Microwave link budget
Microwave Power Link Budget
How much DC power lands at the rectenna after travelling through free space? Friis equation, all loss terms exposed.
How this model works & what it omits
parabolic-dish gain, free-space path loss, EIRP, rectification efficiency. far-field assumption; near-field warning surfaces automatically when violated.
This calculator runs the Friis transmission equation (Friis 1946) end-to-end for a microwave power link: parabolic-dish gains, effective isotropic radiated power (EIRP), free-space path loss (FSPL), pointing / polarisation / atmospheric losses (ITU-R P.676), and the rectenna - rectifying antenna - conversion back to DC. Defaults model a 35 GHz Ka-band satellite-to-satellite link, with every term exposed and the frequency tunable across 0.1–300 GHz. Far-field and beam-spill warnings surface automatically when a configuration violates the Friis assumptions. This tool sizes a microwave power-transmission link, not a data link - for a communications downlink budget (carrier-to-noise, rain fade, and link margin), use the satellite comms link budget calculator.
Microwave power transmission is a 50-year-old idea with renewed engineering relevance for space-to-space and space-to-ground power links. The basic physics is short: a transmitter radiates a beam of microwave energy through a directional antenna, a receiver (a rectenna - rectifying antenna) captures the beam, rectifies it back to DC, and delivers power to a load. The arithmetic between Tx and DC is the link budget.
This tool implements the Friis transmission equation with explicit terms for parabolic-dish gain (G = η(πD/λ)²), free-space path loss (FSPL = (4πR/λ)²), pointing loss, polarisation mismatch, and atmospheric absorption. EIRP is Tx-power times Tx-gain; received signal is EIRP times Rx-gain over FSPL, minus all the loss terms; DC delivered is received signal times rectenna conversion efficiency.
Defaults model an example satellite-to-satellite microwave link: a 1 kW transmitter, 5 m dishes at both ends, 1000 km range (a transmitter at ~500 km to a receiver at ~1500 km), 35 GHz Ka-band. Three preset scenarios cover orbital-to-orbital, short-range densification, and ground-to-LEO with realistic atmospheric loss. Wavelength and FSPL update live as you change inputs.
Two warnings surface automatically when relevant: the Fraunhofer near-field boundary (Friis is invalid for ranges shorter than 2D²/λ), and beam spill when the Tx beam diameter at the Rx exceeds the Rx aperture diameter. Both are real failure modes for any microwave power link design.
What this tool does not model: rain attenuation at 35 GHz (significant for ground-to-space links in storm cells; ITU-R P.838), beam steering and tracking dynamics, sidelobe interference, regulatory power-density limits on the ground, and non-uniform aperture illumination. For flight design, defer to ITU-R P.676 / P.838, the SPS-ALPHA reference architecture (NASA 2012), and Brown's MTT history (1984).
try a scenario, then dial the loss budget for your link.
Frequency + range
35 GHz Ka-band is the default; tunable across 0.1-300 GHz.
Transmitter
parabolic dish; gain = η × (πD/λ)².
Receiver + rectenna
DC efficiency captures Schottky rectification + matching losses.
Loss budget
margins; sum into the link equation in dB.
Link budget
35 GHz · 1,000 km · far-field
63.7 dBi
Tx antenna gain
63.7 dBi
Rx antenna gain
93.7 dBW
EIRP
183.3 dB
Free-space path loss
-26.7 dBW
Received signal
2.1 mW
Rx power (linear)
1.5 mW
DC delivered
0.000%
End-to-end η
4.18 km
Beam diameter at Rx
187.3 µW/m²
Isotropic power density
5.84 km
Near-field boundary
beam spill
The Tx beam diameter at this range (4.18 km) is larger than the Rx aperture. Most of the radiated power misses the rectenna; the link calculation here uses the Friis equation with the configured Rx gain, which captures this loss implicitly through the smaller Rx capture area. To recover more power, increase Tx dish diameter (tighter beam) or move the Rx closer.
received power density vs range
