Link Budget Calculator
Created by: Sophia Bennett
Last updated:
Model the full RF path from transmit power to received level so you can judge margin instead of guessing from wattage alone.
Link Budget Calculator
Amateur RadioCombine transmit power, antenna gains, cable losses, path loss, and receiver sensitivity into a received-signal and margin estimate.
What is a Link Budget Calculator?
A link budget calculator estimates whether a radio path is likely to close by adding the important gains and losses from transmitter to receiver. In amateur radio it is especially useful for VHF, UHF, microwave, satellite, and weak-signal station planning where every decibel can matter. Instead of guessing whether more power or a better antenna will help, the calculator shows where the signal is won and lost.
The calculation begins with transmit power and turns that into EIRP by combining it with transmit antenna gain and transmit-side losses. It then subtracts free-space path loss, adds receive antenna gain, subtracts receive feedline loss, and compares the resulting received power against a chosen receiver sensitivity. The difference is the link margin, which is one of the most useful summary numbers in practical RF planning.
A positive margin suggests the path is theoretically workable, while a negative margin suggests it is short of threshold before real-world variability is considered. That does not make the result perfect. Terrain, clutter, fading, polarization mismatch, and noise can still shift the real outcome. But a disciplined link budget is still far better than trying to reason about the path from transmitter watts alone.
For amateur operators, this is the point where separate calculators such as EIRP and path loss become one coherent station-planning model. It helps explain why modest power changes sometimes do very little, while a few dB of antenna improvement or feedline cleanup can transform the path.
How the Link Budget Calculator Works
The calculator first converts transmitter watts into dBm and then adds transmit antenna gain while subtracting transmit cable loss to compute EIRP. It separately computes free-space path loss from the entered distance and frequency. The receive side is then handled by adding receiver antenna gain and subtracting receiver cable loss. That produces the received signal estimate in dBm.
Link margin is the received signal minus the receiver sensitivity. If the margin is comfortably positive, the path is marked open. If it is close to zero, the result is marked marginal. If it is negative, the path is marked closed in free-space terms. The output also includes an approximate S-unit estimate so the result is easier to picture in ordinary amateur-radio language.
Link budget formulas
EIRP(dBm) = transmitter power in dBm + TX antenna gain - TX cable loss
FSPL(dB) = 20 x log10(distance in km) + 20 x log10(frequency in MHz) + 32.44
Received power(dBm) = EIRP - FSPL + RX antenna gain - RX cable loss
Link margin(dB) = received power - receiver sensitivity
Example Calculations
Example 1: Typical local VHF path
A simple 2 metre station with modest gain on each end can show how quickly received power changes with distance, even when free-space assumptions remain favorable.
Example 2: Weak-signal digital path
When the same hardware is judged against FT8 or WSPR sensitivity instead of SSB, the margin often changes from marginal to clearly workable. That is one reason weak-signal digital modes open paths that voice cannot sustain.
Example 3: Improving the system by dB, not just watts
Reducing coax loss or improving antenna gain can shift the margin more effectively than a modest transmitter-power increase. The budget makes those tradeoffs visible.
Common Amateur Radio Uses
- Estimate received signal strength for line-of-sight and free-space amateur-radio paths.
- Compare the impact of antenna gain, coax loss, and transmitter power on the same path.
- Judge whether SSB, CW, FT8, or WSPR thresholds are realistic for a proposed station pair.
- Support repeater, simplex, weak-signal, satellite, and microwave planning.
- Identify whether the transmit side, receive side, or path itself is the largest limiting factor.
- Turn path loss and EIRP figures into a complete received-power estimate.
Tips for Better Ham Radio Planning
Use realistic sensitivity numbers. A receiver specification and an on-air decoding threshold are not always the same thing, especially when bandwidth, noise environment, and operating mode differ from lab conditions.
Treat the result as a clean baseline rather than a full terrain model. If the path is obstructed, add additional losses mentally or with the path-loss calculator before trusting a small positive margin.
Frequently Asked Questions
What does a link budget tell me?
A link budget adds together the important gains and losses in a radio path so you can estimate received signal level and compare that result against receiver sensitivity. In amateur radio this helps turn vague station planning into something measurable, especially for VHF, UHF, microwave, satellite, and weak-signal work.
Why is EIRP part of the calculation?
EIRP combines transmitter output, feedline loss, and transmit antenna gain into one effective launch-power figure. That lets the link budget cleanly separate what happens at the transmitting station from what happens across the path and at the receiving station.
What does link margin mean?
Link margin is the number of decibels between predicted received power and the minimum signal level your receiver or mode needs. Positive margin means the path is theoretically open. Negative margin means the path is short of the required threshold before fading, polarization loss, and real-world variability are considered.
Why can a path still fail with positive margin?
Because the model may omit clutter, fading, polarization mismatch, feedline inaccuracies, terrain blockage, and man-made noise. A positive margin is useful, but it is not a guarantee. It is best treated as a planning estimate rather than a promise.
Which amateur modes can tolerate the lowest signals?
Weak-signal digital modes such as FT8 and WSPR can decode well below the SSB threshold, which is why they often succeed on paths that sound unusable by voice. That difference shows up directly when you compare link margin against different receiver-sensitivity assumptions.
How should I use this with the path-loss calculator?
Use the path-loss calculator when you want to isolate just the attenuation of the path itself. Use the link-budget calculator when you want the full received-power estimate after adding transmit power, gains, losses, and receiver threshold.
Sources and References
- ARRL Handbook, link-budget fundamentals and receiver-sensitivity concepts.
- Communications-system engineering references for EIRP, path loss, and margin analysis.
- Amateur weak-signal operating references for practical threshold and decoding context.