Field Guide · term

Also known as: link budget, RF link budget, power budget

A link budget is the bookkeeping of every gain and loss a signal encounters on its way from transmitter to receiver, tallied in decibels so the terms simply add and subtract.1 The output is the predicted received power, which is then compared against the receiver’s required threshold; the difference is the link margin. If the margin is comfortably positive the link closes; if it is negative or thin, the link is unreliable.

P_tx +G_tx − path loss +G_rx P_rx sensitivity floor margin
A link budget adds transmit power and gains, subtracts feedline and path losses, and checks that the received power clears the receiver's sensitivity floor by a positive margin.

How it works

The canonical form starts from the transmitter’s EIRP (transmitter power plus transmit antenna gain, less transmit feedline loss), subtracts the propagation losses, and adds the receive gains:

P_rx (dBm) = EIRP − L_path − L_misc + G_rx

  • EIRP — effective isotropic radiated power at the transmit antenna.
  • L_path — path loss; over a clear line of sight this is the free-space path loss computed from the Friis transmission equation.
  • L_misc — everything else: feedline and connector loss, polarization mismatch, atmospheric and rain attenuation, body/foliage blockage, pointing error.
  • G_rx — receive antenna gain, less receive feedline loss.

Compare P_rx with the receiver’s sensitivity — the minimum input power for an acceptable bit error rate or SNR. The surplus is the link margin. A robust design does not aim for margin = 0; it reserves a fade margin on top so that fading, multipath, and weather do not drop the link below threshold.

In practice

Every term is a straight addition once expressed in dB, so a link budget fits on one line of a spreadsheet. Engineers run it in both directions: forward to predict coverage from known equipment, and backward to solve for a missing requirement — for instance, “how much antenna gain or transmit power do I need for 10 dB of margin at this range?” On fading channels the required margin is set statistically (e.g. enough to keep the outage probability below a target), which is where Rayleigh and Rician fade models feed into the budget.

Relevance to SDR

For a receive-only SDR the link budget explains what you can and cannot hear. Starting from a transmitter’s published ERP/EIRP and the distance, subtract free-space (and terrain) path loss, add your receive antenna gain and any LNA, and compare the result against your effective sensitivity — the noise floor plus the SNR the demodulator needs. A negative margin is a quantitative diagnosis: the fix is more gain, a lower-loss feedline, a quieter front end, or a better location, and the budget tells you exactly how many dB short you are.

GopherTrunk does not compute link budgets internally — it decodes whatever reaches its input — but the framework is the right way to reason about why a given trunking site or channel does or does not decode at your location, and what change would close the gap.

Sources

  1. Link budget — Wikipedia, the gains-and-losses accounting from transmitter to receiver and the definition of link margin. ↩

See also