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This is a short video to test the amount of signal loss when using extension cables or adapters to connect the FPV Goggles to the Antenna(s). I’m using the DJI FPV Goggles V1.
The test scenario, like the other ones that I do “in-house” (check my playlist of technical videos using fixed ground station for receiving and transmitting antennas : • Dahsware - How to inte... , consists in the drone (transmitting antennas) in one corner of my room, and the goggles (receiving antennas) in the opposite one, with lots of rooms and concrete in between.
To ensure that the receiving antennas remain in exactly the same position during the tests, I’m using a self built ground station (a microphone stand with some 3d printed parts), with 4 holes to install the cables that subsequently go to the goggles.
The ground station has been set-up with 3 stock antennas, connected to to Goggles via rg58 coax cables, leaving the top left connector in the goggles free (the one close to the channel indicator leds).
In the first test, I connected my best receiving antenna, the TrueRc Sniper II, directly to the Goggles in the free connector, pointing in the direction of the transmitting antennas, and recording a 2 minutes DVR video.
In the second test, I connected the Sniper II at the ground station, in exactly the same spot where I kept the goggles before, and connected the Sniper II to the Goggle through a thin RG174, 2 meters long cable, and made another DVR recording.
Lastly, I replaced the thin RG174, 2 meters long cable that connected the Sniper II to the goggles, with a thicker, RG58, 1 meter long cable ( www.thomann.de/gb/pro_snake_s..., and made the last DVR recording.
I averaged all the samples of each recording, and here’s the results :
1. Direct connection between Sniper II and Goggles : 35,7 mbps
2. 1 meter thick RG58 cable to connect the Sniper II to the goggles : 30.2 mbps ( - 15% signal )
3. 2 meters thin RG174 cable to connect the Sniper II to the goggles : 11.7 mbps ( - 67% signal )
Also, I made a separate test to compare the signal quality when using a small adapter, in this case a 5 cm long, semi-rigid, SMA male to SMA female adapter. The result, as can be seen in the background videos, tells that the usage of an adapter brings a -15% signal loss, passing from 23.9 mbps to 21. That’s a huge loss, and can be avoided buying antennas that have natively the same connector as the VTX / Air unit that they will be connected to. In the case of the DJI system, I suggest antennas that have a MMCX connector. Of course it’s simpler to get a 3d printed TPU part and use MMCX to SMA adapters for the DJI AIR UNIT, but -15% signal is the price to pay. I will definitly switch to native MMCX antennas for the DJI Air Unit, after having seen those numbers.
Result analysis :
If you want to use a cable extension to connect the Goggles to the FPV Antennas, you really need to buy the best cable you can put your hands on. RG58 or LR400 / LR240, large coaxial cable diameter, double shielded, gold plated connectors, etc. Otherwise the signal loss will be simply too much and it wouldn’t work.
In my case, the idea of using my fixed ground station “in the field” for long range flights would have the PROs of always having the receiving antennas pointed in the right direction and elevation (that means to always have the best signal possible), and also relieving me from the stress of keeping my head still. In the CONs, of course, there’s the -15% signal loss.
I don’t have enough elements to evaluate this “on paper”, so I guess I’ll have to try it in the field and see what happens. I’ll try to follow the same path of a previous mission with the same configuration, but with the ground station and 1 meter rg58 cables, and compare the results.
Stay tuned and see you soon