Published on 10/13/2026
An RF link budget adds radio power and antenna gains, then subtracts cable and propagation losses to estimate power at the receiver. A useful link budget example also compares that result with receiver sensitivity for the actual modulation, bandwidth and data rate.
Suppose a 2.4 GHz radio works on a bench but loses packets after its antenna moves outside a metal enclosure. The antenna now has a better position, but the longer coax feed has consumed some of the improvement. Treat the cable as a measured loss in the budget, not an invisible connection.
This guide follows power from the transmitter connector through the antenna and into the receiver. The numbers below are explicit design assumptions, not SigmaRF lab measurements or a promise of coverage.
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dBm is an absolute power level referenced to 1 mW. dB is a ratio used for cable loss and amplifier gain. dBi is antenna gain relative to an isotropic radiator in a specified direction. Add gain in dB to power in dBm; do not add two independent power levels in dBm.
| Quantity | Example input | Meaning in the budget |
|---|---|---|
| Radio output | 20 dBm | 100 mW before feeder loss |
| Cable loss | 2 dB | Subtract from radio power |
| Antenna gain | 5 dBi | Directional gain relative to isotropic |
| Receiver sensitivity | -90 dBm | Threshold for the stated radio mode |
Key idea: A 3 dB improvement is a power ratio of about 2. It is not automatically twice the range, twice the throughput or half the packet error rate.
For one transmission direction, received power equals transmitted power minus transmit feeder loss plus transmit antenna gain minus path loss plus receive antenna gain minus receive feeder loss. Define the endpoints first so losses are not counted twice.
Equation 01
EIRP at the transmit antenna
EIRP (dBm) = transmitter output (dBm) - transmit feeder loss (dB) + transmit antenna gain (dBi). If the equipment specification already gives EIRP, do not add its antenna gain again.
Equation 02
Power at the receiver input
Received power (dBm) = EIRP (dBm) - total path loss (dB) + receive antenna gain (dBi) - receive feeder loss (dB). Extra filters, switches and adapters belong in their relevant feeder total.
Equation 03
Margin above sensitivity
Link margin (dB) = received power (dBm) - receiver sensitivity (dBm). Subtract a separately justified fade reserve to find the margin remaining against your design target. Do not hide fading in path loss and subtract the same reserve again.
Use insertion loss at the operating frequency for the installed assembly length. A cable family name alone does not establish loss: length, cable construction, connector interfaces and manufacturing tolerances all matter. Include both ends and any separate adapters if they are not already included in the assembly measurement.
For an early estimate, feeder loss = cable attenuation in dB/m multiplied by length in m, plus separate connector, adapter and filter losses in dB. Replace that estimate with the complete assembly measurement when available. A VNA measurement of S21 magnitude provides insertion loss; VSWR addresses matching and is not an interchangeable loss figure.
| Change | Budget effect | Practical limit |
|---|---|---|
| Shorter transmit cable | Lower feeder loss increases EIRP | Recheck the applicable EIRP limit |
| Shorter receive cable | More signal reaches the receiver | Also consider noise ahead of an LNA |
| Higher-gain antenna | Adds gain in the chosen direction | Pointing and polarization must match |
| Extra adapter | Adds another interface and possible loss | Specify its frequency and impedance |
Read SigmaRF's Insertion Loss vs Return Loss vs VSWR guide for the measurement distinction, and Coax Cable Loss Calculator for feeder-only arithmetic. This article adds the antenna and propagation terms instead of repeating the cable comparison.
Specify the loss you can afford, not only the cable name
Send SigmaRF your frequency band, connector interfaces, installed length and insertion-loss target. Every assembly is VNA-swept for VSWR and insertion loss before shipment.
Specify a Custom AssemblyFor far-field free-space propagation, FSPL (dB) = 32.45 + 20 log10(distance in km) + 20 log10(frequency in MHz). Keep those units: inserting meters into the km term produces a 60 dB error. The equivalent expression is 20 log10(4 pi d / wavelength) with matching distance units.
At 2.4 GHz over 100 m, the free-space loss is about 80.05 dB. Walls, terrain, metal structures, multipath, blocked Fresnel clearance and antenna polarization can change the installed result. Free-space arithmetic cannot predict these effects just because a spreadsheet returns a positive margin.
Key idea: If total path loss comes from a measurement or propagation study, check which antenna gains and feeder losses it already contains. Do not add an obstruction allowance to a measured total that already includes it.
Receiver sensitivity must match the real operating mode. A threshold for a slow, narrowband mode cannot qualify a higher-rate mode. Use the same performance criterion, such as packet or bit error rate, when comparing radio options.
Step 01
Calculate both directions
A high-power gateway can reach a sensor that cannot return a packet. Reverse transmitter and receiver roles and use each radio's actual power, feeder and sensitivity values.
Step 02
Reserve margin deliberately
The example uses a 10 dB reserve only to demonstrate the arithmetic. Choose the real reserve from the environment, availability goal and installation evidence; there is no universal number that proves reliability.
Step 03
Confirm the installed link
Measure feeder loss, inspect connector condition and check packet delivery at the required rate and locations. A budget above sensitivity can still fail because of interference, collisions or changing obstructions.
Illustrative assumptions: 2.4 GHz, 100 m separation, 20 dBm transmit power, 2 dB transmit feeder loss, 5 dBi transmit antenna gain, 2 dBi receive antenna gain, 1 dB receive feeder loss and -90 dBm receiver sensitivity. The calculation uses free space and no additional obstruction loss.
The extra 3 dB of transmit feeder loss removes 3 dB of link margin. That is the cleanest way to compare two cable options when all other assumptions remain unchanged. It does not establish a new maximum distance, and the 23 dBm EIRP must be checked against local band and equipment rules.
How do I calculate an RF link budget?
Add transmit power in dBm and antenna gains in dBi, then subtract transmit cable loss, path loss and receive cable loss in dB. Compare the received power with sensitivity for the chosen radio mode.
What is the difference between EIRP and transmit power?
Transmit power is measured at the radio output reference plane. EIRP includes feeder loss and directional antenna gain: EIRP = radio power - feeder loss + antenna gain.
Does 3 dB cable loss cut antenna power in half?
Approximately yes: 3 dB loss leaves about half the input power at the output. Its effect on range depends on the propagation environment and link threshold, so it does not mean half the range.
How much fade margin does a wireless link need?
The required reserve depends on the environment, availability target and propagation evidence. A 10 dB reserve can illustrate a calculation, but it is not a universal requirement or proof that a link will work.
Can a better antenna compensate for coax cable loss?
Directional antenna gain can offset feeder loss in the budget along that direction. Check pointing, polarization, radiation pattern and EIRP limits; more gain is not free coverage in every direction.
Is a positive link margin enough for reliable communication?
No. It means the calculated input level exceeds the chosen sensitivity threshold. Interference, fading, packet collisions and an asymmetric return path can still prevent reliable communication.
A useful RF link budget is a ledger with clear reference planes. Use the actual radio mode, complete feeder insertion loss and directional antenna gain, then show your propagation assumptions and margin reserve separately.
For the cable purchase, give SigmaRF the operating band, assembly length, connector interfaces and maximum allowed insertion loss. SigmaRF supports assembly requirements across 100 MHz to 40 GHz, with Vadodara volume and Cleveland-area precision facilities. Lead times can be as short as 2 days, subject to the build and material availability; a particular assembly still needs its own agreed frequency and test limits.
Specify the loss you can afford, not only the cable name
Send SigmaRF your frequency band, connector interfaces, installed length and insertion-loss target. Every assembly is VNA-swept for VSWR and insertion loss before shipment.
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