What is it and why use it?
What’s a link budget?
According to Wikipedia, a link budget is the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. It accounts for the attenuation of the transmitted signal due to propagation, as well as the antenna gains, feedline and miscellaneous losses.
Our version is the accounting of all the gains and losses from an active antenna, i.e. received power, to an amplifier/repeater assembly transmitter into free space or for direct connections to receivers or timig devices.
Why a link budget?
•You need a Link Budget to ensure proper RF coverage in a particular location
•It is used to determine the maximum distance at which a transmitter (amplifier/repeater assembly) and receiver can successfully operate
•If the estimated received power is sufficiently large (remember less is more), the Link Budget is said to be sufficient for sending signals under perfect conditions
•By itemizing all of the substantial parameters it provides a helpful basis for discussion, analysis and design, i.e. feasibility of any given system and risk contingency planning
•A Link Budget is also an excellent means for anyone to begin to understand the various factors which must be traded off to realize a given cost and level of reliability for a communications link
Ratio of Measures (Decibels or dB) and Link Budgets
The signals leaving a GPS satellite (transmitter) can be literally a billion times larger than the signals arriving at the receiver (active antenna) and as such the multiplication and division of these numbers can be quite a challenge. As an example, the receive power level at the receive antenna is -130dBm (L1) or 0.0000000000001 (thirteen zeroes) less than the transmitted power level at the satellite.
To represent these big numbers with smaller numbers, we use logarithms which allow the use of addition and subtraction instead of multiplication and division. This addition and subtraction forms the base of a link budget.
A logarithmic scale is a scale of measurement. The use of the logarithms of the values rather the actual values reduces a wide range to a more manageable size. The decibel (dB) is a logarithmic unit that indicates the ratio of a power or intensity relative to a specified or implied reference level.
Decibel Power Ratios (Example)
100dB = 10 000 000 000 90dB = 1 000 000 000
80dB = 100 000 000
70dB = 10 000 000
60dB = 1 000 000
50dB = 100 000
40dB = 10 000
30dB = 1 000
20db = 100
0dB = 1
-10dB = 0.1
-20dB = 0.01
-30dB = 0.001
-40dB = 0.000 1
-50dB = 0.000 01
-60dB = 0.000 001
-70dB = 0.000 000 1
-80dB = 0.000 000 01
-90dB = 0.000 000 001
-100dB = 0.000 000 000 1