Ummmm... I'll try. You get the short version. What the link calculator does is estimate the "fade margin" or "system operating margin" for a wireless link. Everything is in dBm (decibels above 1 milliwatt reference level).
Looks like the YDI site go hijacked so we'll switch to:
You start at one end of the link with the transmitter. It puts out
+15dBm (about 32 milliwatts) of RF power at 2.4GHz. That's the starting point.The +15dBm gets reduced by about half (3dB) by loss in the coaxial cable pigtail and connectors. That's the TX cable loss figure. The signal is now down to +12dBm.
The signal then goes to the cantenna. The best of the breed cantennas have a gain of about 8dBi. That's 8dBi above a reference theoretical isotropic radiator. 8dBi is 6.3 times gain over this theoretical point source radiator or about 6dBi gain over a cheezy dipole antenna as found on many cheezy access points. The nice thing about using the isotropic numbers is that I can just add the gains. So, we take the
+12dBm signal, add 8dB gain to it, and end up with a signal level of +20dBm.The +20dBm signal level is at the transmit antenna output but hasn't gone anywhere useful. Mother nature and her accomplis, physics, has conspired to make RF communications expensive. Inverse square law says your +20dBm signal gets weaker as it blunders toward the receiver. 5km will yield a free space loss of about 114dBm. So our
+20dBm signal is now decreased to 20 - 114 = -94dBm. The minus sign means that it's 94dB *BELOW* the 1 milliwatt reference level. That's a very weak signal.The cantenna on the receive end adds 8dB gain for a signal level of
-86dBm.
The receive cables and connects eat 3dB so that the signal at the receiver input is now -89dBm.
Digging into the specifications on the WAP11 and similar radios, I find that the receiver sensitvity is -82dBm at 11Mbits/sec. Note that the sensitivity varies with modulation method and connection speed. I'll do this at 11Mbits/sec although a slower speed may be more useful. The difference between the received signal (-89dBm) and the receiver sensitivity (-82dBm) is called the fade margin or system operating margin. In this case, it's -7dBm, or 7dBm short barely functioning. That won't work at all.
Even if the fade margin were 0dB, it still would not work. Sensitivity is measured at a BER (bit error rate) of 1*10^5. That's a
1 bit error every 10,000 bits. That's a rather high error rate and you would not want to run your system at this level. There are also the effects of signal bounce, atmospherics, birds sitting on the antenna, and such to ruin the signal. You need some kind of margin to insure that you have a strong enough signal at all times. 10dB fade margin is considered the absolute minimum. 20dB is a tolerable target. There's also a direct relationship between fade margin and % reliability.You're 17dB short of such a marginal system using WAP11's and cantennas. You need more antenna gain or more tx power or you don't have a chance at 5Km.