Here is a tip for the RC heli crowd on how to repair or extend the antennas of the popular AR7200BX flybarless controller.
AR7200BX is pretty much a Microbeast BeastX flybarless controller bundled with a 7 channels 2.4 GHz Spektrum DSM2/DSMX receiver. This setup is really nice because it saves a lot of wiring and cable management for those flying with Spektrum / JR radios, not to mention it saves space and weight too.
The receiver has two antennas in order to address polarization issues. In the GHz range polarization is very important and on RC helli is a challenge to maintain consistent Tx-Rx antenna polarization. Often the only instance when the heli is is in normal orientation (blades up, skids down) is just until take off ( 3D pilots know exactly what i am talking about). The two antennas need to be oriented in a way to cover at least two different planes (X and Y, and ideally the third plane Z as well via a satellite receiver). The two AR7200BX antennas are normally placed at 90 degrees to each other. The problem is that they have to clear the carbon fiber frame and stick out sufficiently so the antennas are not "shadowed" by the helli's fuselage
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I made this antenna mount for my 450 PRO using a plastic straw from a compressed air can, a cable tie and a cable tie mount (the self-adhesive foam was removed from the mount and replaced by single side adhesive one so the mount does not adhere to the controller - it is held in place by the velcro strap). It works great to maintain good 90 degrees polarization difference. The thin grey 1.13 mm antenna coax can be seen exiting the black grommet of the FBL controller.
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The two antennas are made out of miniature coaxial cable (1.13 mm) and are long 110 mm and 40 mm respectively. The actual antenna element is the end portion of the coax, where the outer jacket and the coax shield are removed leaving exposed about 31 mm (1/4 wavelength @ 2.4 GHz) of the insulated center conductor.
As per Murphy's law: one needs just a few centimeters more coax to clear the heli's frame or other parts. Another potential problem is that these antennas can get damaged easily in a crash or just by touching to the sharp carbon fiber frame edges - such thin coax is very fragile.
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AR7200BX is pretty much a Microbeast BeastX flybarless controller bundled with a 7 channels 2.4 GHz Spektrum DSM2/DSMX receiver. This setup is really nice because it saves a lot of wiring and cable management for those flying with Spektrum / JR radios, not to mention it saves space and weight too.
The receiver has two antennas in order to address polarization issues. In the GHz range polarization is very important and on RC helli is a challenge to maintain consistent Tx-Rx antenna polarization. Often the only instance when the heli is is in normal orientation (blades up, skids down) is just until take off ( 3D pilots know exactly what i am talking about). The two antennas need to be oriented in a way to cover at least two different planes (X and Y, and ideally the third plane Z as well via a satellite receiver). The two AR7200BX antennas are normally placed at 90 degrees to each other. The problem is that they have to clear the carbon fiber frame and stick out sufficiently so the antennas are not "shadowed" by the helli's fuselage
.
The two antennas are made out of miniature coaxial cable (1.13 mm) and are long 110 mm and 40 mm respectively. The actual antenna element is the end portion of the coax, where the outer jacket and the coax shield are removed leaving exposed about 31 mm (1/4 wavelength @ 2.4 GHz) of the insulated center conductor.
As per Murphy's law: one needs just a few centimeters more coax to clear the heli's frame or other parts. Another potential problem is that these antennas can get damaged easily in a crash or just by touching to the sharp carbon fiber frame edges - such thin coax is very fragile.
Here is my Align Trex 450 Sport V2 DFC. The AR7200BX is mounted inside the frame on the gyro plate. I wanted to have the short antenna pointing vertically downwards but alongside the plastic skid frame. This way, the frame will act as a mechanical shield protecting the antenna whip. Unfortunately, I need about 10 mm more coax on the short antenna to really clear the heli's CF frame. Another good antenna location is behind the anti-rotation bracket of the swashplate but the coax is way too short to reach there too.
After some investigation here is what I found out: the AR7200BX is using standard IPX connector on the cable assembly for both antennas
This image is courtesy of Helifreak member sup77095. It shows both miniature IPEX / IPX coax connectors on the receiver board.
As it turns out IPEX / IPX cable assemblies are dirt cheap - just search on eBay for "IPX cable". They are used as interconnects for many wifi devices, inside laptops, cell phones, etc and always come in the form of a "pigtail" (ready-made cable assembly). I got two cables completed with connectors for under $4 (free shipping too).
Just measure and cut the length needed for an extended antenna or repair then very carefully, using a sharp blade, strip the outer insulation and coax shield about 31 mm from the end, to expose the insulated center conductor and that's it. Be very careful not to damage the center conductor and the teflon insulator around it.
(Probably not need, but I'll mention anyway that such modification will void the warranty on the FBL unit and it is mostly for the brave ones)
The coaxial loss is ~3.1dB/m @ 2.4GHz or 0.031 dB per centimeter. One needs to optimize the length to the absolutely minimum needed to avoid signal strength issues in the receiver but generally up to 10 cm extension for the short antenna should be OK.
If you fly your heli as a "dot in the sky", installing a satellite receiver is recommended anyway.
Remember to perform Radio RANGE CHECK after doing any antenna work on your heli.