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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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

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.



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/

Just guessing that Hawkeye is slowing harmonics at 500mW that interferes with the GPS frequencies?

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

Should I add one? How?

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 - 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.


Hi Anton, 

sorry , you need to acknowledge my friend request before I can message my email to you... 


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?

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.

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.

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