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I am building a prototype for a remote controlled vessel, the LoRa Marine Environmental Ranger (LoRa MER) that gathers environmental data and relays it to a shore based station using Arduino MKR WAN 1300 LoRa boards. This post is the sixth in a series, and describes the boat afloat (but no power) and a DIY dipole antenna.
Implementation Status
Changes from last week are in red bold.
Boat
- RC control and motors tested on land
- MKR WAN 1300 firmware tested
- Following sensors / hardware fitted temporarily and tested
- ST3775 TFT Display
- ICP10100 Atmospheric Pressure / Temperature
- NEO-6M-0-001 GPS
- Thermistor Water Temperature
- Thermistor Air Temperature
- KX224-I2C 3 axis Accelerometer
- RPR-0521RSLight Sensor
- TSW-10 Turbidity Sensor
- It floats but the radio control failed during the test
Shore base:
- Enclosure completed
- TFT screen fitted
- Firmware has LoRa reception (polls) and responds with RSSI
- MKR WAN 1300 connected to Raspberry Pi by USB serial
- Python script sends data over internet to adafruit.io
- Dashboard implemented on adafruit.io
- Fitted a DIY dipole antenna
First Test in Water
The MKR WAN 1300 and all the sensors were placed in a plastic bag for a bit of water proofing and then into the boat.
After successfully running the motors and RC controls at home we went down to the duck pond for the first tests in water. Well this is embarrassing.... No response at all from the motors. The poor boat just sat there in the water. At least the LoRa portion and all the sensors were working.
After returning home, everything seemed to be working again. Maybe a loose wire? I plan to remove the electronics in the boat this coming week and see if I can spot the problem.
DIY Dipole Antenna
One of the things that has been bothering me (although they are working) are the antennas. I know I can get better range, and in the case of the "base station" the antenna is squished into a tight enclosure. So I thought I would experiment a bit and build my own dipole to compare to the Molex antennas purchased earlier.
The build is very simple. I soldered insulated 22 AWG wire to the pin inside a uFL coax cable and another to the outside threaded connector. A small piece of heat shrink was placed over the solder joint. The formula for estimating the total length of the dipole in meters is 143 / frequency in MHz. The length of the two elements in the dipole is then half the total length. For 915 MHz the total length would be 15.6 cm and the length of the elements 7.81 cm. The wire is left long and then trimmed back to the optimal length.
My crude tests consisted of bending the wires back over themselves a bit and seeing if that improved RSSI on the other radio. If it did, I snipped off that bit of wire and kept going until there was no improvement. In the photo below the DIY antenna is shown beside a rule and the Molex antenna.
I then compared the DIY dipole to the Molex - the DIY dipole gave approximately 2 dB improvement in RSSI, a pleasant surprise. Also nice, the ground portion of the dipole just fits in the enclosure I am using with the other element sticking outside as shown in the photo below.
Next Steps
- Tear down the boat and look for the cause of the RC failure
- Tests in water
Comments, corrections, and ideas are always welcome!
Links
MKR WAN 1300: LoRa Marine Environmental Ranger - This is the landing page which outlines the project
LoRa MER Week 1: Making it Portable
LoRa MER Week 2: Boat in a Box
LoRa MER Week 3: Building a Shore station and Starting the Boat
LoRa MER Week 3.5: Testing Turbidity and Posting Data to Internet
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