Discussions

915mhz Telemetry

Posted by Finnius on February 18, 2014 at 5:24pm

Hey folks,

Did the original 900mhz telemtetry kits that have the built in usb adapter on the ground radio get the interference or was it just the 433mhz ones that had limited range because of usb interference?

Also, do you think i can get away with 1.2ghz video (1.2ghz inverted vee) and 915mhz telemetry (1.2ghz cloverleaf) on my drone - or will there be interference? Should i stick with the 915mhz antenna for the telemetry instead of a 1.2 cloverleaf i have available, to reduce the amount of 900mhz telemetry getting into the 1.2ghz?

Any help would be appreciated.

Thanks

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Replies

  • Geoffrey Preston February 19, 2014 at 7:01am

    It seems like we can’t cram enough electronics into our UAVs. You might get away with using two bands that are close together if your electronics are well filtered, but I wouldn’t bank on it. You simply can’t get enough distance between the different antennas in a small aircraft to prevent “swamping” the signals in each radio component. Each frequency band needs its own antenna cut or manufactured to resonate to that band so using a 1.2 for a 900 or vise versa just won’t work and would harm the electronics if you try to transmit with an antenna not resonant for the band being used. There are ways to cut down interference such as using filtration and opposite antenna polarization, but once again not being able to separate different antennas with enough distance is a real shortcoming. Hope this helps.

    g

    • Finnius > Geoffrey Preston February 19, 2014 at 3:23pm

      Thanks for the info.

      I will try out my video on 1.2 and my telemetry with 900. Each with their own appropriate antennas. Do you think i will get interference from 1.2 on 900? Also, i have been googling everywhere but can never seem to find out whether cloverleafs or inverted vees have better range for long distance flying? As i am hoping to put an inverted vee from the 1.2 on one wing and an inverted vee on the other for the 900 mhz telemetry radio... then i would have a biquad of each 1.2 and a 900 on the ground to recevie both - sounds good to me??

      Thanks for the reply

      • Geoffrey Preston > Finnius February 19, 2014 at 4:01pm

        The interference can be minimized by separating them apart as far as possible. One for each wing seems like a good idea.  I published something here some time ago.

        http://diydrones.com/profiles/blogs/more-antenna-alternatives

         using copper tape as an antenna. If attached to your wing laying flat they would be horizontally polarized.

        I dont think the cloverleaf is any better than a dipole. The clover leaf has circular polarization and requires a receiving antenna of the same polarization and it takes up more room.

        Make sure your bi-quad is polarized to match your antenna on the UAV.

        g

        • Finnius > Geoffrey Preston February 19, 2014 at 4:05pm

          Do you mean i have to have my biquad sideways if my inverted vee is sideways for correct polarisation? How so? Also, what type of range do you think i could get with a inverted vee and biquad on those 100mW radios at 900mhz? Thanks so much

          • Geoffrey Preston > Finnius February 19, 2014 at 5:07pm

            An inverted V is horizontally polarized with the elements parallel to the earth. You should get good range if your not in an urban environment, perhaps several miles with good line of sight and the UAV at altitude. See the sketch for polarization of the bi-quad.

            39e9ab5383c12cb2381ff9e86169132d.png

            https://storage.ning.com/topology/rest/1.0/file/get/3702817454?profile=original
            • Finnius > Geoffrey Preston February 19, 2014 at 5:15pm

              oh wow!

              thanks for the sketch.

              so i should have my biquad with the elements at right angles to the earth if the inverted vee is parallel to the earth. do you know why this is so? it just looks right when both antennas are parallel to the earth...

              also, several miles LOS with 100mw at 900mhz with an inverted vee - wow! i cant wait to take it for a flight, but i will be testing with the little rubber duckies first.

              i read you have a flying wing? if so, are you running it with an apm, and how much weight do you carry. i have a bixler frame with a lot of mods, because it can carry all my gear, fpv as well as uav.

              Thanks Geoffrey

              • Finnius > Finnius February 19, 2014 at 5:21pm

                also you mentioned cloverleaf not generally being any better than a dipole, is an inverted vee better than the standard diy dipole?

                • Geoffrey Preston > Finnius February 20, 2014 at 10:36am

                  Inverted V is a dipole forming a slight "V" in order to create a load impedance closer to 50 ohms... a better electrical match to a 50 ohm transmitter.

                   Plenty of hobbyists would argue that a clover leaf is better and it may be for some situations. Search the web for radiation patterns and gain for both antennas and draw your own conclusion based on your own needs. A clover leaf takes a lot of room at 900 mhz because it is 3 dimensional in nature but a dipole is 2 dimensional and can be laid flat on a wing or fuselage or stretched between a tail and cockpit with little wind resistance. It doesn't matter if the ends droop downward or go forward or backward as long as the plane of the dipole antenna is horizontal (assuming that you want a horizontally polarized antenna).

                    For me, a 900 mhz inverted V flat on the wings for data and a 5.8 clover leaf (size is relatively small) for video when I'm ready.

                  There are some good web sites and books available on antenna theory that could explain things better than I for your other questions.

                  g

  • Finnius February 18, 2014 at 5:54pm
    I also have a 1.2ghz biquad, do you think i could receive both my 915mhz telemetry and 1.2ghz video through the same antenna?
  • Finnius February 18, 2014 at 5:37pm

    Also, i have notice that the 3DR 433mhz telemetry units have a usb plug that goes straight into them. Wouldn't that mean that the ground one would still be using an inbuilt usb adapter - that causes interference. I thought that they replaced those versions with one where the adapter was out of the module, i just cant see it in the description or images.

    Thanks

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