Work out the free-space path loss link budget for a point-to-point radio link: enter TX power, antenna gains, cable loss, frequency, and distance to get received signal power, link margin, the TX power needed for your target margin, and the maximum usable range.
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Quick Facts
FSPL (dB)
32.44 + 20log(fMHz) + 20log(dkm)
Free-space path loss between the two sites
Link margin
RX power − noise floor − interference
Reserve signal above the noise floor
Distance vs. loss
+6 dB loss per doubling
Both distance and frequency follow this square-law rule
Rule of thumb
10-20 dB margin
Typical target for a stable outdoor link
Remote Site Link Budget Results
RF Budget
Estimated RX Power
0 dBm
Received signal estimate at receiver
Link Margin
0 dB
Margin above configured noise floor
TX Power Required
0 dBm
TX needed for target fade margin
Max Distance @ Target Margin
0 km
Theoretical free-space distance limit
Link Budget Components
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How the RF link margin is calculated
This calculator builds a standard free-space link budget for a point-to-point remote-site radio link. It combines your transmitter power and antenna gains, subtracts the losses along the way, and compares what arrives at the receiver against the noise floor. Enter TX power, TX and RX antenna gain, combined cable/connector loss, operating frequency, and path distance, then click Calculate.
The free-space path loss (FSPL) formula
Free-space path loss in decibels is FSPL = 32.44 + 20·log10(frequency in MHz) + 20·log10(distance in km). This is the standard Friis-derived formula for unobstructed line-of-sight propagation and is the largest loss term in most outdoor links.
Link budget and margin
Estimated received power is TX power + TX antenna gain + RX antenna gain − cable loss − FSPL, all in dB/dBm. Link margin is received power − noise floor − interference penalty — the cushion left above the noise floor before the link starts dropping packets. The calculator also solves the budget in reverse to report the TX power needed to hit your target fade margin, and the maximum free-space distance the current setup can reach at that same target.
Interpreting the results
A link margin at or above your fade margin target is generally considered reliable; a margin near zero or negative means the link is likely to fail during rain fade, multipath, or minor misalignment. If the required TX power or max distance looks unreasonable, double-check antenna gains and cable loss first — those are the values most often mis-entered.
Frequently Asked Questions
What formula does this RF link margin calculator use?
It uses the standard free-space path loss (FSPL) link budget. FSPL in dB equals 32.44 + 20·log10(frequency in MHz) + 20·log10(distance in km). Received power equals TX power plus TX antenna gain plus RX antenna gain minus cable loss minus FSPL. Link margin equals received power minus the noise floor minus any interference penalty.
What is link margin and why does it matter?
Link margin (also called fade margin) is the extra signal strength above the noise floor that a radio link has in reserve. It absorbs rain fade, multipath fading, seasonal foliage changes, and minor alignment drift. A common rule of thumb for a stable outdoor link is 10-20 dB of margin; margins under about 6-10 dB are prone to dropouts during weather or interference.
Why does distance affect signal loss so strongly?
Free-space path loss increases with the square of distance, so doubling the distance between two radios adds about 6 dB of loss (20·log10(2) ≈ 6.02 dB). Loss also rises with frequency: doubling the frequency adds another 6 dB, which is why higher-frequency links (like 5.8 GHz vs 900 MHz) need shorter hops or higher gain antennas to cover the same distance.
What does the maximum distance result assume?
The maximum distance figure is the theoretical free-space range at which the link would still just meet your target fade margin, holding TX power, antenna gains, cable loss, and noise floor fixed. It assumes unobstructed line of sight with no terrain, foliage, rain, or Fresnel-zone obstruction — real-world range is usually shorter.