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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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.

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?




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.

An overview of my different tries so far. Will add more experiments as suggested in this thread.

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....

I think you might try mounting your 433mhz Transmitter , or at least its dipole antenna on back of your plane.

like in this picture.

move your GPS and RX in fuselage , they are both receivers. they wont bother each other. just move Tx away from Autopilot and RXs. and mount your antenna vertical.

I always use a separate battery for electronics , other than flight battery.

you might try this too.

Hope this will help

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. 


Hi Alasdair,

This is one of the more insightful answers! 

So the feeder of my modem is matched for a lamda/4 monopole antenna and it goes haywire on a lambda/4 dipole? I found no 433 antennas on the site you mentioned. This article is about impedance matching, but I understand only half of it. http://www.radio-electronics.com/info/antennas/yagi/yagi-feed-imped...

This site has a nice selection of 433 antennas. How do I know if any of them are matched?


Or should go and build my own antenna, get an SWR measurement device and match it?

(Sadly, where I live 1.2GHz is an illegal frequency to use. So I'm stuck with 433 en 2.4. So no 1.2G video for me. :( )

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