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.
// 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.
How this model works & what it omits
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.
// shareable URL encodes every input. no backend.
// received power density vs range
// ai-generated breakdown of what these numbers mean — with diagrams.
Common questions
What is a rectenna?
A rectifying antenna — the receiving element of a microwave power link. It captures the incident beam and rectifies it directly to DC; the tool's conversion-efficiency input lumps the Schottky rectification and matching losses into one number, so the headline result is DC watts delivered, not RF watts received.
How much power actually arrives at the receiver?
Usually far less than intuition suggests, and the tool shows why. In the default satellite-to-satellite preset — 1 kW transmitted, 5 m dishes at both ends, 1000 km at 35 GHz — the beam has spread to a radius of roughly 2 km by the time it reaches the receiver, so a 5 m dish intercepts only a sliver and the delivered DC lands in the milliwatt range. Aperture diameters and range dominate everything: that beam-spill trade is the central engineering problem of any microwave power link design.
Why does the far-field (Fraunhofer) limit matter?
The Friis equation assumes the receiver sits in the transmitting antenna's far field, beyond the Fraunhofer distance 2D²/λ. Closer in, the beam has not formed and the numbers are invalid — for a 5 m dish at 35 GHz that boundary is about 5.8 km. The tool computes the boundary for your inputs and flags any range that violates it.
Does the model include rain and atmospheric loss?
Atmospheric gaseous absorption is exposed as an explicit loss-budget term (ITU-R P.676) — the ground-to-LEO preset carries a realistic value. Rain attenuation is not modelled: at 35 GHz it is significant for ground-to-space links in storm cells, so for those designs add margin from ITU-R P.838 on top of what this first-order budget reports.
References
- // Friis, H. T. (1946). A note on a simple transmission formula. Proc. IRE 34, 254-256.
- // Brown, W. C. (1984). The history of power transmission by radio waves. IEEE Trans. MTT 32(9), 1230-1242.
- // Mankins, J. C. (2012). SPS-ALPHA: The First Practical Solar Power Satellite via Arbitrarily Large Phased Array. NASA NIAC report.
- // ITU-R Recommendation P.676 — atmospheric gaseous absorption.
- // ITU-R Recommendation P.838 — rain attenuation models.
- // For the power-budget context, see the Spacecraft Power Budget tool's beamed-mode toggle.
