Antenna Gain & Link Budget Calculator
Enter your transmit power, antenna gains, frequency, and path distance to get free-space path loss, EIRP, received signal level, and link margin. Works for ham radio, LoRa, IoT, and FPV links. Nothing uploaded.
Link Parameters
Advanced link parameters
Link Budget
Learn more: free-space path loss and link margin
Path loss follows distance and frequency
The tool's free-space path loss (FSPL) is 20 x log10(distance in km) + 20 x log10(frequency in MHz) + 32.44 dB. At 915 MHz over 10 km that is 20 + 59.23 + 32.44, or 111.7 dB. At 20 km only the distance term changes, from 20 to 26.02, so the loss is 6.02 dB higher at 117.7 dB. That is where the rule of 6 dB per doubling of distance comes from.
The same rule works for power. Oscar Liang's FPV range guide says that raising output power four times, or by 6 dB, can double the maximum range. Frequency matters too: it says moving from 5.8 GHz to 1.3 GHz could increase your range by over 4 times. In the tool, 2.4 GHz at 10 km loses 120.0 dB against 111.7 dB at 915 MHz, 8.4 dB more.
Link margin
The tool adds transmit power and transmit gain, subtracts cable loss, adds receive gain, then subtracts the path loss and any obstacle loss. That gives the received power. The link margin is the received power minus the receiver's sensitivity.
For 20 dBm of transmit power, 3 dBi antennas at both ends, 1 dB of cable loss, 915 MHz, 10 km and a -110 dBm receiver, EIRP is 22 dBm and the received power is -86.7 dBm. The margin is 23.3 dB. With 25 dB of obstacle loss, such as terrain or buildings, the margin falls to -1.7 dB and the link fails.
What the verdicts mean
The tool calls a margin of 10 dB or more strong, 0 to 10 dB marginal and below 0 dB no link. Those cutoffs are the tool's own. Oscar Liang uses 10 dB as an example link margin, and the right figure depends on how much fading and interference your link sees.
The "range at 0 dB margin" figure scales the distance you entered by 10 to the power of margin over 20. It keeps your obstacle loss fixed and assumes free-space loss, so it is an upper bound. In the example above it gives 146.75 km, a distance where the horizon and terrain would block the path.
FAQ
Is more transmit power better than a better antenna?
For the link budget they count the same. Transmit power and antenna gain both add to EIRP, so 3 dB from either gives the same margin. Antenna gain can be cheaper, but gain also narrows the beam, so check what suits your layout.
Why does a link work indoors but fail at distance outdoors?
Free-space path loss assumes a clear path with nothing in the way. Walls, terrain and foliage add loss on top of it. The tool has an obstacle loss input for exactly that, separate from the distance loss.
Is a higher frequency always worse for range?
In free space, yes, at the same distance and gain. The frequency term adds 20 x log10 of the ratio, so 2.4 GHz costs 8.4 dB more than 915 MHz. Oscar Liang makes the same point with 5.8 GHz against 1.3 GHz.