Comms Track: Spacecraft Links and Flight Software Basics · Module 2 of 4 · about 20 min

Decibels, antennas and the link budget

Comms engineers spend a lot of time on one question: will enough signal arrive to be decoded? The bookkeeping that answers it is called a link budget. You start with the transmitter's power, add the gain of the antennas on each end, subtract every loss along the way, and compare what is left with the noise at the receiver. If the margin is positive with some room to spare, the link closes.

The numbers span enormous ranges, so engineers use decibels. A decibel figure is 10 times the base-10 logarithm of a power ratio. Plus 3 dB is about double, plus 10 dB is ten times, plus 20 dB is a hundred times, and minus 3 dB is about half. The handy part is that multiplying ratios becomes adding decibels, so a whole budget is a column of additions and subtractions.

Antenna gain is measured against an imaginary antenna that radiates equally in every direction, called an isotropic radiator, which is why you see the unit dBi. NASA's own example: Voyager's high-gain antenna has a gain of 47 dBi at X-band, a factor of more than 50,000. Feed it a 20-watt transmitter and the effective power along that narrow beam is around a megawatt. The catch is pointing. A high-gain dish only helps when it is aimed accurately, which is why spacecraft also carry low-gain antennas for emergencies.

The biggest loss is distance. A signal spreads out as it travels, and its strength per square meter falls with the square of the distance: twice as far means a quarter of the power, or about 6 dB less. Over interplanetary distances this loss is huge, which is why deep space receivers are cooled, sensitive and attached to very large dishes.

Frequency matters too. Deep space links have moved from S-band (around 2 GHz) to X-band (around 8 GHz) and now Ka-band (around 32 GHz). Higher frequencies let the same size dish form a tighter beam and carry more data, but they are more affected by rain and humidity in Earth's atmosphere, so a budget for Ka-band carries extra weather margin. If you can read a link budget table and explain each line in plain words, you can hold your own in a first comms interview.

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