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Beams of death in the air

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  • Beams of death in the air

    This was posted in a response to a discussion on “dead zones” in another thread. When working on 5G a few years ago, I became aware of the existence of >1300 companies in USA providing Wireless ISP (WISP) services in unlicensed bands, including the 2.4GHz used by the vast majority of our RC equipment. They have to obey the same wave law as everybody else, and the most commonly cited part of Part 15.247 is a limit of 1 watt with a maximum +6dB antenna gain in any direction (in other words, +36dBm max EIRP). However, there is an exception for “fixed point-to-point operations” in 15.247(c)(1)(i), which allows a -1dB reduction in total power for each +3dB of antenna gain. So in other words, for each +3dB of antenna gain you can add +2dB of EIRP. There is no specified EIRP limit.

    I don’t know what is typical or maximum, but I saw a demonstration of a system by a WISP company and saw the link budget, and they were using +56dBm EIRP! In other words 400x hotter than the 1 Watt limit of a non-fixed system. They could send live video 25 miles away with this system. This would obviously create a “beam of death” in the sky. If your plane flew into a beam at +56dBm, your receiver would be swamped.

    Because these are unlicensed, there is no requirement to disclose or record the locations. It could be problematic for UAV C2 links to have unmapped unknown “no-go” lines randomly scattered around, and potentially popping up unannounced. Nobody even knows how common this is. There is a lack of good research data on unlicensed congestion. I formed a “radio research” group within ASTM F.38.02, and this was one (of many) issues on the task list. But since then I have left the communications field and handed it over to others. So I don’t know the latest status.

    But anyway, there was discussion of random “dead zones” on various threads. I would tend to look for fixed point-to-point links. Power lines were mentioned in the Mig-29 thread, and it was mentioned that when power lines were removed the problem went away. It is possible the power lines themselves were causing a problem (they can), but another possibility to consider might be that where there is infrastructure there is often a communications link to that infrastructure, and when it is remote this might very well be a 2.4GHz fixed point-to-point system. So perhaps when the lines were moved the comm link was also moved.

    Anyway, I thought it was interesting enough to mention to RC folks. It would be great if somebody would pay for a general study of unlicensed congestion. FAA always thinks everything will magically get done “by industry.” They seem to constantly have unrealistic expectations about why companies would spend resources on this kind of thing. So last I knew (about a year ago), it was still a wildcard.

  • #2
    Gringo............ even though your tech language of dBm etcetera may go over most of the readers understanding, as an ex microwave engineer I totally get and agree with your writing.
    So the million dollar question (being facetious) is, Do ya think the FCC and FAA collaborate at all ????
    Thanx for taking the time to put this subject out here.
    Warbird Charlie
    HSD Skyraider FlightLine OV-10 FMS 1400: P-40B, P-51, F4U, F6F, T-28, P-40E, Pitts, 1700 F4U & F7F, FOX glider Freewing A-6, T-33, P-51 Dynam ME-262, Waco TF Giant P-47; ESM F7F-3 LX PBJ-1 EFL CZ T-28, C-150, 1500 P-51 & FW-190

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    • #3
      Originally posted by OV10 View Post
      Do ya think the FCC and FAA collaborate at all ????
      If by "collaborate" you mean "fight", then yes, lol. OK, I'm (half) joking. They both have groups and efforts to address the topic of aerial communications (for the coming "UTM"), but it was never quite clear who had final say in what. Aviation has typically used aviation-dedicated spectrum, and the FCC sort of left FAA alone in that sandbox. Now that many UAVs are designed to use unlicensed or cellular, then it puts the two in much closer contact/conflict. How do you weigh FAA's concerns and needs versus other users of the same spectrum? Not an easy question to answer. This is exactly the field I worked in for years, it is not something I can summarize in a post. It's a long story and a still-developing area.

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      • #4
        Very interesting topic. In the Mig 29 thread, when I mentioned the similarity of flying sites between my old site and the one where the first record Mig 29 crash happened, mainly the proximity of the high voltage lines, may have been a factor. I was never stating any conclusions but merely citing the similarities. I'm not expert enough in that area of knowledge to say one way or another. My mind was more on the track of electromagnetic fields that are generated by such high voltage lines (affects ranging upwards of 1000 feet). We all know that in N. America, electrical transmission is in terms of Hertz (cycles/second), however I think this is a different type of Hertz than the ones being compared to (ie, 2.4 Mhz). Where electrical transmission of AC is measured in terms of how many times the current changes direction (120 times or 60 cycles; where DC current has 0Hz yet still emit EMF), radio wave Hertz is in terms of oscillations of a sine wave over time. I don't believe that has anything to do with signal loss but rather the affect of EMF from high voltage lines. The higher the voltage, the higher the EMF. The power lines at my old field were rated at 138,000 volts, while some power lines can approach 1 million volts. I was pondering whether or not EMF (measured in "tesla" or Gauss) could have any affect on radio waves like the kind used in RC aircraft.
        Having said that, your post certainly does put another spin on the situation.

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        • #5
          Originally posted by xviper View Post
          .... could have any affect on radio waves like the kind used in RC aircraft.
          There is only one "Hertz", it is cycles per second. Well, then there is also the rental car company, so I guess there are two, lol. But anyway the discussion was whether an electric field so far away in frequency have any effect. The short answer is yes, it can. If interference is strong enough, the distance in frequency doesn't matter because the interference saturates circuits. Terms used are "clipping", "saturation" or "blocking". Even if it is a very low frequency noise, if that noise is pushing some circuit between its highest and lowest available levels, it essentially "erases" or "clips" whatever was there before. Frequencies only stay separated when added linearly, and if your circuit runs out of linear range, this math is no longer applicable. So yes passing through extremely high intensity oscillating electric/magnetic field can induce interference that saturates circuits. Then it doesn't matter how great is the frequency separation between the noise and the desired signal, because the desired signal is stomped out of existence. Radio circuits are particularly sensitive and susceptible to interference because they have high amplification.

          Also it isn't an "on" or "off" phenomenon. Generally compression is gradual, and you can see high frequency artifacts start to pop-up from a strong low-frequency interferer. In other words you can think of "signals add linearly and therefore are completely separable" and "noise totally saturates circuits and erases desired signal" as two extremes, and real life is usually somewhere in between.

          I would guess you have to be pretty darn close to a power line to experience this with the equipment we typically use, but I can't say I've ever done testing, or even looked up available data. So in short, it is not implausible, but I don't have numbers.

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          • #6
            Even shielding has questionable effect when you get near a giant EMF like those produced by big humming hydro lines. Circuits just get swamped with Eddy currents...

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