Link Budget

The idea

A link budget is like planning a road trip: you need enough fuel (signal) to reach the destination — with some in reserve. It is one line of arithmetic that decides whether a radio link succeeds, and because everything is in decibels, the whole chain collapses into addition and subtraction.

On the plus side: transmit power in dBm (how loud you shout), transmit antenna gain in dBi (a megaphone), and receive antenna gain in dBi (a hearing aid). On the minus side: free space path loss (the big one, often 70–120 dB), cable losses (a few dB in the wires), and obstacle losses (walls, trees, rain).

The result must beat the receiver sensitivity — the quietest signal the receiver can still decode, typically around −80 to −95 dBm for WiFi. The amount by which it beats sensitivity is the margin, your safety cushion. Positive margin means the link works today; engineers demand an extra 10–20 dB as insurance against rain, crowds, and bad luck. Zero margin on a sunny day means a dead link in a storm.

The math

$$P_{RX} = P_{TX} + G_{TX} - L_{path} + G_{RX} - L_{cables}$$ $$\text{Margin} = P_{RX} - \text{Sensitivity}$$

where P_TX is transmit power in dBm, G_TX and G_RX are antenna gains in dBi, and the L terms are losses in dB.

Worked example: transmit power +20 dBm, TX antenna +6 dBi, path loss 100 dB, RX antenna +6 dBi. Received power = 20 + 6 − 100 + 6 = −68 dBm. With a receiver sensitivity of −80 dBm, the margin is −68 − (−80) = 12 dB. Positive and reasonably healthy — ship it. Note how the path loss dwarfs everything else; that is typical, and it is why antenna gain and receiver sensitivity matter so much.

The same arithmetic tells you how to fix a failing link. Delivering −85 dBm to a receiver that needs −80 dBm? You are 5 dB short: swapping a 2 dBi antenna for an 8 dBi one adds 6 dB and saves the link, while doubling the distance or the frequency would each cost another 6 dB.

Common misconceptions

• Myth: If the transmitter is powerful enough, the link must work. Reality: Transmit power is only one entry in the budget. A 100 dB path loss swallows power fast — antenna gain and receiver sensitivity are often cheaper fixes than raw watts.
• Myth: A received signal below zero dBm is a failure. Reality: Almost all received signals are far below 1 mW. What matters is clearing the receiver's sensitivity, not the sign of the number.
• Myth: Any positive margin is good enough. Reality: A 2 dB margin dies the first time it rains or a crowd walks in. Professional designs carry 10–20 dB of headroom.

Try it

Open Link Budget Lego and assemble a link from blocks — transmitter, antennas, path, receiver. Try to break the link by stretching the distance, then rescue it with a higher-gain antenna and watch the margin bar move. The Link Budget Calculator handles the same arithmetic for real designs.