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Thread: Researching a 100Km video link

  1. #1
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    (Researching a) 100Km video link: DONE

    Since Roberto Montiel showed us his new Fascination I started playing with an idea.

    Given that:
    - In the 63Km record he did not use the motor.
    - With the solar panels that he has installed, in the summer he should be able to cover the consumption of the electronics on board, which means that he would have a virtually unlimited flight time (depending on the sun, of course).
    - Using Yagis in the UHF transmitter the RC link can easily exceed 100 km. (Sander is looking after this part).
    - The video of his record shows that from the 40Km every time the plane banked to turn a thermal there were losses, and at 63Km and video quality was just inexistent.
    I concluded that the only thing limiting him to beat his own record was the video link, and I decided to contact him to offer my help.

    The configuration he used for the record with the Grafas was:
    - Video Transmitter 1.3GHz, 1500mW.
    - Standard dipole antenna in the video transmitter.
    - 1.3GHz Video Receiver LawMate.
    - 14dB Yagi antenna in the video receiver.
    Hence the possible areas for improvement were:
    - Reduce power to 800mW to reduce consumption on board. The 2.5dB lost are easily retrievable.
    - Switch to circular polarization, to avoid losses by banking the plane when it approaches the range limit.
    - Improve video receiver sensitivity by changing the SAW filter by a lower bandwidth one.
    - Build a circular polarized antenna for the receiver with the maximum gain, within practical limits (the size of the trunk of my car ...).
    With this starting point we fixed a meeting in Madrid, to introduce each other, see in detail his setup and exchange impressions. It was a very pleasant meeting. As Roberto said when he shake my hand: "What a pair of fools!"

    From there I went to work, I built a Skew Planar Wheel for the transmitter, and began to build the receiver antenna.




    Obviously here it was just a matter of size, so I bought a 100cm offset dish, along with a portable tripod:





    Then I started to build the feed for that dish, and opted for a 3.5 turns helical. I did several tests with different construction techniques, as well as different methods of impedance matching, and the result was this feed, built with PVC corners and metal reflector, and matching the impedance by driving the conductor parallel to the reflector for the first quarter wave:









    However, the final result did not convince me, so I made a second version, with a construction method more "elegant" and adjusting impedance by a copper triangle. By the way I discovered the nobility of Plexiglas, with whom I had never worked. It's like very soft metal, very friendly. The result was this feed:









    I measured the helical in the laboratory, and the impedance matching was just perfect with the triangle, and is the only method consistent enough to avoid tuning and testing each single antenna.

    After that I modified a receiver by changing the SAW filter for a lower bandwidth one to improve sensitivity, taking out the RSSI signal to point the dish towards the plane with something more accurate than the subjective quality of the video, and making a video cable with a female 3.5mm jack for compatibility with Roberto’s ground station.

    Also I added a fitting to allow the dish to be fine-tuned on elevation, welding a nut, and using a piece of threaded rod as a buffer to regulate the angle of elevation. The azimuth could already be adjusted by the design of the tripod itself:






    And this is the result:








    With a bit of luck on Friday I will make some 6.6km field tests. First I have to adjust the position of the feed on the focus of the parabola, and then check if it improves efficiency by eliminating half turn of the helical to open a little more the beam and better feed the dish. From there, it will be a matter of adding attenuators to see how much attenuation can be added, and convert this maximum attenuation into theoretical range increment.


    To be continued...
    Last edited by sircana; 12th January 2012 at 08:07 PM.

  2. #2
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    watching, of course. amazing work as always mi amigo.
    seeing the world through a security camera

  3. #3
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    Looks amazing ! Looking forward to the results...

    Ro

  4. #4
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    aww crap, don't tell me I need to beat another record this summer ...

  5. #5
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    Looks awesome!

  6. #6
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    who would have ever guessed a record attempt with CP!!!!!!!!!!!!!!?????

    LOL

    are you expecting gain over 24db!? with such low gain on the TX i would expect you are going to need over 30db.... which would make aiming it a really hard job,

    just my first impressions, looks like an awesome project done with real skill and passion

    cannot wait for the vids

  7. #7
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    assuming lawmate-like sensitivity, and similar antenna-performance, at 800mW he will need about 24dbi to beat our record. I doubt this is 24dbi for 1.2GHz, but I'm really not good enough with antennas to guess the gain through a couple of pictures

    in any case, we've done our homework, looks like sircana and rmontiel are doing it right now. I'm looking forward to the challenge this summer

  8. #8
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    you might need to bust out the supra again trappy, asking over 90km of the zephyr really is a big ask!!!

  9. #9
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    naaah, no composites to test limits. we've got a plane capable of doing 300km, that should be enough :P

  10. #10
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    let me guess, launch at the top of everest ....... ? lol

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