What is free-space path loss and why does it increase with frequency?

Free-space path loss (FSPL) is not actually a loss in the traditional sense; no energy is absorbed. It represents the geometric spreading of the wavefront as it travels outward from the source: the power density decreases as 1/r^2 because the same power is distributed over an increasingly large sphere. The frequency dependence arises from the receive antenna's effective aperture shrinking at higher frequencies (Ae = lambda^2/(4 pi) for an isotropic antenna). FSPL in dB = 20 log10(4 pi d / lambda) = 32.44 + 20 log10(f_MHz) + 20 log10(d_km). At 1 GHz and 1 km: FSPL = 92.4 dB. At 10 GHz and 1 km: 112.4 dB. The 20 dB increase per decade of frequency is entirely due to the smaller effective aperture of the receive antenna.

How much fade margin should I include?

Fade margin depends on the required availability and the propagation environment. For 99.99% availability on a terrestrial microwave link: 30 to 40 dB margin (to cover multipath fading). For 99.9% cellular coverage at 2 GHz in an urban environment: 8 to 12 dB log-normal shadow fading margin (standard deviation 6 to 8 dB). For satellite links at Ku-band in a rain zone: 5 to 10 dB rain attenuation margin depending on geographic location and frequency. Indoor WLAN at 5 GHz: 10 to 15 dB margin for wall penetration and body shadowing.

What is the difference between EIRP and ERP?

EIRP (Effective Isotropic Radiated Power) is the product of the transmitter power and the antenna gain referenced to an isotropic radiator (dBi). ERP (Effective Radiated Power) references the antenna gain to a half-wave dipole (dBd), which has 2.15 dBi of gain. Therefore EIRP = ERP + 2.15 dB. A base station with 20 W transmitter power and a 15 dBi sector antenna has EIRP = 43 + 15 = 58 dBm (or equivalently, ERP = 58 - 2.15 = 55.85 dBm). FCC regulations often specify limits in ERP (for FM broadcast) or EIRP (for cellular and satellite), so knowing the convention is critical for compliance.

System Design

Link Budget

Before building any wireless system, you need to answer one question: will the receiver be able to hear the transmitter? A link budget answers this by adding up every gain (transmit power, antenna gain) and subtracting every loss (cable loss, path loss, fade margin, atmospheric absorption) to determine how much signal arrives at the receiver and whether it exceeds the receiver's sensitivity. If the margin is positive, the link works. If negative, no amount of clever coding or signal processing can fix it. The link budget is the first calculation in any wireless design and the last one checked before deployment.

Category: System Design

Key Result: Link Margin (dB)

Pass Criteria: Margin > 0 dB

Gains Minus Losses Equals Your Answer

Link budget equation:
P_RX = EIRP − FSPL − L_misc + G_RX
Link Margin = P_RX − Sensitivity

EIRP = P_TX − L_cable,TX + G_TX
FSPL = 32.44 + 20·log(f_MHz) + 20·log(d_km)

Worked example: 5G NR macro cell downlink at 3.5 GHz:
P_TX = +46 dBm | L_cable = 2 dB | G_TX = 18 dBi
EIRP = 46 − 2 + 18 = 62 dBm
Distance = 1 km | FSPL = 32.44 + 70.9 + 0 = 103.3 dB
Body loss = 3 dB | Shadow fade margin = 8 dB | Penetration = 15 dB
G_RX = 0 dBi (handset) | L_misc = 26 dB total
P_RX = 62 − 103.3 − 26 + 0 = −67.3 dBm
Sensitivity (64QAM, 100 MHz) = −85 dBm
Link Margin = −67.3 − (−85) = 17.7 dB ✓

Typical Link Budget Parameters

Parameter	Cellular (3.5 GHz)	Microwave PtP (18 GHz)	LEO Satellite (12 GHz)
TX Power	+46 dBm	+30 dBm	+43 dBm
TX Antenna Gain	18 dBi	42 dBi	35 dBi
FSPL (at range)	103 dB (1 km)	140 dB (15 km)	185 dB (1,200 km)
RX Antenna Gain	0 dBi	42 dBi	38 dBi
Fade/Misc Losses	26 dB	35 dB	5 dB
RX Sensitivity	−85 dBm	−75 dBm	−120 dBm
Link Margin	17.7 dB	14 dB	12 dB

Common Questions

Frequently Asked Questions

What is free-space path loss?

Not actual absorption; it is geometric spreading (1/r²). The frequency term comes from the receive antenna's effective aperture shrinking at higher frequencies. FSPL = 32.44 + 20·log(f_MHz) + 20·log(d_km). 20 dB more loss per decade of frequency.

How much fade margin?

Depends on availability. Terrestrial microwave (99.99%): 30 to 40 dB. Urban cellular (99.9%): 8 to 12 dB. Satellite Ku-band rain: 5 to 10 dB. Indoor WLAN: 10 to 15 dB for walls and body shadowing.

EIRP vs. ERP?

EIRP references isotropic (dBi). ERP references dipole (dBd). EIRP = ERP + 2.15 dB. FCC uses ERP for FM broadcast, EIRP for cellular/satellite. Know the convention for compliance.

Related Terms

← Linear Region LNA →

System Planning

Interactive Link Budget Spreadsheet

Editable link budget template with pre-loaded parameters for cellular, satellite, microwave PtP, and IoT scenarios. Calculates margin, availability, and maximum range.

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