Homemade waveguide antenna for FPV racing timing gate

Quadcopter FPV racing has quickly grown into a competitive sport in the past few years. As a result it has become necessary, like in all other racing hobbies, to keep track of lap times. This is usually done using IR transmitters and receivers. However, in the case of FPV vehicles this timing can be done without extra IR or RF beacons. By making use of the already present video transmitter on each vehicle, it is possible to keep track of every vehicle in the race using their specific video channel assigned to them. However, there are some problems associated with using the 5.8 GHz emissions to track the vehicles. The first is the spacial accuracy where a timing event is triggered, the second is the effort needed to continuously monitor the full FPV RF band. This then turns into a antenna and receiver problem.

A very easy and cheap solution exists for the antenna. Using some cardboard, foil and aluminium or copper tape it is possible to build a high-gain wave-guide antenna at 5.8 GHz. A slotted waveguide antenna has a thin disc shape antenna pattern which is perfectly suited for use as a timing gate. Below are some photos showing the construction of the antenna.

Cardboard structure

Cardboard structure

Adding the feed structure

Adding the feed structure

Feed in place

Feed in place

Wrapping the rest of the waveguide in foil

Wrapping the rest of the waveguide in foil

Cutting waveguide slots

Cutting waveguide slots

Testing the antenna

Testing the antenna

The receiver needs to be broadband to be able to monitor all of the vehicles at the same time. For this I would suggest mixing down the antenna signal to below 1 GHz and use multiple RTL-SDR dongles as a low cost solution to detecting all of the multi-copters in real-time.

Unfortunately I have run out of time to work on the receiver. I will upload further data if there is interest in reproducing the antenna or continuing the project.

FPV Quadcopter Racing

After slipping into the world of quadcopter as few years ago, mostly for research purposes, I recently stumbled across a sport called FPV racing. This most commonly involves a small quadcopter equipped with a forward facing camera. The pilot then straps a monitor to his face and attempts to fly a designated race track. This is like Red Bull Air racing, just less expensive and less dangerous.

I built a 3D printed quadcopter based on the MHQ 2.0 on Thingiverse. All of the electronics were sourced from bandgood.com which made this probably the cheapest racer on the planet. Instead of buying the expensive Fatshark FPV goggles, I equipped myself with a boscam transmitter and receiver along with Quanum foam goggles (Poor mans fpv option). In the end I really became comfortable with this setup with no regrets.

Yesterday I took part in the first race of the first official FPV racing competition in South Africa. My 3D printed underdog even made it past the first heat…

Below are some photos of my 3D printed fpv quad and South Africa first historic FPV race:

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The intelligent ski-course

Last year I started building an intelligent ski-course, which was basically a set of buoy drones that would swim to there positions in a public river to form a ski course. This would then allow tons of interesting possibilities apart from easy setup, such as changing the shape of the course. Most of the project is documented on hackaday.io. As I am a very eager slalom skier in dire need of a course for the local public lake I would very much like to finish this project. However, at this point I find myself without time working on finishing my PhD. I will one day restart my efforts, if anyone is interested in developing the project from where I left it, you are more that welcome, just keep me in the loop.

The state of the project:

  1. Most of the code has been written.
  2. The electronics for the buoy and base station has been mostly finalized
  3. 3D printed parts needs to be revised
  4. Propulsion system needs to be tested
  5. Control system must be programmed
  6. Support must be added for the rest of the buoys

Birth of FEMU 2.0

As part of my PhD research, characterizing the propagation environment of the SKA Karoo site, a multi-copter RF metrology vehicle was developed. A dramatic autopilot failure in our early prototype caused us to lose the entire vehicle. This gave us a clean slate to do a full redesign upon what we have learned. The main problems with RF metrology using a multi-copter is the effect of the multi-copter itself on the measurement which at this point has not been properly addressed in research. Therefore, we set out to design a vehicle that could be properly de-embedded from a measurement.

The performance of antennas on-board these vehicles are in most cases unknown or assumed. These antennas have a certain characteristic pattern which could cause significant fluctuations in the measured signal, depending on its orientation. Even if the orientations were kept constant, the antenna patterns are sensitive to changes in metallic structures of the vehicle. A good example of this is the replacement of a battery after flight. The replacement battery might have slightly different dimensions, position and will most certainly perturb some of the large power cabling.

Our approach was to shield all of the subsystems of the vehicle in a metallic enclosure. This gave us a vehicle which had a predictable antenna pattern over time. Also, by closing the complex metallic environment, accurate antenna simulations have been made possible. A paper will be published on this shortly. Additionally, FEMU 2.0 also boasts a quasi-isotropic antenna pattern and a bandwidth of 260 MHz to 960MHz (See the paper for more information on this).

Hopefully this will pave the way for RF metrology using multi-copters. If done correctly this could significantly speed up measurement time and deliver measurements that are spatially continuous. The entire vehicle has been constructed from 3D printed parts and local hardware supplies. The electronics, receiver and antenna systems can all be made available if another research group is interested in further developing the project.

Below are 2 images showing FEMU 2.0 during setup and measurement.

Setting up FEMU 2.0 before flight FEMU 2.0 during measurement

 

Don’t leave me…

Since the last post we have completed a year of successfully RF measurement campaigns. Sadly, the quadcopter seen in the last post (FEMU 1.0) underwent a autopilot failure  during a measurement dry-run which caused it to fly straight up into the air until its battery died. It has never been seen since… This forced us to build FEMU 1.5 in 3 days. being a temporary vehicle, FEMU 1.5 was decommissioned shortly after his first measurement campaign (25 flights). All this happened in the earlier part of the year and gave us the opportunity to do a complete redesign which will be discussed in the next post. This redesign formed a large part of my PhD degree and made attempts to break new ground in RF metrology using Multicopters.

Reward if found poster for the disappearance of FEMU 1.0

Reward if found poster for the disappearance of FEMU 1.0

This was also the start of our quest to 3D print antennas, see the 900 MHz antenna mounted at the bottom.

FEMU 1.5 with a 900 MHz antenna

FEMU 1.5 with a 900 MHz antenna

My 3D Printer

I decided it would be a good idea for our lab, and for me, to have a 3D printer at our hands. After some consideration I ordered a kit from a local supplier DIY Electronics whom I would gladly recommend. The kit, a relatively new design, is named a Prusa i3. Up to now, after the printer has been assembled and improved. I have yet to decide on a proper name, in the mean time he will be called wall-E.

Below is a very crude time lapse I made using my GoPro 3, suction-cupped against the wall, taking some stop-motion photos.

After installing the Repetier host and firmware and getting the end stops and motors working in the correct directions I was able to successfully print a cube. Which I thought was way too easy. After some numerous calibration guides and models from thingiverse I have calibrated my machine enough to print its own parts. The list of which I will share at the end of this post. Below is 2 photos of wall-E, he was originally just white, but now has a few black parts which were all self printed with PLA.

printer2 printer1

MG at Langebaan

Here is the MG close to our favorite kite-boarding spots Langebaan and Sharkbay. This was taken at the mill house close to the local Club Mykonos casino.

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RFM22B on a Raspberry Pi

After about 3 weeks the Raspberry Pi RFM22B has arrived. The boards have been soldered and tested. There is some setup needed to get the SPI interface working, this is all explained at the top of some example code I am attaching at the bottom of this post. I built a total of 3 of these boards, each with a differently tuned RFM22B board. These boards are being used on my Quad copter as a small lightweight spectrum analyzer. I have attached both the code that I am currently using to drive the modules as well as a .zip file which can be directly submitted to Oshpark for fabrication. If you manage to try it, let me know how it works for you. The code I have written at the moment only covers setting the frequency, IF filter and pre-amp and allows the programmer to request the current RSSI level. I have also attached a LED which is directly connected to the GPIO pins just to play with.

Boards arrived from Oshpark

Boards arrived from Oshpark

Board mounted on GPIO port of Raspberry Pi

Board mounted on GPIO port of Raspberry Pi

Python driver code

Fabrication files

Schematic

Schematic

32 bit spectrum analyser update

The 32 bit spectrum analyzer has been used for multiple tasks in its dodgy prototype form. However it has been decided to slightly change the final design. This choice is motivated mainly by the application I am using for my Masters degree. Therefore, this is the end of the development for the 32-bit analyzer and the birth of the raspberry-pi analyzer. The specification will be much the same as previously listed, however the micro-controller will be replaced with the raspberry pi which will communicate directly to the RFM22B chip via its own SPI channels. The software will all be written in python and will have much the same functionality as my current 32bit spectrum analyzer design.

In the mean time I will attach the MPLAB project file and python source code used to interface with the device below. The MPLAB project file clearly lists the pin allocations which eliminates the need for a schematic. Finally see the board layout for the raspberry pi spectrum analyzer board ordered from OSHPark an rendered by their website.

Top and bottom board layout of new raspberry pi spectrum analyser

Top and bottom board layout of new raspberry pi spectrum analyser

MPLab project for 32 bit spectrum analyser

Python interface code for host PC

Note that his was not by far a final design of the code, there are still many small bug fixes and streamlining needed for example, the communication protocol between the device and host still needed to be sent byte wise and not by character, however feel free to use it for experimentation. Below is a photo of the 32 Bit spectrum analyzer measuring a sweep with a resistively loaded mono-pole antenna built by my colleague, Matthew Groch.

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New set of legs for SKA measurements…

With a lot of help from a friend, Johan Frank, new legs and a platform was built for the quad copter. These legs allowed for a antenna to be mounted below a platform supporting a spectrum analyzer and single board computer. This platform was specifically built for a measurement campaign at the SKA site which turned out very successful.

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