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.
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.
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