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Thread: Link Margin, Transmitter Power, Theoretical Range, and Antenna Gain

  1. #61
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    Have you thought about a Fresnel-zone addition to account for antennas near the ground? I just had a remote-landing event when I went too far, too low for generic RC 2.4GHz gear. Video at 5.8GHz was still good. Range was about 500m. Usually I can go more than 4 times this far if I am at altitude, but at 1-2m off the ground - no link. The good news - it landed fine, but I had to walk out and fetch it.

    http://youtu.be/quGz7HADVyQ

    I think it's time to switch to UHF. ;-)

  2. #62
    Young Skywalker Rfriedman's Avatar
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    This thread is amazing, I am going to have so much fun
    I'm going to compare my rangelink to the pepperbox 13dbi on 1.3 using mad mushroom, and compare theoretical ranges.
    FYI if you want more receiver sensitivity, get your video rx from iftron. I saw firsthand how my bevrc 1.3 rx sensitivity was 91 dbm, while the iftron was 99dbm. I'll test the readymade v4 next time and post results (I currently fly readymade V4 + pepperbox)

  3. #63
    I see you... Derrick's Avatar
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    Quote Originally Posted by Rfriedman View Post
    This thread is amazing, I am going to have so much fun
    I'm going to compare my rangelink to the pepperbox 13dbi on 1.3 using mad mushroom, and compare theoretical ranges.
    FYI if you want more receiver sensitivity, get your video rx from iftron. I saw firsthand how my bevrc 1.3 rx sensitivity was 91 dbm, while the iftron was 99dbm. I'll test the readymade v4 next time and post results (I currently fly readymade V4 + pepperbox)
    Receiver sensitivity is only part of it, local noise floor is the other... if you have a receiver sensitivity of -99dbm but the local noise floor is -88dbm, the iftron receiver will preform no better than the -91dbm receiver.

    Edit:
    This is where the "Theoretical" portion of the title comes from... under ideal circumstances. To determine your likely max range you would replace the RS term of the equation in post #2 with the measured local noise floor at your receivers location.
    Last edited by Derrick; 24th October 2014 at 09:31 PM.
    When nothing else out there will suit your needs... design and build it yourself.

  4. #64
    Young Skywalker Rfriedman's Avatar
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    I'll grab the rf explorer next time I go out and check the noise floor (approximate) . I understand noise floor can be higher than sensitivity.

  5. #65
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    I'm surprised this thread went dead because there's a little bit of explanation left to do in the areas of noise and fresnel zones. Also no-one has yet talked about polarization which can be pretty important. I'm a bit of a layman in this area; any expert review is much appreciated.

    About noise:

    Lumping all noise by calling it "background noise" is a misnomer, some noise sources can be directional and can be filtered out by intentionally placing the noise source in a lossy direction of the antenna's gain. A certain level of environmental noise is truly omni-directional and can't be filtered out even with a perfectly directional system; this is usually called the noise floor. The gain/directionality of a system can be changed by using a different antenna shape, or by electrically combining multiple antennas into a single system. Large electrically steered phased arrays have a high directionality in a very specific direction, and reject signals from other directions. Larger phased arrays aren't practical to the hobbyist but simpler systems can be constructed using just a couple antennas.

    Noise may be of a specific frequency or it may be a 'white noise' that more evenly permeates the frequency spectrum. Noise of discrete frequencies outside the desired signal band may be filtered out using electric filters on the receiver or the antenna design itself (the bandwidth of the antenna). Cheap receivers may ignore filters altogether and be subject to a lot of otherwise avoidable noise. Engineered systems may electrically modulate an extremely tight filter with a known pattern and are extremely resistant to noise (aka jamming, when the noise is intentionally generated). Certain antenna shapes have tighter bandwidths and reject more frequencies, but this also means they can only receive signals in a tighter band as well. Wide bandwidth antennas will pass a large frequency range to the receiver, which must be electrically filtered, this is why you 'tune' an FM radio to a certain station.

    Complex filters (and their amplifiers) can also introduce additional electrical noise into a system. Near-field interference (aka local magnetic fields) from nearby electronics, or even internal to the receiver itself, can also wreck havoc on poorly designed or unshielded receivers. Cheaper unenclosed inductors are notorious as internal noise sources and will even be amplified by shielded enclosures. All these factors can combine to make some atrocious receivers if no QA has been performed on the design.

    About polarization:

    EM waves may also be polarized in a certain direction. Most antennas emit linearly polarized signals and by rotating the antenna 90 degrees you can greatly impact the signal strength. If your receiver and transmitter are 90 degrees out of phase, you will experience a big reduction in signal. Also if noise is a linear polarization you can reduce the received noise by rotating your antenna (and transmitter!) so that it is 90 degrees out of the noise's phase. If you know of a fixed transmitter (or if your buddy right next to you is operating in the exact same frequency band!) is the source of your noise, you might try rotating your antenna and transmitter so it's 90 degrees out of phase.

    Mankind really loves to point dipole/whip omni-directional (omni) antennas towards the sky. Dipole, whip, and most patch antennas emit linearly polarized waves. If you operate your own antenna & receiver 90 degrees out of phase with all the others, so that they are parallel to the horizon, you may greatly reduce your noise levels. Just don't point your dipole/whip antenna directly at the target, that's the dead zone for a dipole! Most patch antennas have gain perpendicular to the antenna surface, so you DO actually want to point patch antennas towards your target. Patch antennas that are strip fed are linearly polarized in the direction of the feed, coax fed patches are linearly polarized from the input of the feed towards the center of the patch. Try to keep the line of sight perpendicular to a dipole/whip orientation, or reorient your base station antenna as you fly around it. For hi-gain omnis as soon as you have altitude separation you're flying outside the gain of the antenna! This can also be avoided by using horizontally oriented antenna & receiver. (Because horizontal antennas are directing their entire gain at the ground they may also be more susceptible to multipath dead zones, but you can also find regions of constructive interference!).

    About fresnel zones:

    EM waves don't only travel along the line of sight, they actually flex and re-converge inside the .5 fresnel zone as they propagate. If you have any obstruction (like trees, a wall, or even the ground itself) within .5 of the first-order fresnel zone, your signal will experience fading due to the obstruction. The fresnel zone, and therefore fresnel losses, become larger with transmission distance. Raising the height of your antenna will provide clear space and reduce the fresnel losses. If you are operating with a large obstruction (any wall) directly below your line of sight, you are going to experience significant fresnel losses at longer ranges. Additionally if you have any large reflective surface parallel to an even fresnel zone (or a fractionlly even fresnel zone!) you will experience some destructive interference from multi-path propagation. Multi-path interference is pretty negligible out in the country, but inside a city the metal wall of a building could annihilate your signal in select spots (but actually boost your signal in other areas!).

    TLDR: The orientation of your antenna matters a lot, and each of these phenomenon could cut your signal strength just as much, or more, than free space loss.
    Last edited by zerohourrct; 30th April 2016 at 03:17 AM.

  6. #66
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    Quote Originally Posted by zerohourrct View Post
    I'm surprised this thread went dead because there's a little bit of explanation left to do in the areas of noise and fresnel zones. Also no-one has yet talked about polarization which can be pretty important. I'm a bit of a layman in this area; any expert review is much appreciated.
    Thanks zerohourrct, good information!
    I saw recently this video "The Fresnel Zone explained"
    It might add some more information about what you explained already.


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