Link Margin is no longer a mystery - it is currently surfacing as possibly something more sinister; a manufacturer's term of art.
A standard Google search for the phrase inundates you with fairly consistent descriptions of what it is and how to calculate if from other receiver specifications that would e expected on a 'Data Sheet'.
From Wikipedia: "In a wireless communication system, the link margin, measured in dB, is the difference between the receiver's sensitivity (i.e., the received power at which the receiver will stop working) and the actual received power. A 15 dB link margin means that the system could tolerate an additional 15 dB of attenuation between the transmitter and the receiver, and it would still just barely work.
It is typical to design a system with at least a few dB of link margin, to allow for attenuation that is not modeled elsewhere. For example, a satellite communications system operating in the tens of gigahertz might require additional link margin (vs. the link budget assuming lossless propagation), in order to ensure that it still works with the extra losses due to rain fade or other external factors.
A system with a negative link margin would mean the system is insufficient to transfer data, usually this means a better receiver is needed, with improved sensitivity."
Many of the more comprehensive definitions equate Link Margin with 'Fade Margin' or 'Fade Budget' which does make contextual sense. This makes me think airLink should be telling me what the 'Link Margin' value *is* vs requesting me to input it. It is a relative number , expressed in db, of received power between what is good (a working and useful link for a theoretical amount of data throughput) and what is not good - a dropped link with no data throughput.
The wikipedia definition, if it is correct, states it is attenuation in db of an unspecified actual received power expressed in db below which the link will drop.
airLink wants me to input a value that would appear to be either empirical or below some *specified* db of received signal strength.
I'll try this again tomorrow. I was first ignorant, then ignorant and confused, now I'm ignorant, confused and I have headache.
The good news is - at https://www.ubnt.com/videos/ there is a page full of Ubiquiti videos alleging to explain stuff. I expect one of them will either make clear what airLink wants or will confirm my headache is warranted.... tomorrow
73
...dan wl7coo
PostScript - the ubnt.com/videos while mildly useful, are not at all instructional wrt to the technology so much as sharing (touting) their products' capabilities.