Mactadpole
Member
Discovered an interesting but disconcerting interference issue recently after adding a 1.2 GHz video transmitter to my hexacopter mapping rig. You can see my build here.
I have a Garmin Astro 220 dog tracker (transmits in the 151-154 MHz range) on the copter that I have in case of a fly away. I have used this for some time and never noticed an issue. So recently I added a 1000mW 1.2 GHz vtx to this copter. I don't always remember to turn on the dog tracker, bad habit. During testing of the vtx I never turned on the tracker.
I go out the other day to do the first flights with the new vtx and I forget to turn on the dog tracker for the first flight, all goes well. I change locations and remember to turn on the dog tracker for the second flight. I take off and immediately have periodic twitches and mission planner starts screaming 'compass variance error' (I fly Pixhawk). I land, unplug everything, fire it back up, takeoff and observe the same issue. I look around and see that I am near a fair number of power lines, including a main transmission line about 500 meters away (I am doing university and govt. agency environmental monitoring work so that is the only reason I am restricted to some flight areas, otherwise I would not fly near power lines). I think that is the issue so I change locations. Same behavior at the new location. I'm near empty on this set of batteries so decide to change batteries thinking maybe something weird with one of the cheap multistar batteries I fly is causing compass issues. I put the new set of batteries on and takeoff, I still see a slight twitch but less frequent and no compass variance error. I complete what I need to do and go home. I take a look at the logs and see compass issues on the external compass but not the internal, and they are associated with current drops to near 0 amps. In the logs the current drops are happening at a regular rate, the compass variance doesn't happen as frequently, and there are drops on the baro as well. I should mention that I have two separate battery leads to the PDB with only one being monitored by the attopilot power module I made, while the other is monitored by a Frsky current module. I tore the copter apart checking all connections and wires and found nothing. I tested my batteries and found nothing wrong. I recalibrated everything and turned off the internal compass even though it show no signs of error in the logs. I did a test flight with everything on and still had the twitch behavior and it was actually pretty severe on that flight. Don't know why I didn't think about turning off the thing I had added first, but when I turned off the vtx there were no flight issues. I then made the connection that it had flown fine with the vtx on and the dog tracker off. Flew it that way and sure enough it was fine. So, there is some sort of interaction occurring between the vtx and the dog tracker. It appears to be worse as the batteries deplete. I tried moving the dog tracker and its antenna all over the copter and it got a little better in certain positions but could not get rid of issues all together. I am a bit perplexed as to how this interaction is generated at these very different frequencies and I don't know enough about harmonics, etc. to figure it out. I would love to hear about any theories.
Now I have to decide whether to change vtx frequencies or change the type of tracker I use. There are positive and negatives of both and I would like some advice from others. My first thought is to change the tracker to the Marco Polo system or the RCbeacon system. Marco Polo works in the same frequency range as my RFD900 radio modem for telemetry but the Marco Polo is not transmitting unless pinged by the locator unit. But if the copter goes down and the RFD900 is still powered it might desensitize the MP tag's receiver meaning I might need to wait for the copter batteries to die to get the tracker to work. The RCbeacon system is more rudimentary but works on the 433 MHz frequency. The bonus of changing the tracker system is the weight reduction, but I must say that the Garmin dog tracker system works really well in forested areas. The downside is that I think I will experience some range loss, I just don't know how much. The bonus of changing vtx systems is the smaller antenna size for the 5.8GHz systems but I worry about loss of signal penetration and distance. I should mention that I run the Frsky 2.4GHz system alongside these other device frequencies. Would like some ideas on this decision too.
Thanks, Shawn
I have a Garmin Astro 220 dog tracker (transmits in the 151-154 MHz range) on the copter that I have in case of a fly away. I have used this for some time and never noticed an issue. So recently I added a 1000mW 1.2 GHz vtx to this copter. I don't always remember to turn on the dog tracker, bad habit. During testing of the vtx I never turned on the tracker.
I go out the other day to do the first flights with the new vtx and I forget to turn on the dog tracker for the first flight, all goes well. I change locations and remember to turn on the dog tracker for the second flight. I take off and immediately have periodic twitches and mission planner starts screaming 'compass variance error' (I fly Pixhawk). I land, unplug everything, fire it back up, takeoff and observe the same issue. I look around and see that I am near a fair number of power lines, including a main transmission line about 500 meters away (I am doing university and govt. agency environmental monitoring work so that is the only reason I am restricted to some flight areas, otherwise I would not fly near power lines). I think that is the issue so I change locations. Same behavior at the new location. I'm near empty on this set of batteries so decide to change batteries thinking maybe something weird with one of the cheap multistar batteries I fly is causing compass issues. I put the new set of batteries on and takeoff, I still see a slight twitch but less frequent and no compass variance error. I complete what I need to do and go home. I take a look at the logs and see compass issues on the external compass but not the internal, and they are associated with current drops to near 0 amps. In the logs the current drops are happening at a regular rate, the compass variance doesn't happen as frequently, and there are drops on the baro as well. I should mention that I have two separate battery leads to the PDB with only one being monitored by the attopilot power module I made, while the other is monitored by a Frsky current module. I tore the copter apart checking all connections and wires and found nothing. I tested my batteries and found nothing wrong. I recalibrated everything and turned off the internal compass even though it show no signs of error in the logs. I did a test flight with everything on and still had the twitch behavior and it was actually pretty severe on that flight. Don't know why I didn't think about turning off the thing I had added first, but when I turned off the vtx there were no flight issues. I then made the connection that it had flown fine with the vtx on and the dog tracker off. Flew it that way and sure enough it was fine. So, there is some sort of interaction occurring between the vtx and the dog tracker. It appears to be worse as the batteries deplete. I tried moving the dog tracker and its antenna all over the copter and it got a little better in certain positions but could not get rid of issues all together. I am a bit perplexed as to how this interaction is generated at these very different frequencies and I don't know enough about harmonics, etc. to figure it out. I would love to hear about any theories.
Now I have to decide whether to change vtx frequencies or change the type of tracker I use. There are positive and negatives of both and I would like some advice from others. My first thought is to change the tracker to the Marco Polo system or the RCbeacon system. Marco Polo works in the same frequency range as my RFD900 radio modem for telemetry but the Marco Polo is not transmitting unless pinged by the locator unit. But if the copter goes down and the RFD900 is still powered it might desensitize the MP tag's receiver meaning I might need to wait for the copter batteries to die to get the tracker to work. The RCbeacon system is more rudimentary but works on the 433 MHz frequency. The bonus of changing the tracker system is the weight reduction, but I must say that the Garmin dog tracker system works really well in forested areas. The downside is that I think I will experience some range loss, I just don't know how much. The bonus of changing vtx systems is the smaller antenna size for the 5.8GHz systems but I worry about loss of signal penetration and distance. I should mention that I run the Frsky 2.4GHz system alongside these other device frequencies. Would like some ideas on this decision too.
Thanks, Shawn
