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Thread: Lawmate 2.4GHz Tuned Video Receiver

  1. #11
    "illegal" is a bit harsh. I prefer "gray area" seriously, though, the noise floor is similar on all frequencies. the proper gain comes from an antenna tuned to the frequency. I've noticed some antennas do better at the higher frequencies and totally crap out at the lower ones, or vice versa.

    the only way to successfully reduce the noise floor is to get away from noise sources. period.

    The SAW filter has the Digi-Key Part Number: XC993-ND

  2. #12
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    Trappy, thanks for the part number.

    IBCrazy: thanks for the info, it clarifies things a lot.

    By the way... I just noticed in the new proximity video that TBS doesn't fly with CP antennas, but uses vertical polarized ones. The other post about the 86km record mentions that maintaining the video signal became problematic and that for this extreme range CP antennas are next to impossible (for now). having said that, when you'd stay within 30-40km or so (or perhaps closer), what then are the disadvantages of CP's / cloverleafs? Is it that there is no suitable high gain directional antenna for CP yet? (which has similar perf. to the grid?)

  3. #13
    multipath rejection is the reason why CP is favored over linear. for range and, unfortunately also for reliability, linear antennas are the way to go on the plane. When we touch branches or generally during transportation the CP antennas are much easier to bend. with that said, there were 2 people with cloverleafs at the meet, also on zephyrs. the performance was comparable to linear, even with linear groundstation antennas.

  4. #14
    Engineer for Jesus Christ IBCrazy's Avatar
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    Circular really is best for short and mid-range. Once you start going out over long distances, linear begins to rule. I think that circular is far more reliable than linear, but it is certainly more fragile. Hangar rash is usually what breaks my antennas (poor handling in transport).

    CP gives a cleaner video feed when used on both ends. Linear is easier and gives longer ultimate range. However, if you fly in an area with a high noise floor, your polarization is not going to matter. Your video is going to suffer no matter what you do. The best thing you can do for your video is as Trappy states: get away from noisy sources. that doesn't mean you can't fly near them. It just means your ground station should be far away from them. This is when the SAW mod will help.

    There are various reasons why one type of polarization rules over another. Believe it or not, horizontal (antenna laying flat) gives the best ultimate range, but it doesn't allow your plane to turn...

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  5. #15
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    Allright. So there are certainly reasons for going VP all the way instead, just for stability, ease-of-use, ease-of-replacement sake and avoiding snags in branches. If CP is used to counter multi-pathing, we need to ask the question whether multi-pathing is still an issue at varying distances. If it is irrelevant, there really is no compelling argument to go CP anyway and one just chooses one based on preference.

    Transport issues and bending can be mitigated, for example by a protective dome-like structure. Before you start yelling that this would generate huge amounts of drag in comparison to the VP antenna, since obviously this has a larger surface facing the wind, I'd like to point out that drag is mostly about how you manage what the wind does after it hit your surface. The numbers here are that a single round wire generates the same amount of drag of an aerodynamic airfoil that is ten times its size (in terms of surface area facing the wind). A standard casing for a VP antenna is 1cm wide versus 10cm high. You could therefore also imagine this antenna as a 10cm high airfoil of 10cm wide...

    So instead of making the dome perfectly round as well, it should be made into a tear-drop shape, exactly like the futuristic looking aerodynamic helmets of professional cyclists. No airfoil is perfect, so let's assume 75% efficiency here in construction. One end is resting on the wing, so this reduces efficiency by half. However, the dome is half as high as the antenna, so it's twice as efficient again. This means that it's very likely this teardrop dome would have the same performance in terms of drag as the standard ducky, even though from the looks of it, it's wider and seems to have a larger surface area.

    This provides us with two possible experiments:
    - The tearshaped dome would give the required protection against snags, hangar rash and would still generate the same drag as a standard VP. It would certainly generate *less* drag compared to its own bare wires. However, not sure what the material will have to be and whether there are issues with the signal unable to penetrate the demo or getting attenuated or whatever. Having said that, the VP's also have a protective casing, so that should not be an issue?

    - A simple (laminated?) paper/cardboard sleeve around a vertical antenna would significantly reduce the drag of that antenna, down by 60-75% or so. In comparison to an aerodynamic airfoil, this would mean reducing the 10*10cm vertical airfoil down to 2.5cm*10cm comparative airfoil or so.

    On larger aircraft this makes less of a difference, since relative gain is lower. If noticeable at all, it would be the total consumed mAh or the top speed...? If anyone decides to try this, I'm interested in seeing the numbers.

  6. #16
    Engineer for Jesus Christ IBCrazy's Avatar
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    Perhaps this will explain it better:

    Think about all the dynamics of a plane at long distance. Generally if you selected you flying site well, you should have few objects to bounce off of and a fairly low noise floor.

    You also have a high gain antenna. This naturally rejects multipathing as many of the reflections enter the antenna's nulls rather than it's beam. Now if you chose your path well, you aren't flying into a high noise area. Aiming your highly directional antenna into an area of high interference still increases your noise floor. The beauty of interference far away is that your signal gets amplified the same amount, thus your signal needs to be as weak as the noise level at the noisy location (which is already pretty far away from your base station) before you drop out.

    This is the exact reason I have more or less given up on 900MHz. There are cell towers all over the place and if I'm not beside one already, I inevitably aim my antenna at one. I actually fly 1.3GHz as my preference and 2.4GHz as my back up. They nice thing about these bands is the noise floor is very predictable and typically quite low.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  7. #17
    I Like Waffles... SENTRY's Avatar
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    Quote Originally Posted by Trappy View Post
    multipath rejection is the reason why CP is favored over linear. for range and, unfortunately also for reliability, linear antennas are the way to go on the plane. When we touch branches or generally during transportation the CP antennas are much easier to bend. with that said, there were 2 people with cloverleafs at the meet, also on zephyrs. the performance was comparable to linear, even with linear groundstation antennas.
    Are you saying that people flew w/ CP on the plane and linear on the ground? There is still a 50/50 shot that the reflected signal will hit the rx out of phase - so what's the point? W/ only 50/50 odds - give me linear all the way - or if they are REALLY wanting to do CP then put CP on both ends. At least this way the rx can process "most" of the out-of-phase returns.
    "I Like Waffles" : FPVLab on Facebook and FPVLab on Twitter

  8. #18
    the problem was that the CP antennas we had on the ground only worked well on CH8, CH5 not so much. So since we switched around all the time (different people on different frequencies on various groundstations) we had to go with linear otherwise we would get problems.

    CP ground antennas are still realatively low gain. I'm used to perfect, uninterrupted pictures at 15km out. CP doesn't give me that yet, I can go 12km then the first signs of grainy picture start to appear. The cool thing about CP, though, is I can do close to 12km in the city, too

  9. #19
    I Like Waffles... SENTRY's Avatar
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    Makes sense - I would have made the same decision given the circumstances and given the need to share the band(s).
    "I Like Waffles" : FPVLab on Facebook and FPVLab on Twitter

  10. #20
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    The SAW filter you said is the same used by rcexplorer in his modification of a 1.3ghz Rx. He also said that this upgrade was not necessary for 2.4/5.8 ghz as they already had good enough SAW filters. Is that correct?
    Link http://www.rcexplorer.se/page14/saw/saw.html

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