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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  • OK I solved it! Thanks everyone - and especially Joe - for your suggestions. Powering the modem via a separate UBEC - like Joe suggested - worked like magic. No balun needed, no ferrite core. 

    The modem came with a little 4 wire cable that became a 6 wire plug at the autopilot side. I removed the red wire from that socket by lifting the little plastic finger keeping it in place. I then soldered a hitec female connector to the red wire. Next I cut the black wire and made three way junction, one continuing to the autopilot, the other to hitec. My UBEC had a hitec male connector. I forgot to add the ferrite core again, but it worked so I didn't bother. 

    I suppose moving the dipole to the tail and making it a half wavelength dipole would be even better, I could then move the GPS & Rx to the front. But I didn't have the right cables and mounting surfaces to do that. And this just worked.

    Here's an update overview of my tries. If you have more questions, just ask!


  • The hb antenna is basically the coax from which I removed the shielding for L/4 165mm (so 300/433/4*.95), at the point where the shielding stops a soldered a wire of the same length. The antenna is taped on the tail of the plane and vertical polarized.

    My TX/RX is not shielded and close to the GPS receiver, the antenna feed coax runs over the GPS receiver. I did not experienced any drawback in GPS performance. On the picture with the RX/TX you can see the ferrites around the coax feed and signal wires.

    Yes you can use a 4el Yagi, but you have to point it towards your plane, my Diamond X5000 antenna has full round radiation pattern in a shape of a "pancake" with 8dBi gain. So my antenna is fixed at the roof of my house. Height of he antenna is key!!!

    After several long distance experiments my main conclusions where: Avoid man made disturbance (select empty frequency), Antenna height, Plane antenna far from other electronics.

    No beyond 20Km I did not go, the "distanceRSSIremain" showed over 100Km reserve, but I doubt if the rounding of the earth is taken in account .

    I'm currently experimenting with a video camera in the plane and a FM link at 1296MHz, also here my goal is 20Km, but here I use an rotated 55el vertical Yagi at 20m height. This test is not finished due lack of time and good weather.  


    • Hi Anton.

      I don't post often but have been a member here for many years. Please take the time to consider the advice of  Gisela and Joe. I have never directly needed their advice but have a great respect for their methodical problem solving skills. I have however been an engineer for 39 years and their method is a tried and tested one.

      Regards Lyn. 

  • Anton,

    I have more or less the same setup as you and never experienced any issue; my setup is an Ebay 500mW 433 Telemetry module(s). My antenna however is an homebrew vertical dipole (very simulair as the picture of Tipu), with an ferrite around the feeding coax to block the outer coax currents (in Dutch matelstroom filter), these currents you get as the dipole is symmetrical and the TX and coax are A-Symmetrical, this leads to currents flowing on the outside of the coax. You can also mount a quater wave pipe around the coax, soldered at the feeding point of the antenna, this has same effect for 433Mhz, not for the 3rd harmonic however.
    My home antenna is a vertical Diamond X5000 antenna on 10 meters, I get easily an range of 20Km. My Home TX is on max power (500mW), my plane is 250mW (but on 500mW, no issue with the GPS) I have this asymmetrical powers as my plane high in the sky picks up much man made spurrius.

    • Can you add a picture of your hb antenna? Or a link to the tutorial on how to make it? And a parts list?

      I was thinking of DIY 4 elements yagi antenna for the GCS. Would that work?

      Did you ever fly beyond 20 km?



  • I apologize for not reading the entire thread before commenting, but I wanted to put something out there...

    Use copper foil tape for RF shielding, rather than aluminum foil.  You don't need multiple layers of copper foil tape, although you can use multiple layers if you create a pseudo Faraday shield with them (insulating between each layer, and decoupling ground between them).  Still, that would be overkill and likely not necessary in your case.

    Copper foil RF shielding tape.

    copper foil tape - Google Search
  • Had problem similar to yours. Came up with a two solutions:

    1) reduce the power output of the 433MHz transceiver. That's a poor man's fix.

    2) replace dipole with ground-plane antenna (nose-down, ground-facing). That virtualy doubles 433MHz TX power (as it stops from being beamed into the sky), plus isolates sky-facing GPS from UHF frequencies.

    Google for: "UHF ground plane antenna"

    • John, the problem is that achieve the radiation pattern you allude to, you have to have a ground plane that is at least a 1/2 wave long per side, or 1/2wave diameter if a circle. You can make the ground plane from 3 or better, 4 wire elements, each 1/4 wave long. The size is somewhat unwieldy on the aircraft though - around 350mm or 175mm long wire elements...


      • Also, a ground plane may not help you in distant maneuvers, like a banked turn either way from heading out will likely black out your signal until you level off, unless you are at extreme altitude relative to distance. As Joe alluded to, keep it simple, unless you want the experience of it, spending fifteen bucks on a tuned dipole with a current balun is a lot easier and less risky, and likely lighter than trying to build and tune your own. Purchasing a different brand telemetry system with a factory supplied antenna will likely get you a super cheap rubber duck 8th wave monopole which will achive less range anyway.

  • Hi Anton,

    Prior to reaching the page showing the insides of your antenna connection, I thought, I'll bet it is direct fed unmatched helical, and there it was.. A large problem exists with that type of antenna, in that the mismatch between feeder and driven element causes the entire shield and ground circuit to become a radiator, and at different parts of your circuit interference will be unavoidable, even with ferrite rings. You should acquire a dipole with a matching transformer at the feedpoint, not only will it reduce interference and harmonics [some are enhanced by mismatched feeders], your range will improve, your radiation pattern will be more uniform and your transmitter will run cooler with better reliability and less current draw. I use the FPV Pro Dragonlink dipoles for this reason, they are cheap, lightweight, available with direct or lead connection, and have very low VSWR. Also Try to mount dipoles vertically for long range.I also have a Ranger, but use 433 for control and telemetry is multiplexed with video on 1.2ghz. 

    FPV Pro Dipole.jpg

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