I wanted to do long range telemetry so I got myself a dipole antenna for the modem.
I never could check the range because the plane couldn't get a gps lock anymore, with the modem powered on. With the modem powered off, I had a 3D GPS lock, 17 sats in seconds.
What can I do to get both long range telemetry and good GPS?
Replies
BECs have low RF noise, so replacing it, won't make any difference.
The RF Explorer units are great for troubleshooting this sort of interference problems, but they aren't cheap.
I have the same type of aircraft as you have.
Its flying with the Pixhawk and the 3dR GPS module.
I am also using the 3dR 433Mhz telemetry radio and have no issues with the GPS getting a satellite lock.
I would certainly, make sure that the telemetry module is not overstepping on the GPS frequency.
Try to keep both units as far away as possible from each other.
I noticed you mounted the GPS unit on the tail session, try mounting it on top of the main center wing.
And the modem antennas, where the GPS unit was previously mounted.
Just guessing that Hawkeye is slowing harmonics at 500mW that interferes with the GPS frequencies?
Hi Anton,
i don't have any clue how to solve your problem but i have some stuff for the ground side. I think it is very convenient.
Check out this site: https://433bluetooth.wordpress.com/
Please describe in some detail the layout, where the GPS is located in relation to the telemetry antenna, What telemetry modem you are using, its output power and frequency,What type of dipole antenna ( mnfr and part number?) , what type of GPS you are using and what type GPS antenna. Is appears the modem transmission is blocking/desensitising the GPS receiver, which means there are strong harmonics from the telemetry transmitter being detected by the GPS receiver. If the dipole is not matched properly to the modem TX frequency, or the Dipole has no Balun, there would be a high level of RF all over the wiring, etc, due to the poor SWR. If you can give as much info as possible on the setup/layout, maybe I can help.
Regards
Joe
The NamPilot
I opened the antenna casing. And it looks like there is no balun indeed...
Should I add one? How?
Hi Joe!
Thanks so much for taking an interest in the situation. This is my setup:
- A ranger 757-3 with GPS antenna all the way out on the back of the plane.
- 44cm in front of the antenna, about in the middle of the plane, the HawkEye 433 Dipole Antenna. Mounted horizontally. You'd say the least power is sent along the axis of the plane, where the gps antenna is...
- The antenna is connected to a no-brand 500mW telemtry modem (from aliexpress or banggood). APM planner works great with it.
- 24cm in front of the dipole, all the way in the nose a 2.4 MHz turnigy 9x receiver.
- What more information could I give you?
The instant I power down the 433 telemetry modem, I get a GPS fix.
Yesterday I flew without telemetry and used the onboard wifi hotspot to download a mission to the autopilot. The range of the wifi is of course limited and the connection is flakey, but the plane flew great.
Now I'd love to have live long range telemetry too.
Thanks,
Anton.
Hello Anton,
Anton, I suspect there are a number of issues here, but the main culprit is most likely high levels of RF harmonics getting into the GPS receiver.
However, before digging deeper, a few more questions:
What is the make/model of the telemetry module? I would like to try find some tech info on it if possible, esp if the output signal has a low pass filter or not...
Do you know what the Telemetry module signal baud rate is?
A wide band RF signal, with no low-pass filtering on the RF output will have terrible harmonics, well into the GPS frequency. The third harmonic already lies close to the GPS channel.
I presume you have no means of measuring the Dipole SWR? If that is not decently matched to the TX output, it will exacerbate the problem, as does the lack of a balun on the dipole. However, we need to first understand the modem issues before worrying to much about the balun..
How are you supplying power to the various units - GPS, Modem, servos, etc. If the items are powered from the same source, then any RF that is finding its way from the modem or its dipole, back into the modem power lead, will end up on the power leads of all the other items. This could also influence GPS performance.
There are ways of resolving this, by means of ferrite beads ( toroids) with the various leads wrapped a number of time through the ring, forming an RF choke, one ring at the RF module ( GPS, Modem, etc) and one at the power source, The same should also be done to the signal leads ( serial link to GPS, Modem, etc) . All this can be done once we understand the modem itself a little better..If the Modem has no output filtering then it will be very difficult to solve the problem..if at all.
By the way, the way you have your dipole placed means the max signal reached the GPS module - the dipole radiation pattern is a toroid or doughnut, with the ends or tips of the dipole being in the direction of minimum signal.
Let me have some of these answers, and I will try help further..
Joe
This is what I see when I open up the modem. Does it help?
I have a spare ferrite coil which I will try tonight.
The modem connection is 57K baud serial.
I'm asking around in my neighborhood if anyone has a SWR measurement device...
Hi Anton,
sorry , you need to acknowledge my friend request before I can message my email to you...
Joe
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