A link budget answers a practical question: given transmit power, antenna gain, propagation loss, and receiver sensitivity, is the signal arriving at the receiver sufficient for reliable demodulation? Most two-way radio systems are half duplex, handheld antennas are low to the ground, and operating environments vary widely. A link budget therefore provides engineering intuition and a planning starting point, not a substitute for field measurement. Actual values should be calibrated against equipment measurements and the real environment.

Power is commonly expressed in dBm: 0 dBm equals 1 mW, and every additional 3 dB is approximately a doubling of power. Antenna gain in dBi is referenced to an isotropic radiator, while dBd is referenced to a half-wave dipole; a common conversion is dBi approximately equal to dBd + 2.15. When writing a link table, the reference basis must be kept consistent. Mixing dBi and dBd can easily introduce errors on the order of 3 dB. Feedline loss, connector loss, and duplexer insertion loss are all counted under "other losses."

The received available power can be abstracted as:

[ P_r = P_t + G_t + G_r - L_{\text{path}} - L_{\text{other}} ]

Here, (P_t) is transmit power, (G_t) and (G_r) are transmit and receive antenna gains, (L_{\text{path}}) is path loss, and (L_{\text{other}}) covers feedline losses, human-body blockage, multipath fade margin, weather margin, and related effects. A usable link requires (P_r) to remain above the minimum usable signal required by the receiver for the target modulation and bandwidth, while also preserving fade margin to tolerate shadowing, fast fading, and equipment aging.

Free-Space Path Loss and the Two-Way Radio Scenario

Under line-of-sight and far-field approximations, free-space path loss increases with both distance and frequency:

[ L_{\text{FSPL}} = 20\log_{10}(d) + 20\log_{10}(f) + 20\log_{10}\left(\frac{4\pi}{c}\right) ]

Urban, mountainous, and indoor environments often deviate substantially from FSPL. Diffraction, penetration, and multipath make the effective loss higher than free-space loss at the same distance. Planning based on FSPL alone is therefore often overly optimistic; empirical models or field measurements are needed. In UHF and VHF two-way radio, antenna height often matters more to practical connectivity than a 1-2 dB adjustment in handheld transmit power.

Half Duplex and Talk-Around

In simplex same-frequency operation, one side transmits while the other receives, so the budget can be calculated in one direction at a time. When the roles reverse, differences in antenna height, blockage, and how the radio is held may make communication effectively one-way: one direction has sufficient margin while the other does not. A repeater improves the downlink from a higher site, so coverage "through the repeater" is often significantly better than handheld-to-handheld communication.

Two-Hop Repeater Links and the Bottleneck

In repeater scenarios, the system consists of an uplink (handheld to repeater) and a downlink (repeater to handheld). Actual user experience is often limited by the weaker hop. Even if the repeater transmits at high power, a weak handheld uplink can still produce the familiar outcome: "you can hear the repeater, but the repeater cannot hear you." Inadequate duplexer isolation or antenna isolation can raise the noise floor or create intermodulation, which is equivalent to additional (L_{\text{other}}).

Fade Margin and Digital Error Correction

Multipath causes fast fading. Digital systems may improve intelligibility through error correction and interleaving, but they still require design margin. Human-body proximity and handheld posture change antenna radiation and matching; fluctuations of several dB are common. Regulatory power limits and the approved equipment configuration are hard constraints. One must not exceed power limits or modify equipment unlawfully in pursuit of extra range.

Relation to Sensitivity and Bandwidth

Receiver sensitivity is usually specified together with channel bandwidth, modulation type, and a target BER or SINAD criterion. Narrower bandwidth can reduce thermal noise, but may be constrained by regulation and adjacent-channel requirements. A change in modulation also changes the threshold (P_r) that is required. Digital systems often show a gap between "static sensitivity" and "dynamic sensitivity"; in real motion, multipath and frequency offset raise the effective threshold. If a link budget uses only the static value from a datasheet, an implementation loss and an environment margin should be added.

Noise-Limited and Interference-Limited Cases

In suburban or otherwise quiet RF environments, a link is often limited by thermal noise. In cities or areas with strong electromagnetic interference, the link may instead be limited by co-channel interference or intermodulation. In that case, raising transmit power does not necessarily improve intelligibility and may even worsen interference. Spectrum planning, filters, and antenna isolation are network-level matters that a single-end link budget cannot fully capture by itself.

References

For private-network engineering design, use the propagation model and vendor planning tools specified by the project. The formulas here are simplified for instruction.