How to read a link budget before you buy the radios
A point-to-point link either closes or it does not, and the arithmetic that decides which tells you long before any hardware arrives on site.
WZC Networking
Every wireless bridge is a sum. Power leaves one radio, the air takes some away, and what arrives at the other end either clears the receiver's threshold with room to spare or it does not. The whole design lives in four numbers.
The four numbers
Transmit power is what leaves the radio, in dBm. Antenna gain is what the dish adds by concentrating that power into a narrow beam instead of a sphere, in dBi. Free-space path loss is what distance takes out, and it is the largest number in the budget by a wide margin. Receive level is what is left.
A typical 60 GHz shot over just under two miles might read plus 23 dBm of transmit power, plus 34 dBi of antenna gain, minus 128 dB of path loss, arriving at minus 45 dBm. If the radio's threshold is minus 65, that link has 20 dB of fade margin.
Margin is the number that matters
Path loss is fixed by physics and distance. It does not change. What changes is everything else: rain, humidity, a hot afternoon, an antenna that shifts a fraction of a degree on its mount over a winter.
Fade margin is how much of that the link can absorb and still stay up. A link with 3 dB of margin works on the day it is commissioned and drops in the first heavy rain. At higher frequencies, where rain absorption is severe, margin is not a luxury.
Line of sight is a volume, not a line
The most common design error is treating clear line of sight as a straight line between two dishes. It is not. It is an elongated ellipsoid around that line, the Fresnel zone, and it has to stay clear of obstructions.
At two miles this zone is metres across at its widest point. A roof parapet or a tree line that sits well below the visual line of sight can still intrude into it, and the loss that causes does not show up in a survey done by eye.
Do the arithmetic first
None of this requires the hardware to be on site. Distance comes from a map, path loss from the frequency and that distance, gain and transmit power from the data sheet. The budget closes on paper or it does not.
Ordering radios and finding out afterwards is the expensive way to learn the same thing.
