RFD900, RFD900+ - New long range radio modem

Hi All,

I would like to introduce you to a new radio modem that we developed for very long range datalinks!


Some of the key features of the RFD900 are as follows:

  • Multi point and point to point link capability.
  • Long range >40km depending on antennas and GCS setup.
  • 2 x RP-SMA RF connectors, diversity switched.
  • 1 Watt (+30dBm) transmit power.
  • Transmit low pass filter.
  • > 20dB Low noise amplifier.
  • RX SAW filter.
  • Passive front end band pass filter.
  • Open source firmware / tools, field upgradeable, easy to configure.
  • Small (30 x 57 x 13 mm), light weight (14.5g).
  • Compatible with 3DR / Hope-RF radio modules.
  • License free use in Australia, Canada, USA, NZ.


These modems are designed to support long range applications, while being easy to use and affordable.  

These modems have been flying in various platforms and have demonstrated excellent performance in real applications. 

RFD900 modems are now available at: http://store.rfdesign.com.au

Support within APM planner and the radio configurator from Michael Oborne is already available.

It works seamlessly with APM planner, all radio Mavlink parameters are available.

Update, December 2014:  The RFD900+ with improved specifications is available now at:


Seppo Saario



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Hey guys, I have a noob question here.. If i purchase the RFD900, do I need anything else to attach it to APM 2.5 for it to work? I keep seeing "compatible with 3DR and Im kind of confused if you need both.

The RFD900 is a direct replacement for the 3DR radio - each end of the radio link will have either an RFD900 *or* a 3DR radio, not both.

i.e. you can have an RFD900 on one end and a 3DR on the other if you want, and things will still work, although the range will be lower than having RFD900's on both ends.

Thanks.. I just ordered the bundle after I saw a youtube video on it.. thanks for the help


Hi Seppo,

I purchased 2 of your RFD900s.  They look very well made :)

I have some questions:

1.  I was planning on using the  "PL2303HX USB to TTL to UART RS232 COM Cable module Converter" for the ground station modem ($8.80 USD at Amazon).  This will only provide +5V,Ground,TX and RX.  The clear-to-send and request-to-send signals aren't used by MissionPlanner anyway, are they?

2.  RTS and CTS aren't used by the autopilot (APM) either, are they?

3.  Is there any harm if I only connect one antenna to the modem and leave one connector open?  I was only planning to use a single antenna on the ground station modem and use both antennas on the aircraft modem. But I know some RF amps can be damaged by transmitting into an unmatched impedance.

4.  Is there a way to tell the modem to only use antenna connector A1?


Hello Dan,

Thanks for the message ! 

1) for the Cable, TX, RX, +5 and GND are the only lines required, so it should work OK.    As long as the cable can supply enough current for the modems then it is OK.

2) The current generation of APMs don't support flow control (CTS/RTS) so they're not connected.

3) No harm in that case, the modem will automatically switch to the best antenna.

4) No way at the at the moment to do that - there is options at build time to enable or disable diversity, but not through S registers at the moment.  The modem will use the best antenna so will switch as required..

Let us know how they go with your setup :)



Seppo, thanks for the quick answers!  I'll let you know how it goes.

Best regards,


Hi Seppo, In my bench testing I've noticed that the RFD900 looses very few packets. [about  0.1% packets lost] very nice!

Compared with the Xbee radios which loose way more packets 10 to 20 times more. 

Which antenna port is the TX port?  I'm assuming that only one antenna port transmits and both will receive.

On my ground-side antenna tracker setup I want to use an omni-directional antenna on the TX/RX antenna port and a high gain patch antenna on the RX only port.

Seppo, is there a wiring diagram for this, utilizing Minim OSD with it? Im still hunting, but it looks as though I can only use either telem or osd, not both at the same time.. but Im betting thats incorrect. Any ideas where to find a similar setup. Im not trying to fry anything.




PS, I did get the package. I had to track it in USPS after it got state side.

Has anyone noticed jumpy voltage readings with this radio? I have upgraded to it and appreciate the extra range, but my Attopilot 90A voltage/current sensor reports voltages randomly and continuously jumping between about +/- 0.4v when the radio is in use (eg. it will jump from 11.8 to 12.5 to 12.1 to 11.9 etc). This is while the plane is sitting on a desk with the motor off, but also when in the air. With my previous XBee setup, the voltage reading was stable.

I know the RFD900 draws a lot more current, but should the voltage be jumping around like this? I am powering my APM via a 8-15A UBEC on the output servo rails.

I have done some limited in air testing and it doesn't seem to impact performance (other than shorter flight times). Does anyone else see this behavour?

Are you using an APM 2.5? Try connecting the BEC to the inputs PWM. I'm know that would be much better as you don't have the voltage drop across a diode and solid state fuse.

Hi Bill,

I am using APM2.5. I tried your advice and switched the UBEC to the input rails. Mission Planner reports a much more stable voltage now (+/- 0.05 rather than 0.4). The only downside now is that the servos on the output rail are no longer powered since apparently powering the system from input rails will not power the servos, even with JP1 in place. I have disconnected the BEC of my ESC as it was a crappy linear one with only 3A capacity. 

My UBEC has two outputs, so I think what I will do is power both sides of the APM2.5 with the UBEC outputs and remove JP1.

Your symptoms sound quite similar to what I describe here.  I don't know anything about this particular radio, but if it is consuming significant power intermittently then fluctuating readings on voltages measured by the APM are what I would expect.

Bill, I think James's result strengthens your case that the fuse may be the primary culprit here.

James, clearly your UBEC is effective at providing a reasonably constant voltage even with the radio connected, so I think your solution may be workable.  If your UBEC's outputs are isolated, however, I would suggest that it might be even better if you can power the TM radio and the servos both via the output rail and then provide the cleanest possible power to the APM via the input rail.

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