MR60

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How to optimize the choice of your UAV's radio frequencies

 

1. Introduction

 

As we make our UAV fly we use multiple radio frequency bands for different uses : RC control of the craft, eventual telemetry band, eventual video downlink band (FPV or camera monitoring), GPS.

An issue we have all already encountered are interferences between used radio frequencies.

So how could we try to optimize, in a simple manner, the choice of our UAV's various radio frequencies (rc control, telem, FPV, ...) ?

 

2. Basic radio wave principles background

 

Radio waves are sinusoidal signals or a combination thereof. Any radio signal can be reconstructed by an addition of sine waves of various amplitudes and frequencies. Alternatively there are mathematical functions we can use to decompose (analyze) an apparently very complex signal in a number of pure sine waves (of various amplitudes and frequencies), so as for example to extract from the lot those who have the largest amplitude. 

The point is that a radio signal is never a pure sine wave propagating on a unique single frequency.

Here come the "harmonics".

Every radio emission does not only emit on its base frequency but also on a number of "harmonics". Harmonics are radio waves propagating at a multiple frequency of the base frequency. The following figure, copied from the amateur radio wiki, illustrates what harmonics are:

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(if you'd like more info google "radio frequency harmonics" to find the radio amateur's wiki site)

 

The issue with harmonics is that they are not on your mind when you decide to use channel 5 of your FPV transmitter. You selected channel 5 on your FPV Tx because it seems to be apparently far enough from all the other frequencies you have on your drone; so it should be right ? guess what : not good enough.

Because of the "upstream" harmonics you might still suffer from interferences that will impact your RC control range, disturb your GPS reception (bad HDOP) and or create lines and interferences in your video downlink.

The most annoying harmonics are mostly the ones resulting from the emission of a lower base frequency having an impact on higher base frequencies. An "impact" means that an harmonic's frequency is too close to another base frequency we want to use.

 

3. And so what ?

 

So it becomes quite a burden in practice to define for example which FPV Tx channel should I best use, because thanks to these harmonics we can't simply compare the base frequencies of rc control, telemetry, FPV, GPS. We must also consider all of their harmonics !

Don't worry, except if you're a genius,  you should normally not be able to compute which FPV channel is best, just like that, on top of your head.

So we need either to write down on paper all of the upstream harmonics of all the base frequencies and then compare all of these values manually to find what is the optimum frequency combination.

The best combination is the set of (base) frequencies that maximize the frequency distances (in mhz) between not only these base frequencies themselves but also all of their (upstream) harmonics.

You might ask : up to which harmonics should we compute ? 3rd ? 4th ? 5th ? Well that depends on your base frequencies. For example, if you use very high frequencies such as a FPV 5.8Ghz band, then you will have to compute the harmonics of your 900Mhz telemetry link up to the 8th order.

 

4. A tentative tool

 

I tried tonight to produce a simple excel calculation tool (there was nothing on TV) that will automate the computation of which is the best choice for your FPV channel, in function of your defined RC control, telemetry and GPS base frequencies.

I chose the FPV channel as the variable to optimize in the equation, because in practice this is the only radio component on your drone for which you can choose different channels (GPS is fixed, Telemetry is quasi fixed, RC control is fixed in its frequency hopping band).

 

This is a screenshot of the excel tool :

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You enter your own frequeny data in function of your configuration : the yellow cells are the cells where you enter your data.  All frequencies are expressed in Mhz.

You get the ability to enter up to 10 frequencies to analyze (to choose from).

The "Results" frame shows you a color coded computation result for each of the 10 frequencies :

-Green means the best frequencies,

-Orange means it is ok but not optimal (there are other frequencies which are better to use),

-Red means a bad frequency to use because it interferes with another radio component on your UAV.

 

Optional parameters allow you to define two things:

-Minimimum wished frequency distance (in Mhz) : this is the minimum amount of frequency distance between two frequencies that is required to avoid interferences (100 mhz by default).

-Max capping value for distance calculation : this is an internal computation parameter, just leave it at 100.

 

I tried this method (manually until this tool) for myself to choose my FPV freq among the 9 available channels and could observe indeed a clear improvement on my GPS HDOP stability (I often had an issue that my 1.3Ghz band FPV would disturb the GPS HDOP). I am curious if this method/tool could also improve your own setup; in any case let the community know what your improvement results are !

 

You can download the excel tool here: HDFreqoptimisationV1.zip

 

Any suggestion for improvements welcome,

 

Hope it will help,

Cheers,

Hugues

 

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Comments

  • Developer
    @hugues. You can filter out an unwanted signal, unless its on the channel frequency you are using. If the harmonic ie. Frequency is 'out-of-band' you can filter it.

    @paddles: You are either transmitting a signal as a number of frequency changes in a fixed band. The rate of change of the signal gives you variable., or you are sending a fixed frequency, with changing amplitude. FM and AM, (with it more complex versions SSB and LSB which save wasting power). Frequency Hopping, Spread Spectrum, Time Division Multiplexing etc help split the available bandwidth amongst many users, especially in higher bands like 2.4GHz, when more bandwidth is available. And help with reducing the impact of interference. Just look at 2.4GHz radio gear these days. Who needs a crystal board.

    You are correct in stating filters help de-sense from local interference by removing the unwanted signals from the receivers front end. a low pass/high pass/band pass filter on the output can reduce the harmonics your are sending. Which your other antenna don't want to receive. Harmonics are generally less power than you main carrier

    See this discussion on band pass filter to do just whats needed with so many transmitters co located. http://www.arrl.org/files/file/Technology/tis/info/pdf/9805044.pdf

    Antenna design also helps with out of band harmonic rejection.

    http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6068557&a...

    Filters do add complexity and some power reduction, but a quality antenna and receiver design with good rejection of harmonics and high sensitivity to in-band signals is what decides the distance that you can achieve. They can really help with problem configurations like small multirotors and planes
    http://www.arrl.org/files/file/Technology/tis/info/pdf/9805044.pdf
  • Admin

    Good work and excellent points  to ponder..

  • front end desense is easy to accomplish accidentally on the L1 GPS freq with ublox and other recievers..

         just not sure copper is the right medium at 1.2ghz silver coated yes but bare copper not so sure...

    looking for some l1 freq shielding braid

  • totally agree with the above.. and in my case I noticed appreciable e-field and mag field differences when arming a AIO pro 1.1 in an GLB RTF HJ450 (yeah right) where the rtf folks had mounted the AIO pro with on board compass and mags RIGHT ON TOP OF THE ESC power wiring.. freaking IDIOTS, glad it hasnt flown yet(awaiting GPS)  as it would have been instant toilet bowling given the difference in readings had to order a kit of nylon M3 hardware to mount the AIO on the top deck instead and leave the middle deck for the LIPO.

          HZL

    ps on the IRIS was going to mount the VTX low on one of the arms hopefully far from the FC GPS MAGS and Gimbal controller.

  • Yes that's definitely the right approach. Many people put on senders with high watt and have no shielding on FC GPS etc and don't even think of that issue.

    I tried around with different setups to actually disturb GPS reception (simple ublox without build in special filtering) and it is VERY easily done (don't use a groundplate for best disturbance, of course :) ).

    Even arming could reliably drop GPS reception and induce even position jumps (don't know wheter the plush style ESC or the FC changed the EMI footprint). I think proper shielding of the equipement is vital for a reliable function.

    Didn't test with videoequipment.

  • MR60

    Hi Pedals, thx for this interesting explanation, I did dont know that harmonics were not cut/filtered by RF filters.

  • Great easy to use tool.  Harmonics like this are often overlooked even in the professional world of RF.  Fortunately, there is no interference between 2.4ghz control, 5.4ghz video, and GPS.

    Using high/low cut filters is usually not very helpful. The harmonic is ON your frequency. You can't filter out the harmonic without filtering out the signal you WANT as well.  The only way is to change frequencies or alter the effective radiated power.  High / low cut filters are only useful for cutting de-sense (overload) from nearby frequencies on nearby transmitters.

    Also bear in mind that adding filters induces RF loss.  We aren't talking about much power to begin with here.  Inducing 3db of RF loss effectively cuts your power in half.

  • Moderator

    @Hugues yes it will be worldwide for all UAS, private or commercial. Its protected frequencies for our use only. At the moment looks like a chunk above 5Ghz. The next World Radio Council is hoped to rule on it.

  • Crady, give 5.8GHz a shot.  You can go very long range with the right setup.

  • Cool.  Thanks, Hughes.

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