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Thread: Developing the Ultimate FPV Video Link

  1. #1

    Developing the Ultimate FPV Video Link

    I am working on a “zero video dropouts” system, even doing stunts at the range limits, or over my head. I am bored of being constantly reminded by the video dropouts that I am not “inside” the plane.

    The first step is to understand the different issues that degrade the video link quality, and also to be able to cause them, in order to determinate later if the adopted solution corrects the problem.

    So, I have built a small test platform, using a Lawmate 2.4GHz transmitter, with standard omni vertical polarization antenna, and a Lawmate receiver, with the same antenna, but using one 18dB SMA attenuator, to simulate long range.

    The following video gives examples of all the issues:



    1. WIFI INTERFERENCE: When using channels inside the wifi band, if there are wifi networks in the neighborhood, you get this kind of horizontal lines.

    2. MULTIPATH: with good quality signal, we get suddenly a video drop out, due to the reflected propagation path cancelling the direct propagation path. This happens in a specific point of the space, and disappears just moving one of the antennas (typically less than one wavelength).

    3. PROPAGATION LOSS: when the distance between Tx and Rx increases, the video quality start to slowly degrade.

    4. POLARIZATION LOSS: When using vertical polarization, if the Rx and Tx antennas are at 90º, in theory there are infinite losses. So, if we bank the plane, we start adding polarization losses, and at a perfect 90º we lose the video.

    5. ANTENNA RADIATION PATTERN NULL: The omni antennas are not isotropic, and typically they have got nulls in their radiation pattern, aligned with the antenna axis. If we point the Tx antenna to the Rx antenna, the effective gain of the Tx antenna is much lower, and the effect is adding suddenly some extra dB loss to the video link. The most common situation is when flying over the ground station.

    6. SIGNAL OBSTRUCTION: If we place between the Tx and Rx antenna some conductive elements (a battery in the video), the signal gets partially obstructed, and the effect is adding some dBs loss to the video link. This happens when the camera, motor, batteries, etc. get in the middle of the transmission path in a specific orientation of the plane.

    Now that I can replicate all these issues, is time to start playing with Circular Polarization and Diversity, to see how we can solve all these issues one by one.

    I will keep posting.
    Last edited by SENTRY; 16th September 2011 at 01:14 PM.

  2. #2
    Obviously, there are two other factors much more critical: flying LOS, and having a proper power system, with no ESC noise. I am assuming these two factors are under control.

  3. #3
    I Like Waffles... SENTRY's Avatar
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    I like where this is going.
    "I Like Waffles" : FPVLab on Facebook and FPVLab on Twitter

  4. #4
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    Your unfortunately seeking the impossible (zero reception problems), but yes certain things can be done to improve matters.

    Circular polarization can and does help when it comes to reflections/refractions causing signal nulls at the receiving antenna. But, circular polarization can also work against you.

    If say the only signal you can receive (no direct LOS path) is a reflected signal (bounced an odd number of times of objects) then circular polarization is your enemy. In this situation a diversity receiver with each circular polarized antenna on the opposite polarization will allow you to continue to receive the reflections and help maintain reception.

    Loss of signal due to path loss can only be overcome by increased antenna gain (antennas wth directivity) and/or increased transmitter power and/or increased receiver sensitivity.

  5. #5
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    Moving your receiver antennas higher up above ground level can help a great deal - hence why commercial/hams use towers.

    One way to do this might be to use a baloon on a string/rope to get your little video receiver together with it's little antennas up above tree/building level. That may sound silly but it's not, some hams do it when out and about dx'ing. You can also get portable telescopic towers to get your antennas up higher.
    Last edited by TallyHoe; 15th August 2011 at 04:09 PM.

  6. #6
    Quote Originally Posted by TallyHoe View Post
    If say the only signal you can receive (no direct LOS path) is a reflected signal (bounced an odd number of times of objects) then circular polarization is your enemy. In this situation a diversity receiver with each circular polarized antenna on the opposite polarization will allow you to continue to receive the reflections and help maintain reception.
    In my experience, signal bounces does not help us a lot in the places we use to fly. They cause multipath, which is a big disadvantage compared with their only advantage: alternative path when loosing direct LOS path. The technique you describe is widely used in urban Wifi and GSM deployment, but for us I assume we always fly with direct LOS.

    Quote Originally Posted by TallyHoe View Post
    Loss of signal due to path loss can only be overcome by increased antenna gain (antennas wth directivity) and/or increased transmitter power and/or increased receiver sensitivity.
    Totally agree. This is the easy one.

  7. #7
    Quote Originally Posted by TallyHoe View Post
    Moving your receiver antennas higher up above ground level can help a great deal - hence why commercial/hams use towers.

    One way to do this might be to use a baloon on a string/rope to get your little video receiver together with it's little antennas up above tree/building level. That may sound silly but it's not, some hams do it when out and about dx'ing. You can also get portable telescopic towers to get your antennas up higher.
    Sounds good, but not the most portable system in the world...

  8. #8
    As the practical tests will have to wait a little bit, let’s start with the theory of the ways to solve each issue:

    1.WIFI INTERFERENCE:
    - Use the out-wifi-band channels in the Lawmate transmitters.
    - Fly in non-wifi areas...

    2.MULTIPATH:
    - Circular polarization: it naturally rejects multipath, as the polarization change from left hand to right hand (or vice versa) with the signal bounce.
    - Diversity at the receiver end, with two antennas with a separation of at least one wavelength.

    3.PROPAGATION LOSS:
    - Higher gain receiver antenna.
    - More transmission power
    - More receiver sensitivity.

    4.POLARIZATION LOSS:
    - Circular polarization: obvious, no polarization losses.
    - Diversity at the receiver end, with one antenna vertical and the other horizontal.

    5.ANTENNA RADIATION PATTERN NULL:
    - Diversity at the transmitter end, with two antennas (obviously with two video transmitters working in different frequencies) with each axis tilted 45º from each other.

    6.SIGNAL OBSTRUCTION:
    - Diversity at the transmitter end, with two antennas (obviously with two video transmitters working in different frequencies) placed in both wing tips. This way there will always be one antenna unobstructed.

    This is the theory...

    My idea is using two video transmitters, in different channels, with two skew-planar-wheel antennas (one LHCP and the other RHCP), in each wing tip, with the axis of the two antennas tilted 45º to the left and to the right.

    Would this work? Let’s see. I need to build the test platform.

  9. #9

    VERTICAL vs CIRCULAR polarization

    Finally I had the time to build my first two Skew Planar Wheel antennas, tuned for 2510 MHz:





    From the video issues listed, I have made a video comparing vertical and circular polarization for issues 4, 5, and 6.




    So, the current status is:

    1.WIFI INTERFERENCE:
    Solved with Lawmate channel 8 (2510 MHz). (Yes, I know it is not strictly legal in some countries... )

    2.MULTIPATH:
    Circular polarization is supposed to avoid it. I haven't been able to force multipath with my test platform, so I couldn't test how circular improves this issue.

    3.PROPAGATION LOSS:
    I have used a Lawmate 500mW 2.4GHz transmitter, with the standard whip omni antenna in the receiver. With a 18 dB attenuator in the receiver, I have found that the maximum range (when video signal starts to get slightly noisy) is 1Km. So, the above video is recorded with 1Km between the transmitter and the receiver (placed at the end of the road displayed in the video). Removing the 18dB attenuator would mean increasing the range by 8 (18dB=6+6+6 --> 2x2x2=8), so I can assume that the maximum range with video quality with this setup is 8Km. For going further, I would need a higher gain receiver antenna.

    4.POLARIZATION LOSS:
    With vertical polarization, the loss is huge (almost total) when placing the transmitter antenna at 90º of the receiver antenna.
    With circular polarization, there is no loss at whatever angle.

    5.ANTENNA RADIATION PATTERN NULL:
    With the vertical polarization antenna, pointing the antenna to the receiver means missing the video link.
    With the circular polarization antenna, when pointed to the receiver, there are big losses, but not as bad as with vertical. The null in the antenna axis seems to be not so deep in the SPW.

    6.SIGNAL OBSTRUCTION:
    It is not so terrible. I need to place the battery very close to the antenna, just in front, to get some signal degradation. However, the effect is even smaller with circular polarization.

    CONCLUSIONS:
    Circular polarization is a must for FPV (IBCrazy: you are a leader, thank you very much ).
    However, antenna’s nulls are still an issue, and the conductive elements in the plane (motor, camera, batteries, etc.) could slightly block the signal.

    NEXT STEP:
    Adding diversity in the transmitter side, with two antennas tilted 45º to the right and to the left, to see if we can solve issues 5&6.

    To be continued…

  10. #10
    Navigator Asomaro's Avatar
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    Very nice and informative video - thanks for posting.

    Cheers,
    Simon

    Think it, Build it, Fly it: www.desirerc.com

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