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:

Pilots flying Race start image2

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

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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Crashed…

After flying in loiter mode for a while the quad copter suddenly flipped over. I immediately switched to Stabilize mode and increased throttle to try and save the situation. It stabilized itself just before hitting the ground but sadly had too much horizontal velocity when lightly snagging the grass. This caused the copter to roll and break 2 motor mounts and a landing gear. The cause of the flip was because of a bad connection to 1 of the esc’s from the APM 2.5 output.

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Upgrading to APM2.5…

Upgraded to the APM 2.5. Height and GPS accuracy are significantly better than the APM 1.0. The APM 2.5 also has a much higher telemetry data throughput due to some of the signal processing happening off of the Atmel chip. Below is a photo of the much more neat setup containing the APM 2.5.

APM 2.5 setup

APM 2.5 setup

Stellenbosch from above…

After receiving my raspberry pi camera I thought it to be a good idea to to take some aerial photos with the quadcopter. The first few photos are from Stellenbosch University campus followed by the last few which are from my girlfriends farm near Worcester.

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First diffraction measurement with fEMu

After getting the quadcopter to fly in loiter and auto missions with confidence it was decided to use the quad copter for what it was originally intended for. To measure RF signal propagation. This measurement was done with a transmitter on one side of a 13m high man made berm and flying a vertical path up to 50m on the far side measuring the diffraction of the continuous wave signal at 400MHz. Below is a video with the payload strapped to the quadcopter, note that the antenna (yellow block) was exchanged for a small stub antenna at the actual measurement. The measurement was done using my RS232 spectrum analyser logging onto a Raspberry Pi. The effect of the quad copter on the antenna pattern was ignored and the measured data was treated as relative. The measured data can be seen below and clearly resembles a diffraction pattern. This pattern has been verified against some prediction code of a colleague. As a first test this proved very successful as a proof of concept and was hereafter named fEMu (flying Electromagnetic Metrology unit).

Measurement on upward and downward flight

Measurement on upward and downward flight

Quad-copter first ever successful loiter…

After the vibration issues were sorted out it was time to test the loiter and auto modes. A video of the first successful loiter test can be seen below. The APM 1.0 seems to hold its position reasonably well given that there was a slight breeze that day, the quad-copter was also landed in loiter mode, in this video, by simply decreasing the throttle. The video was taken behind our engineering building at Stellenbosch University. We have also tested the auto mission, takeoff and landing successfully.

The new motor mounts were made by Johan Frank from Malmesbury on his own CNC machine with simple number-plate plastic. These new motor mounts are much less brittle and thicker, therefore much stronger than the original perspex parts.

New Motor Mounts

New Motor Mounts