Module 1 — Radio & communication links

Everything between your thumbs and the drone — and between the camera and your eyes — is electromagnetic waves. This module is why "range" is physics, not marketing.

🟢 Foundations. Radio waves are oscillating electromagnetic fields traveling at the speed of light $c$ . Frequency $f$ and wavelength $\lambda$ are locked together:

$\lambda = \frac{c}{f}$

At 2.4 GHz, $\lambda \approx 12.5$ cm; at 5.8 GHz, $\lambda \approx 5.2$ cm. Antenna dimensions scale with $\lambda$ — that's why video antennas are small. Lower frequencies bend around and punch through obstacles better; higher frequencies carry more data. FPV convention: control on 2.4 GHz (or 868/915 MHz), video on 5.8 GHz.

🟡 Practitioner. Radio engineers work in decibels because signals span billions-to-one ratios:

$P_{\text{dBm}} = 10\log_{10}\!\left(\frac{P}{1\,\text{mW}}\right)$

so 100 mW = 20 dBm, 25 mW = 14 dBm, and every +3 dB doubles power. Free-space path loss (FSPL) in convenient units:

$\text{FSPL(dB)} = 20\log_{10}(d_{\text{km}}) + 20\log_{10}(f_{\text{MHz}}) + 32.44$

The link budget is one line of accounting:

$P_{rx} = P_{tx} + G_{tx} + G_{rx} - \text{FSPL} - L_{\text{extra}}$

If $P_{rx}$ stays above the receiver sensitivity with 10–20 dB of margin, the link holds.

Worked example — ELRS at 5 km. TX power 100 mW (20 dBm), both antennas ~2 dBi, 2.4 GHz: FSPL = $20\log_{10}(5) + 20\log_{10}(2400) + 32.44 \approx 114$ dB. $P_{rx} = 20 + 2 + 2 - 114 = -90$ dBm. At a 250 Hz packet rate (sensitivity ≈ −108 dBm) that's 18 dB of margin — a comfortable link. Approximate ExpressLRS sensitivities (see official docs for current values):

Packet rate	Sensitivity	Notes
500 Hz	≈ −105 dBm	lowest latency
250 Hz	≈ −108 dBm	everyday default
150 Hz	≈ −112 dBm
50 Hz	≈ −117 dBm	long-range mode

Every −3 dB of sensitivity ≈ 1.4× more range; going 250 Hz → 50 Hz roughly doubles it. Same math governs the video link, except video needs far more bandwidth, so it always dies first — by design.

🔴 Advanced. Real links are not free space. The first Fresnel zone — an ellipsoid around the line of sight — must stay mostly clear; its radius at mid-path is

$r \approx 8.66\sqrt{\frac{d_{\text{km}}}{f_{\text{GHz}}}}\ \text{m}$

At 2 km on 5.8 GHz, $r \approx 5.1$ m: fly a few meters above obstacles or the ground itself eats your signal. Add polarization (circular RHCP/LHCP for multipath rejection on video; linear for ELRS), antenna radiation patterns (a dipole's donut has a null straight up — don't fly directly overhead of a vertical antenna), diversity receivers, and multipath fading. Digital video (DJI/Walksnail/HDZero/OpenIPC) trades analog's graceful static for H.265-compressed 1080p with adaptive bitrate — understand what the "bars" in your OSD really measure (SNR vs RSSI).

Link budget waterfall (example, 2.4 GHz @ 5 km)
TX power        +20 dBm ────┐
TX antenna      + 2 dBi     │
RX antenna      + 2 dBi     ▼
Path loss      −114 dB   ═══════►  P_rx = −90 dBm
Sensitivity    −108 dBm  ─ ─ ─ ─   margin = 18 dB ✔

⚫ Master. You can compute a full budget including feedline losses and fade margin, choose packet rate and antennas from requirements (not habit), read a spectrum analyzer, build your own antennas ( $\lambda/4$ ≈ 31 mm ground-plane for 2.4 GHz), and reason about OpenIPC/wfb-ng — a fully open, hackable digital video link — at the packet level.

Mastery checklist

Given TX power, antenna gains and distance, predict RSSI within ~5 dB of measurement.
Explain why 50 Hz ELRS reaches farther than 500 Hz using only the noise-bandwidth argument.
Diagnose "video dies behind trees, control doesn't" from first principles.

🖼️ Image ideas: your own photos of dipole vs patch vs helical antennas; Wikimedia Commons "Antenna radiation pattern" diagrams (many are PD).

📚 Free resources: ExpressLRS documentation (range & telemetry pages); Oscar Liang antenna guides; Chris Rosser's YouTube RF series; ITU-R free study texts on propagation.