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:
These modems are designed to support long range applications, while being easy to use and affordable. Target pricing is in the region of $90 per unit.
They are 100% compatible with 900MHz Hope-RF based radios (3DR) and similar.
These modems have been flying in various platforms and have demonstrated excellent performance in real applications.
RFD900 modems are expected to be available in a few weeks time, before the end of June in production quantities.
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.
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.
Thanks Dan, I'll give that a go at some point.
I've just returned from 2 hrs of flying with my new setup. It seems to be working for me now, I am getting stable voltage readings while using the RFD900. I have reduced the RFD900 output power on both ends to 23dBm to save some current draw - that is enough to give me the range I want anyway.
I ended up connecting one of the UBEC plugs to the output rail and the other to the "Sensor" rail on the side since the cable was too short to reach the input end. This seems to work the same as using the input rail. I removed JP1.
The UBEC puts out a steady 5.25v. I measured it with a multimeter and it does not fluctuate much at all (5.24-5.26v is what I get) when the radio is plugged in and I go crazy on the sticks to work the servos. The UBEC is a big one that is rated up to 15A (it's the Turnigy 8-15A model) so I think it will have enough capacity.
One question I have - do both ends need to be set to the same power output with these radios? If I had a big high gain antenna on one of the ground aerial ports, could I reduce the power of the air radio and keep the ground one higher?
The schematic does show the PWM and IO/sensor 5V pins all being connected, so it makes sense that this works for you.
About the radio output... it is indeed good radio practice in general to not transmit with any more power than you need. As far as using a high-gain antenna on the ground, keep in mind that the antenna will provide that gain in for both transmit and receive. So all other things being equal, if you change to a high-gain antenna on the ground you could reduce the power on both ends to see the same overall performance. At least that's the simple answer.
In reality, it can be a bit more complicated if you really want to optimize things. Things you will want to consider are receiver performance and noise at both ends, which may or may not be equal to begin with. If you happen to introduce other non-symmetric elements into your RF chain, you would want to compensate for those as well. Some of the diagnostic information (RSSI, etc) is reported for the 3DR radios and these look to be compatible, so you might be able to dig into the particular meaning and optimize if you want to.
One additional (and important) consideration is to check your laws regarding the allowable power output. If the limit is based on ERP (effective radiated power), then using a high-gain antenna on the ground may require you to turn the ground transmit power down (if you haven't already).
Thanks Dan. I did a long range flight today out to 2.5km and the signal strength in mission planner didn't drop below 98%, so I may even drop the dBm another notch to save more power.
Here in Australia, the ERP limit is 30dBm. I'm well below that with a 22dBm setting on the radio and a 3dBm omnidirectional antenna in use. The high gain antenna I am thinking of testing is a 6dBm Yagi.
Awesome. Is it compatible with the ArduStation with a bit of soldering?
I have had a look at the Ardustation and it looks like it should interface nicely to the RFD900 with the correct hookup: +5V, TX, RX, GND... - Will be interested to hear if you give it a go!,