If we use tethered balloons (aerostats) with wireless gateways fitted on them (let's say for Cellular or LPWAN communication), they can provide much more coverage owing to the height they can reach (e.g. while towers usually reach a maximum of 2000 feet, tethered balloons can usually reach a maximum of 4.5 km). And increasing the height of a gateway can increase the coverage surprisingly, since at a sufficiently good height, the Fresnel Zone gets good clearance giving clear line of sight. And it has been recently demonstrated using LoRa when someone created a record of 702 km coverage with a high-altitude balloon.
I wonder why are tethered balloons then not used for communication?
While there are many instances of high-altitude balloons (like Google's Project Loon, Space Data SkySite® or this question on this StackExchange), I could find scarce resource on low-altitude tethered balloons. What I could find was this work done in 2008 (IEEE Conference Paper) by a research lab. Another recent conference paper (2016) is this from another lab. The first paper also demonstrates that the tethered balloons turn out to be lower cost in the long run.
Some reasons that directly come to mind are:
- Balloons would require more maintenance (and may turn out to be expensive)
- Cost of balloons would increase dramatically as the weight of the wireless equipments is quite high
- Balloons would struggle under winds (however since they'd be tethered, exactly how would wind affect them?)
What I would like to know is some solid engineering answer on this, with some numbers to back the claim. What exactly restricts their usage from RF, Communications, Aerodynamcis or Mechanical point-of-view, as compared to fixed towers? Or perhaps is it just that beyond 2000 feet (maximum tower height), you do not get any benefit in Fresnel Zone clearance?