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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    • MR60

      Thx for posting this, very interesting.

      It is strange that, when not powering your 433 modem via the autopilot but with a separate power source, you get a better GPS reception. I do not see any logical reason for this result.

      The only one I can think of as a result of your experiments is that the autopilot does not have enough amperage capacity to power both your 433modem and the GPS/compass unit simultaneously very well ?

      • I agree with your thought about not enough amperage... (I've only got about 2 years modern flight controller experience, but about 20 years electrical engineering experience). 

        I also want to mention... I've been building some small copters lately, with telemetry modems, GPS, UHF control, relatively high wattage everything (600mw video, 500mw telemetry), all shoved into a 250mm sized copter...  I definitely have issues with it, in comparison to my bigger vehicles.  But, it works...  I can fly and do auto missions most of the time.... 
        So, I think there has to be something unique (or wrong, like a bad product) in your situation...  Personally, I'd just buy another modem and antenna, before I do anything else....

  • Hello.

    You can get GPS working with following things:

    1- Put a "Notch Filter" on 433mhz transmitter. this will kill most of harmonics.


    2- Wrap you transmitter with Aluminum foil, if transmitter is emitting RF noise, it will be shielded too.

    3- Put an Aluminum foil piece beneath your GPS unit.

    4- If still getting noise on gps, then put a ferrite ring on GPS > Autopilot cable.


    hope this will help

    • - Is a notch filter the same as a balun? It looks similar.

      - Would a 15nF condensator over the modem power lines help? I still hope to avoid a separate battery for the modem...

      - I will try a separate BEC, maybe that helps too.

      I will update this thread later with a systematic approach like Nampilot suggested.

      • 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.

  • MR60

    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/

    433bluetooth Bridge
    Besuche den Beitrag für mehr Info.
  • 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.



    The NamPilot

    • I opened the antenna casing. And it looks like there is no balun indeed...

      Should I add one? How?


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