433 Mhz dipole vs. GPS: total decimation

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?

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    • 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.



      • 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..


        • 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... 


            • I was thinking of building a 433 MHz antenna myself and trashing the hawkeye. I found some tutorials while doing research on baluns... Thanks for that pointer.

            • Sorry I was out in the pub. A man's gotta do what a man's gotta do. :)

              I took two pictures with my DSLR:



              Are they detailed enough?

              • HI Anton,

                Photos are a little fuzzy - I guess you could not get focus closer with the lens? 

                Th photos are however clear enough to show there is no lowpass filtering at all, just a simple Pie matching network to the antenna connector, so that is not good. 

                Anton, before you junk anything, lets do a simple test to try determine if the interference is directly from the antenna/radiated harmonics, or due to radiation from the rest of the interconnecting wiring.

                We need to remove the modem and its connections from the rest of the plane.  Use a separate battery to power the modem (via a suitable volatile regulator, BEC, etc - ensure the modem receives the correct voltage, at sufficient current). Connect all the items up - Antenna to modem, Modem to voltage regulator, voltage regulator to battery.

                Then insulate all the electronics - modem, regulator, battery and its terminals, etc, with insulation tape. Then wrap all the items in aluminium foil, to form a faraday cage, leaving only the antenna and its coax protruding, The Al. foil must be well connected to the coax connector at the modem. Make sure your telemetry receiver ( the one you use to receive the telemetry data from the modem..) actually sees the modem signal before wrapping in Al foil.  There is no data from the APM into the modem, so the telemetry will not have any data, of course..

                Then power on the aircraft with GPS etc, with the modem antenna positioned as you had it in the pictures you posted, and see if the GPS Fixes.  If not, the modem harmonics are killing the GPS. Try move the antenna away from the GPS by 500mm, and try again.

                If it still prevents Fixes, then the Modem is poor...

                If the GPS does fix, then the problem lies in the RF getting into the aircraft/GPS wiring, or where the Modem gets its power from. We can look at that all once you have done these tests.

                Here are some pics of how to connect it all, and wrap it all up...




                All Wrapped up


                Make sure connector makes good contact with the foil to form the Faraday cage.



                • That is a very nice step-by-step process you suggested! And it works!

                  Here are the results:

                  - Only a ferrite core in the modem power wire: not much improvement.

                  - Packing the modem in aluminum foil: noticable improvement: 7 sats after 10-20 seconds.

                  - Packing the modem and powering it from a separate BEC: maximum improvement! 13-14 sats, pretty quickly. 

                  Some weirdness that I still have to figure out:

                  - Why is all of this much worse on a dipole than on a monopole antenna?

                  - Somehow communications went only one way. I couldn't execute commands from the GCS. Maybe a badly connected Tx wire.

                  Question: Should I connect GND, Rx, Tx wires to the Autopilot? +5V and GND to the separate power unit? Won't the EM waves go through the GND wires?


                  • Hi Anton, 

                    Well, seems there are things to be learnt here. What is important is to be methodical - Maybe you can just explain more precisely what you did-

                    - Packing the modem in aluminum foil: noticable improvement: 7 sats after 10-20 seconds.

                    Where was the modem when you did this? Still in the fuselage? Was it still connected with the original ( 'non-working') wiring in place? Was the modem wiring still connected to the autopilot? Where did the modem get its power from?

                    Was the dipole antenna still in it original ( non-working) position?

                    Is the GPS and its antenna located on the tailboom as in the photo, or are the GPS and antenna separate?

                    It begins to look like the interference is entering the system via the wiring, rather than directly from the dipole into the GPS antenna, but this does depend a little on your answers to the above. 

                    If so, then the next steps are to place everything in the position you had, but doing this now -

                    Keep the modem wrapped in foil, making sure the foil is grounded as best as possible to the antenna connector - even wrap some thin stiff copper wire around that junction a few times.

                    Power the modem from a separate BEC, which in turn power from the original battery ( aircraft battery) as before ( when things did not work).

                    That will give a separate Power and Ground reference to the BEC and to the Modem. If you have a (small) handfull of ferrite rings, wrap the BEC leads from the battery, and to the modem, a few times through the ring. Try to place a ring at the BEC, and another at the modem, in that power lead.

                    Then, take GND wire, the TX wire and the RX wire from the modem, wrap through a ferrite ring a few times, close to the modem, and take that to the autopilot. Then try the whole thing again...

                    I am not sure I understand the question re the dipole versus Monopole - did you do this test with a monopole ( what kind - 1/4wave, 1/2wave, what make?), and if so was the monopole located at the same place as the dipole now is?  If so, did it work fine with the monopole? If so, then we may have two problem areas here - one being the RF in the wiring which all the above should address, and the other is that all that stray RF may be a result of a poorly matched Dipole.

                    Lets see how this goes?



          • Hi Anton - Can you take a good close-up photo of the component side of the modem please? I have sent you a message with my email address so you can send the photo(s) in high res please...

            Take some of the complet modem, and then of the area close to the antenna connector area.


This reply was deleted.


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