Thread: New Product RFD900 - Long range radio modem
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Old Aug 20, 2012, 08:02 AM
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Australia, QLD, Brisbane
Joined Nov 2011
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Regarding which is better - the answer is : it depends !

Depending on the antenna and mounting I would generally expect that having an external antenna on the airframe would give the best result.
Reasons being that for a 1/4 wave monopole - a ground plane is required.
The module itself does provide some RF grounding, but, having the antenna external, with a groundplane, is a far better and consistent solution for a monopole. It will also give the best gain and radiation pattern, which will far outweigh the loss in a short cable. Longer cables - it depends on the installation and requirements. As a general rule, shorter RF cables are better, but, depending on the installation, a longer RF cable may make it so much tidier.

Also having a degree of separation between antennas will generally improve the perfomance of all the radios as they typically generate noise in other bands.

Another thing to consider is desensitisation (desense). This is the reduction of sensitivity due to coupling of noise in the RF bands from the electronics on UAV into the antenna. Ideally, all the RF noise producing units (microcontrollers, servos, brushed motors, ESCs etc.) should be shielded and the antennas far enough away not to couple noise into them. Practically, having enough separation between the electronics and the antennas will help enough in most cases. There are some products in development, like servo line RF chokes, GPS band notch filters for transmitters, Antenna groundplanes, etc., which will give another way of reducing noise.

The modems from memory have quite little power drop with voltage drop, giving rated power down to around 4.5V or so. I can check the measurements tomorrow. That said, reasonably thick wire is better.

On the RF cables, I will be able to measure the cables once I get them - the estimated loss for a 15cm cable is around 0.2-0.3dB, for 50dm cable, maybe 0.5dB.

Yes, only 4 lines are required, GND, +5v, TX, RX.

Hope it helps !

Originally Posted by small_rcer View Post
What is the output loss caused by connecting multiple and longer coax cables between the antenna and the module?

Is it a better solution to keep the antenna directly connected to the module and extend the 4 lines that would be required from the UART on the autopilot? i suspect that only +5, GND, Tx and Rx are required. Wouldn't a braided or shielded 4 wire cable to the remotely mounted module and antenna be a better RF solution rather than extending the coax?

As well the module and any RF it generates would be moved farther away from things like GPS and the aircraft receiver and any possible video camera and video Tx caused RF leakage. The only downside is the 800 ma current that the unit draws might require an 18 or 20 gauge wire if any distance is involved.
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