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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  • I tried to achieve the same results using an android device. Check out my blog post.

  • MR60

    Hi Trung, ok I understand your question now : your current ground module has a fixed USB connector instead of the pins, right ? In that case I could not say for sure because the european version I have does not come with a usb connector. What you should try : look closely at the printed circuit board of your ground module to see if you could find these markings  : GND, 5V, CTS, RX, TX, RST. If so they must be a way to solder wires on the four that interest us, i.e. GND, 5V, RX, TX. There should be a serial to usb chip on the board where you could find these pins.

    Hope this helps.

  • 100KM

    Sorry Hugues, I was not clear with my question. I completely agree a bluetooth link would be preferable to a hard wired usb connection. I wish to do the same, but I do not have a spare telemetry module. I would like to use my ground module. What I intended to ask is, could the same be done by connecting the bluetooth module to the "ground" version 3DR module (usb connector) vs. an "air" module (with the pins as you have used)? I hope this is clear. I am not sure how to attach photos in comments, otherwise I would try to illustrate.

  • MR60

    Hi Trung,

    the difference would be that i would then have to hang this usb cable and ground module on my tablet which is not confortable to fly in the field. With a Bluetooth connection, i place the BT+ground module on a mast and I can move around freely with my Bluetooth tablet running droidplanner (mission planner on android). This frees my tablet from any cable and makes it convenient to fix it on my radio tx.

  • 100KM

    Hi Hugues,

    How different would this project be if you were using a "ground" 3DR module with a USB connector?

  • MR60

    @daniel, at the moment I only broke one 3dr module, so I am using the dipole on one unit; the second one still has its duck antenna. However seeing the result I intend to build a second dipole too.

    @Nick, thx for the link.

  • Good write-up! I have been doing something similar for sometime.

    I believe that Mission Planner has issues with bluetooth. See the following link for description and workaround; 

  • Great work Hugues. I've been interested in improving the rubber ducky antenna every since it got ripped off mine. Are you using the custom Dipole for both sides? I hope it increases range too, since mine cuts out at 20 feet.

  • you are right.. my purpose doing this is to get rid off all cable hanging around my laptop while I'm on the field. And i can switch to my android at any time. As long as my 3dr-radio-bluetooth-relay-station is near :D

  • MR60

    @bennybee, the solution to your problem would be to connect a second ground Bluetooth module with ftdi cable on your PC. But then I guess it keeps a cable dangling on your PC.

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