Some of you might remember a previous blog post last summer about my trainer 3DR quad going into the trees.

As a result I broke the 433Mhz telemetry module and antenna, as you can see on the picture above (SMA connector was ripped off the board).

This module was in my drawer since then. At the price of such a module I really did not want to throw it away.

On the other hand I wanted to try using bluetooth to connect my Nexus 7 tablet (with Droidplanner, thx Arthur!) to my X8 Octo. I use it for AP and I want to have my tablet and my TX radio together (not possible to frame a picture with a laptop and control the drone at the same time).

So here was an idea : Build a new antenna for the broken 3DR module, then connecting a bluetooth module to this 3DR module.

1-Let's start with the antenna built :

Due to the damage of the 3DR module, it was not possible to solder a new SMA connector. Plus I do not like very much the standard 433Mhz duck antennas. In these duck antennas we have a quarter wave antenna.

I decided to build a half-wave dipole antenna. Here are the parts to build it:

-A a regular servo lead (>200 mm long). Any other wire will do. it must be thin enough to be inserted in a plastic antenna protective tubing

-A 300 mm long (or longer) antenna plastic tubing like these:

3689560092?profile=original-Solder & iron

-Hot glue

-a Ruler (>30 cm)

Start cutting a 300mm long plastic tube. Mark its middle. Use a hot solder iron to melt a hole in the middle. This hole will be used to pass the servo wires through each half of the tube.

As it is diffciult to pass soft servo wires in such a tube, you can use a small rigid iron wire (take the one your wife uses to attach the roses in the garden...) on which end you solder the tip of the servo lead. Then pull the servo wire through the tube.

It should look like this afterwars:

3689560321?profile=originalAt this stage the two servo wires are extending out the tube.

We now need to cut each servo wire to measure exactly a quarter of a wavelength (i.e. total dipole length is a half wavelength):

Based on this relation between the speed of light (c), the radio wave frequency (v) , the wavelength is:

3689560281?profile=originalSo for a radio wave of 433Mhz, the lambda calculation gives (in MKSA standard units, sorry for inchies):

3689560342?profile=originalSo we have theoritically to cut the servo wires to have a total length of 34,62 cm.

HOWEVER, there is a bit of wire length extending outside the tube (to the 3DR module) AND

the plastic tubing contains carbon. Carbon is an electricity conductor. Therefore carbon based plastics have an impact on radio wave frequency tuning. A rule of thumb : the more plastic (carbon), the more the theoritical length should be shortened. I went for an horizontal length of 30 cm (+ about 1cm of wire outside of the tube, so a total length of 31 cm). 

Thus, take your ruler and measure 15 cm on each side of the hole (300 mm total length) and trim both ends.

Final step : solder the two wires on the 3DR module. At this stage we have to solder one servo lead (half of the antenna) to the ground, and the other servo lead (the other antenna half) to the antenna port of the module.

Be careful when soldering that the wires should never touch!

3689560286?profile=originalOn the picture, the red servo lead is soldered on the ground port of the 3DR module. The white servo lead is soldered on the antenna port.

Now it is time to check is this home made antenna is worth something. I compared two configurations as follows:

1-the 3DR module with its default duck (quarter wavelength) antenna (and a FTDI cable to the computer on which I'm running mission planner).

2-the 3DR module with this home made dipole (and a FTDI cable to the computer on which I'm running mission planner)

Of course the physical setup/relative positionning between the telemetry modules & antennas remains identical in both cases.

1-Chart with standard duck antenna: RSSI measured between [170-190]


2-Chart with home made dipole (half-wavelength) : RSSI measured  >=200

3689560358?profile=original The home made dipole wins !

(The second part on the bluetooth connection follows below)

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  • MR60

    @Bennybee, ash I understand what you were asking. No, for measuring RSSI, I connected the 3dr radio via ftdi cable on the ground side. Sorry mate.

  • APM > 3dr Radio > Bluetooth > PC Mission Planner?

    I tried that with no success on PC. When Bluetooth is paired, there are two serial port created and detected on Mission Planner. I have tried both. No success with 57600 baud-rate. How you manage this two COMs serial over Bluetooth problem on PC?

    However, it did successfully connected to my android (sometime forced closed).

  • MR60

    @bennybee, yes this is how I measured the RSSI !

  • Have you tried connecting on PC Mission Planner? 

  • MR60

    About the Arduino software:

    I used it to connect to the Bluetooth module via the FTDI USB-COM port:

    -connect the FTDI cable to the PC and the Bluetooth module

    -check in device manager which COM port has been defined by Windows

    -Launch Arduino IDE

    -In the Tools menu, select the right COM port

    -In the same menu launch the Serial Monitor screen

    Then in the Serial monitor command line, enter the "AT" command -> it should respond "OK". If not, check that the RX/TX wires of the FTDI cable are crossed (TX on RX and vice versa)

    Then configure the Bluetooth module to function at 57600 bauds with the command : "AT+BAUD7"

    It should respond with "OK57600".

    All done and ready for droidplanner!

  • MR60

    2-Let's go on with the bluetooth connection (using this dipole antenna):

    The objective is to use this custom 3DR module with a bluetooth module to connect to APM (on my X8 Octo) with droidplanner running on a NExus 7.

    List of materials:

    -a 3DR module (with an antenna!)

    -a bluetooth module such as the multiwii bluetooth module sold on Hobbyking for 7 dollars

    -a 5V UBEC

    -an electronic breadboard (or diy shield)

    -the arduino software (download for free from to use it serial monitor functionality.

    -Male PINs to solder

    The bluetooth module is shown below. (it has a molex connector in my case but you could have a 4 breadboard PIN connector depending on the model).


    I use an arduino proto board to solder three lines of six pins next to each other :
    Red=5V, Black=GND, Yellow=TXD, Green=RXD (CTS & RST not used as we do not use flow control)
    -a first line of 6 pin consists in the 6 pins of the 3DR module (just the four 5v-gnd-Txd-Rxd are used)
    -a second line of pins consists in the pins used to connect the bluetooth cables :


    At this stage be careful to CROSS TXD-RXD between the 3DR module and the bluetooth module. Why ? Because the 3DR module is seen by the bluetooth module as a terminating chain device to who it must speak to : so what is transmitted by the 3DR module (Txd) is received (Rxd) by the bluetooth module and vice-versa.

    -a 3 line consists only of 2 pins : 5V & GND. These are used used to power the 2 modules (3DR module and bluetooth module). They are fed a voltage between 3.6 and 6V with a 5V UBEC :


    The assembled system looks like this:


    Use a bit of hot glue to cover the antenna middle hole that you made to pass the servo wires and to reinforce the attachment point to the 3DR module.

    And final results works great with droidplanner ! :




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