Signal Loss is the loss of the signal transmitted from the source AP or wireless card through the cable and connectors to the antenna broadcast, and is measured in dBm.
As far as cables go, LRM-400 antenna cables have less than 7dBm signal loss per 100ft of cable and are considered the most bang for your buck. This is what most people are using and is highly recommended if you want to keep signal loss to a minimum while keeping the cost reasonable [cf. CablesAndConnectors page]. Connectors (like the N-type on the antenna cable) are estimated to increase signal loss by 2.5dBm each.
Typically, total signal loss L will be
- L = 2.5n + 7X/100
where X is the length of cable measured in feet and n is the number of connectors.
Times Microwave has a Coaxial Cable Attenuation Calculator and various RF-math Online_Conversions (Flash). QSL has Perl-CGI design utilities, source available. See also QSL Intro, terms, and useful equations. LNS has a path loss calculation example for Loss Gain in a Path.
When estimating the total strength of the signal from your AP or wireless card:
- AP - SL + AG.
Where AP = Access Point signal, SL = signal loss in cables, AG = antenna gain.
You then have a "Free Space Loss" (at 2400 MHz, it is about 100 dB for one kilometre, 100 dB for 1094 yd. (104 for a mile?); add 6 dB each time you double the distance.
And again, you have the antenna gain, the cable and connectors loss to reach the receptor, which has a sensibility around -100 dBm. The quality of reception is the ratio Signal/Noise which translates into a dB difference. See CardboardHorn for an antennae comparison (signal in red, noise in green, both in dBm).
To reduce the signal loss caused by long antenna leads, many people are now mounting APs in WaterProofBoxes near where they mount their antenna. Doing this eliminates the signal loss due to extreme cable lengths. This will also allow for less efficient cable types to be used without increasing signal loss too drastically.
