One-sentence intuition
A link budget answers one question: after all the losses along the way, does enough signal arrive at the receiver to be decoded?
Plain-language picture
Imagine shouting across a field: your volume is “transmit power,” a megaphone is “antenna gain,” the air and walls absorbing your voice is “path loss,” and how good the listener’s ears are is “receiver sensitivity.” A link budget adds and subtracts these to check whether the listener can still understand you.
The formula
$$ P_r = P_t + G_t + G_r - L_{\text{path}} - L_{\text{other}} $$
| Symbol | Meaning |
|---|---|
| $P_t$ | Transmit power (dBm) |
| $G_t$, $G_r$ | Transmit / receive antenna gain (dBi) |
| $L_{\text{path}}$ | Path loss |
| $L_{\text{other}}$ | Feeder, body blockage, fade margin, etc. |
The link works when $P_r$ exceeds the receiver’s minimum decodable signal for the target modulation, with enough fade margin left over for shadowing, fast fading, and equipment aging.
dBm and dBi quick reference
- 0 dBm = 1 mW; every +3 dB doubles the power. Handhelds typically run 1–5 W (30–37 dBm).
- dBi is relative to an isotropic point source; dBd is relative to a half-wave dipole. Conversion: dBi ≈ dBd + 2.15. Always unify your reference in budget tables.
Free-space path loss (FSPL)
Under line-of-sight, far-field conditions:
$$ L_{\text{FSPL}} = 20\log_{10}(d) + 20\log_{10}(f) + 20\log_{10}!\left(\frac{4\pi}{c}\right) $$
Double the distance → +6 dB loss. Double the frequency → +6 dB loss.
Reality is much worse. Urban, mountainous, and indoor environments add diffraction, penetration, and multipath losses far exceeding FSPL at the same distance. Planning with FSPL alone is dangerously optimistic.
Why antenna height beats extra watts
Going from 4 W to 5 W adds roughly 1 dB. Raising the antenna from 1.5 m (waist level) to a 30 m rooftop repeater simultaneously improves line-of-sight and diffraction geometry—an effective gain far exceeding 1 dB. Professional network planning prioritizes site and antenna height, then power.
Repeater two-hop: the weaker leg wins
In repeated communication the link splits into uplink (handheld → repeater) and downlink (repeater → handheld). System performance is limited by the weaker leg—a high-power repeater doesn’t help if handhelds can’t reach it on the uplink.
Fade margin and regulatory limits
Multipath causes fast fading; body and grip posture introduce several dB of fluctuation. Budgets must include margin. But regulations cap maximum transmit power—never exceed type-approved limits to chase range.
Further reading
For network engineering, use project-specific propagation models and vendor planning tools; formulas here are pedagogical simplifications.