How do I calculate free space path loss at millimeter wave frequencies above 24 GHz?
mmWave Path Loss
The Friis equation shows that FSPL increases with the square of frequency (20 dB per decade). This is sometimes misinterpreted as mmWave signals experiencing more 'loss' during propagation. In reality, the FSPL increase with frequency reflects the decreasing effective aperture of a fixed-gain antenna: Ae = Gλ²/(4π). If both the transmit and receive antennas maintain constant physical aperture (dish or array size), the received power is actually independent of frequency.
The real disadvantages of mmWave propagation are: (1) atmospheric absorption (oxygen resonance at 60 GHz adds 15 dB/km, water vapor at 22 GHz and above 100 GHz), (2) rain attenuation (significant above 10 GHz, dominant above 30 GHz), (3) reduced diffraction around obstacles (waves at mmWave do not bend around corners effectively), and (4) increased penetration loss through building materials (concrete walls: 30-40 dB at 28 GHz vs 10-15 dB at 2 GHz).
Link budget example at 28 GHz, 200m range: FSPL = 107 dB. Atmospheric absorption: 0.1 dB (negligible for short range). Rain attenuation (heavy rain): 1-2 dB. Total path loss: 108-109 dB. To close this link with -80 dBm receiver sensitivity: required EIRP = 28-29 dBm, achievable with a 64-element phased array.
FSPL (dB) = 32.44 + 20log₁₀(fMHz) + 20log₁₀(Rkm)
At R = 1 km:
28 GHz: FSPL = 32.44 + 89.0 + 0 = 121.4 dB
39 GHz: FSPL = 32.44 + 91.8 + 0 = 124.2 dB
60 GHz: FSPL = 32.44 + 95.6 + 0 = 128.0 dB
Friis link equation:
Prx = Ptx + Gtx + Grx - FSPL
Frequently Asked Questions
Is mmWave really worse than microwave?
For the same antenna aperture size: no. The higher FSPL is exactly compensated by higher antenna gain. For the same antenna gain: yes, mmWave has higher path loss. The practical difference is that maintaining large apertures at microwave frequencies requires physically large antennas, while compact mmWave arrays achieve high gain.
What about indoor propagation?
Indoor mmWave propagation is severely affected by wall penetration loss (20-40 dB per wall), furniture blockage, and human body shadowing (20-35 dB). Indoor 5G mmWave coverage requires line-of-sight or near-line-of-sight paths, with dense small cell deployment.
How far can mmWave links reach?
Point-to-point links at 28 GHz: 200-500m for 5G small cells, 1-3 km for fixed wireless with high-gain dishes. At 60 GHz (with oxygen absorption): < 1 km for most applications. At 70-80 GHz (E-band): 1-3 km for backhaul with 0.3m dishes (47 dBi gain each end).