FrSky R9 radio system is storming the RC hobby. It's super cheap after all. But there are problems: EU version is facing a lot of problems and people like to replace original antennas with something else.
Last week I was asked to check what is going on with antennas shipped with FrSky R9 Mini. After all, the same hardware can be used both for FCC 915MHz and EU 868MHz version. Is the antenna tuned somewhere in between? Or not so much?
When FrSky R9M long range transmitter and FrSky R9 receiver series started to pop up some time ago, there was a lot of speculations "it is any good?" "is it a Team Black Sheep Crossfire killer" "cheap Crossfire alternative?". In time, new firmware started to appear, new receivers and so on. And oh yes, EU LBT version finally also appeared. And what… FrSky even was giving R9M for free with X10S Horus.
After weekend tests of Betaflight 3.5 and some discussions with Betaflight developers, here are two things you have to (yes, have to) change in Betaflight 3.4 and 3.5 if you did not did that yet. Worth to do it!
FPV, right? Long range maybe? With a 5.8GHz system, it’s really hard to call it a long-range but what if you put a ridiculous amount of power on transmitter module? 500mW? 800mW? Nope, not enough. Let’s go big and have a practical look at AKK X2-Ultimate which says it does 1200mW of RF output power! 1.2W put into the aether. Wow…
Sometimes you win, sometimes you lose. This time I've lost. Not by much, but always. During testing of my DIY LoRa RC link, radio caught a glitch during a flip few meters above a ground. It was not even a failsafe situation. Link recovered a few milliseconds later, but it was too late and quadcopter crashed into the ground.
During a roll, while being behind a tree, RX antenna got hidden behind a carbon fuselage and both antennas were at 90 deg. That was enough.
Damage is not severe, nothing I can not 3D print in one evening. It's more like a discredited honor or something.
The glitch was so short that is was not even recorded in blackbox log. RSSI was fine, no locked rcData. Quadcopter just kept 90deg attitude for too long.
There is a slight chance it was not faulted in software or hardware. Maybe there was a strong rouge TX polluting the aether. Why? I've caught a failsafe on a different quad (2.4GHz FrSky link) while being only a few meters away and a friend caught a failsafe on a TBS Crossfire. So maybe it's not entirely my fault after all.
Looks like I was able to solve all major known problems with my DIY long range radio system Crossbow. I'm writing known, since no idea what lies beneath… Anyhow… What changed? Quite a lot:
I've extended Arduino-LoRa library with ability to transfer full packet in single SPI transaction. Right now, each read of write to SX1276 uses single transaction. Previously, there were 2 transactions per byte…
The same library now has ability to send packets in async mode. Previously it was blocking code execution until LoRa packet was transmitted. Huge waste on processing time
With OpenTX 2.2.1 on the loose, I was finally able to drop PPM input from Taranis to TX module and replace it with S.BUS. But not without problems. According to specification, S.Bus should be SERIAL_8E2. But my Taranis clearly outputs it as SERIAL_8N2
For now, OLED display is disabled. It was taking too much time to update it using I2C and TX module was loosing S.Bus packets
I've improved RC channels processing time, time required for encoding/decoding went down by 1ms
LoRa modulation has some advantages. Like superb receiver sensitivity and immunity to interference. Has some problems, true, but at the end, it's a great way to send small packets of data to long ranges using low power.
Anyhow, today only one picture: how LoRa spectrum compares to FSK signal spectrum? Like this:
Those two peaks are nearby FSK stations, while plateau is 250kHz wide LoRa signal. Difference is at least clearly visible 😉
Hey, don't leave yet, there is more!
Do you know that there is a YouTube channel with awesome, drone and FPV related video? Why don't you give it a try?