Aerial RF Measurements
Early in 2017, Anritsu demonstrated that it is possible to operate a 70 GHz spectrum analyzer while it was being flown on a 500 class photo drone. This allowed the users to take RF channel power measurements at 24 GHz, view the spectrum while in-flight, and map the resulting RF channel power measurements. This demonstration is a proof-of-concept and not a commercial product.
This information provides the “how it was built” data for anyone who might want to do something similar. Please note that country-specific government regulations concerning commercial drone usage may apply.
For this example, we chose a commonly available drone, the DJI™ Phantom™ 2*. It is not very automated, but it is representative of the 500 size photo drones, and comes without a camera. For the purpose of this demonstration, the lack of a camera saves cost, reduces potential failures, and gives us a bigger payload.
The Phantom 2, when its GPS is synced, has a number of automatic features, typical of its class. It has auto-height hold, making it somewhat independent of payload, auto-leveling, making it somewhat indifferent to center-of-gravity concerns, and auto-position-hold, which makes it independent of wind, within limits. Other drones have obstacle avoidance, making the flight task even easier. So flying the auto-drones is a “steer it to where you want it to be” exercise. Drones in this class are not meant to exercise your flying skills, they are meant to put your camera, or other payload, in a particular position.
The Phantom 2 does not have a payload specification from the manufacturer. However, a check of on-line turned up some user tests showing that it could hover with up to 28 Ounces (785 grams) of payload for up to 14 minutes. This is strictly user experience, not an official specification, but the postings provided an indication that the Phantom 2 would work in this application.
Flight Package Wiring Diagram
The illustration above shows the flight package wiring diagram. Let’s look at each component starting from the drone’s flight battery and going towards the MS2760A Ultraportable Spectrum analyzer.
- The regulator chosen was a Castle Creations 10 amp Battery Eliminator Circuit (BEC). This particular provider has a good reputation in the radio control hobby.
- The BEC comes set to 5.1 volts. That’s not the best voltage for the Intel Compute Stick. To set it to 5.0 volts you will need the Castle Link USB Programming kit.
- Power Supply Cable
- To get voltage from the BEC to the Compute Stick, you will need to create a custom cable.
- We started with a USB type C cable and cut one end off. Two of the wires found in the cable proved to be red and black, corresponding to the +5 volt wire (Vbus) and the ground wire (GND) respectively. Your cable may be different. Please check your result with a Voltmeter before connecting the Compute Stick! Smoke is bad!
- Please refer to on-line documentation for the USB Cable pinout.
- The Compute Stick draws about 1A at 5 V, while the MS2760A draws about 0.7 A at 5 V.
- Intel Compute Stick m5
- The MS2760A requires a PC of some sort for control. In this case, we sourced an Intel Compute Stick, the STK2mv64CC. Lacking a display, it is very light weight, coming in at 2.1 Oz. (59 grams). This Intel computer comes fully configured, with the exception of the operating system. Once loaded with Windows, it is more than sufficient to run the MS2760A hardware and software.
- We chose, and purchased, Windows 10 professional for our operating system.
- Windows 10, like most Windows product, becomes unhappy if you suddenly remove power. Sudden power removals are likely to happen in a flight environment.
- Recommendation: a separate payload battery might make extended flight operations simpler and more reliable. It would require a drone with a larger payload capability than the 500 class drone we used for the proof-of-concept.
- Recommendation: It would be worth exploring Windows IoT, as it seems likely to be able to handle hard power-downs, though it is unknown if it will work with the spectrum analyzer software.
- USB Hub
- The compute stick has only one USB A port, so we need to use a USB hub to allow connection to the MS2760A, and the USB based GPS unit. The four port hub chosen allows us to also connect a mouse and keyboard as needed.
- The USB hub is 3.0 compliant, as shown by the blue plastic on the connectors. The link between the Compute Stick and the MS2760A must be USB 3.0 compliant.
- USB Cable
- The USB cable we used was 1 foot long (30 cm) and was v3.0 compliant. It was type C on one end and type A on the other.
- USB GPS
- We used a separate GPS for the data collection. In this case, it was a BU-353S4, a GPS powered, Windows compatible, GPS sensor.
- We used custom software to combine the GPS readings with the RF Channel power readings from the MS2760A.
- Tuned Wire Antenna
- The flight antenna, with attention paid to weight, was constructed from a length of semi-rigid coax. The cable was cut to be about 3 inches long (7.6 cm) and then the sheathing was stripped back until the newly constricted antenna was resonate at 24 GHz. We used an Anritsu S820E in Return Loss mode for the antenna tuning.
- The semi-rigid coax we used had a SMA connector. To adapt it to the 70 GHz MS2760A’s type V connector (male) we needed an adapter. We used an Anritsu adapter, part number “34VFKF50” which is a DC to 40 GHz, V(f) to K(f), 50 Ω adapter.
The ground package consists of a control station, display and keyboard, extra batteries and chargers, and a transit case.
- Compute Stick Accessories
- The keyboard chosen has an integrated touchpad. This eliminates the need for a mouse when the drone is on the ground, and also eliminates the need for a smooth surface to run a mouse on.
- The Keyboard/Touchpad is wireless, eliminating a connection when the drone returns to the origin.
- The display is an HDMI compatible display, and battery powered. Since the HDMI cable supplied with the Compute Stick is quite short (6 inches, 15 cm) we recommend an extension HDMI cable.
- We had difficulty seeing the full windows display on two models of HDMI display. The Compute Stick display ran off either the right and left sides or the top and bottom of the display at any resolution. We recommend looking for a display that is very adjustable.
- Laptop Setup
- The laptop and the Compute Stick both have Wi-Fi, and Windows is rather Wi-Fi friendly.
- The Pocket router shown as an example is the ZyXEL MRW102. It’s powered by the USB cable shown.
- Once the Compute Stick and the Laptop are both linked to the pocket router, they are on the same network and it becomes much simpler to link applications between the two computers.
- It is strongly recommended to make all Windows Wi-Fi links “Auto Connect” so they will automatically reconnect if the drone flies out of range and then back into range.
- Remote Control
- Once the Compute Stick and the laptop are on the same network, it’s possible to remotely monitor and control the MS2760A.
- Install MS2760A software on both the laptop and the Compute Stick
- Open the Laptop version of the MS2760A software
- Click on System Settings, Auto Connect, and turn it off.
- Click on Host, and enter the IP address of the Compute Stick.
- On the Compute Stick, you can get the Compute Stick’s IP address with the Command Prompt “ipconfig” command from the Command Prompt window.
- Click Connect once the IP address is set.
- The Laptop version of the MS2760A software is now controlling the instrument physically connected to the Compute Stick.
- With the ZyTEL pocket router, we were able to monitor and control the MS2760A as far away as 100 meters. Other routers may have greater range.
- Flight Batteries
- It’s a good idea to buy two or three extra batteries and charger to go with them. The battery charging time is typically 30 to 45 minutes, so with a set of 3 or 4 flight batteries, you can keep flight operations going almost continuously.
- If you are flying in an area without AC (mains) power, you will want to consider a way to charge batteries from your vehicle, or simply buy enough batteries to accomplish your task.
- Transit Case
- There are a wide variety of transit cases or back packs available on Amazon for the popular drones. The one shown was designed specifically for the Phantom 3, but it worked for the Phantom 2 as well.
Assembling the Payload
The payload consists of the MS2760A, the Compute Stick, the USB Hub, and the GPS. The first three items can be grouped by use of the 3D printed assembly shown above.
Download the Printable 3D files (STL file format) at:
Mounting the Payload
When mounting the payload (MS2760A, Compute Stick, and USB Hub) to the drone, it’s good to think about crash-proofing. While it’s not possible to completely guard against the most serious impacts, providing an impact absorbing payload mount will help in many cases.
In this case, we created a custom aluminum plate that mounted on the Phantom 2 using the bolt holes intended for the camera. This plate had a ¼ inch hole drilled for the nylon bolt used for the MS2760A mount, pictured above. The bolt is ¼ inch in diameter with 20 threads per inch pitch. The thread is ¼ by 20. The nylon bolt attaches the payload to the drone and also acts as a mechanical fuse, or break point, for the payload if an unplanned landing is rough. The Hook and Loop fastener made by Velcro®** also stabilizes the payload and has some give in the event of a crash.
Secure all the cables to keep them from moving during flight
It’s important to make sure that all cables are secured to the drone. A cable may become loose or unplugged during flight. The flight package is subject to prop-wash, vibration, and impacts.
A more serious issue is cables getting in the propellers. That can, and will, bring the drone down suddenly. This may result in personal or property damage as well as the loss of the drone and/or the MS2760A. It is critical to make sure that cables cannot move into the arc of the propellers.
During the initial setup you will be powering the payload up and down quite a bit. Since the Compute Stick is loaded with Windows, you need to make sure you power it down properly. Sudden power removal is also known as a crash.
The proof-of-concept discussed here used the drone flight battery as a voltage source for the payload. This means that we needed to establish a link to the Compute Stick and power it down every time before changing the flight battery. If we had flown the flight battery to exhaustion, it’s possible that the drone would power down on its own, without our intervention.
While the Compute Stick held up fairly well to the inadvertent sudden power downs it experienced, there is no guarantee that it will recover. We recommend a separate payload battery, if the drone payload margin allows the weight. A 2 cell 1300 mAh LiPo battery would last about 40 minutes if you allow it to draw down to 80% of its capacity. These batteries are available on-line and weigh approximately 3.5 Oz (100 gm). You will need a dedicated LiPo charger for these batteries.
It’s possible and practical to fly an ultra-portable spectrum analyzer, such as the MS2760A on one of the many drones on the market today. With the current move towards 5G and the high frequency range anticipated for backhaul, the ability to position a 70 GHz spectrum analyzer in the air will have many uses, some anticipated, and many not anticipated. It’s going to be interesting!
*DJI™ is trademarked and the manufacturer of the Drone Phantom™ 2. For more information, please visit their website at: http://www.dji.com/terms
**Velcro is a registered trade mark. For more information, please visit their website at: https://www.velcro.com/legal-and-privacy/legal