Relieable 3+km HD FPV solution

This post presents my first experiences using the wifibroadcast video transmission.

—

To be honest, this post is a bit contradictory.
The title uses the word “reliable” and later in this post I’ll describe my first three FPV flights where the last one ended in a really bad crash.
Analyzing the cause showed that it was actually my fault, not the transmissions one.
But before I come to the bad part let’s start with the good news which is a range test.

Extended range test

The last range test was a bit limited in terms of range since the area I was in was limited to 500m line of sight (LOS).
This time I had a car available and drove to a small hill where I’ve placed the video sender.
You can see a picture of the valley down the hill here:

I used the ALFA AWUS36NHA with the omni antenna that came with it as a transmitter.
The H264 data rate was 7mbps with a double retransmission.
On the rx side I used the TP-LINK TL-WN722N with my home-made double biquad antenna:

The results were really astonishing.
I drove down the valley all the way to the end right before I lost line of sight.
At this point (at 3km distance to the transmitter) I had rock-solid video image.
Since the double biquad antenna is a directional antenna its sensitivity decreases when the transmitter is not in the center of the receive coil.
And again I was surprised that I could move the antenna +-30° both horizontally and vertically until I lost too many packets.
This is really good news: In case your quad stays in a radius of 3km you are safe to fly withing a 60° sector.
This also indicates that the maximum distance is way beyond 3km.
My guess would be that at least 5km should be possible.

I also tested a dipole antenna at the receiver.
This gave a maximum distance of 900m with the advantage of a 360° coverage.

The end of my last FPV flight

As I have mentioned before my last flight ended quite brutally.
And I have learned a lot from it.
Two main problems have caused that crash: Strong wind (30km/h) and blocked line of sight.

I was flying at a distance of around 200m against the wind.
When I turned around the quad was moving with the wind and suddenly lost height which blocked the LOS between the antennas.
The stream was quickly so disrupted that I could not guess its current position and height.
While I was trying to move the quad back to my position (I did not know that the LOS was blocked due to the height) the strong wind pushed the quad away horizontally.
When I (blindly) increased altitude the quad was already pushed away so far that the LOS was now blocked horizontally due to trees close to me.
I still received data but that was mostly unusable.
Maybe each 5 seconds a good frame blinked quickly and was then disrupted again.
When I lost contact completely I was forced to “land”.
The problem now was: Where is it? I knew that the battery was almost dead so the RC activated beeper would soon be quiet.
I ran around like a mad dog listening for the beeping.
After one hour I gave up and “accepted my loss”.
Back home I converted the received video stream of the crash flight to single images using gstreamer.
This way I was able to recover a more or less intact image 10s before the crash:

This image allowed me to localize the approximate position and finally I have found my quad sitting in a field (luckily the green antenna stood out of the long grass).

The impact took place at a distance of 700m to the rx station.
This means that the wind pushed the quad in 30s around 500m away from me…

Aftermath and conclusions

Although the crash was quite hard all the electronics and motors stayed intact.
Most likely this is a result of the sandwich construction that protects the electronics between two plates.
However, all of the 8mm carbon fiber rods that hold the motors broke quite brutally.
Under normal circumstances these rods are really durable.
So the crash seemed to be quite hard.

My conclusion is that a blocked line of sight using 2.4GHz transmission is deadly.
In the future I’ll take care that my rx antenna has a clear field of view of at least 180°.
Also my first reaction in case of a lost video connection will be to gain altitude.
This incident also motivates me to buy a GPS receiver that transmits the position of the quad and also provides an automatic “return to home” function.

PS: A little cheer up

For not letting this post end with bad feelings I show you the video of my first FPV-only flight.
I shot it using the ALFA AWUS36NHA in the air with a dipole antenna and a TP-LINK TL-WN722N also with a dipole on the ground. The video is decoded and displayed using a raspberry pi as shown here.

The occasional errors in the video are because I had the receiver in my pocket. Due to that my body blocked quite often the line of sight between the two antennas. The images are wobbly because it was quite windy that day (around 20km/h). I know, compared to other FPV videos this is sooo boring. But hey, it was my first flight 😉