop, phone hotspot, rc transmitter and a cable.
For short range you can use standard 2.4g rc link which can do telemetry but not full mavlink stuff unless you go the fullsize tbs module or rfd900. All the other rc telemetry ideas are do not allow you to upload a new mission to the flight controller for example. All the different systems available can almost be assigned to the different groups of people who buy it for example, fpv quad ppl want long range with minimum latency to compliment their lighting quick racing reflexes,