Raising lightweight radio equipment just a few dozen metres above the ground fundamentally transforms the geometry and propagation physics of wireless communications. In mountain-rimmed valley basins like Skopje, dense urban reinforced concrete and rugged topographical obstacles attenuate ground-level signals within two to three kilometres. By lofting an autonomous LoRa repeater on a tethered aerodynamic kite or sounding balloon, direct optical Line-of-Sight (LoS) expands across tens of kilometres—establishing an ultra-resilient communication umbrella free from commercial cellular carriers or the electrical grid.

LoRa 868 MHz & Meshtastic Protocol

At the core of the airborne system is LoRa (Long Range) chirp spread spectrum modulation operating within the license-exempt European 868 MHz ISM band, adhering to CEPT/ETSI regulations: - Frequency Channels & Duty Cycle Compliance: For standard community packet traffic, we utilize channels in the 868.0–868.6 MHz sub-band with a maximum effective radiated power of 14 dBm (25 mW) and a 1% duty cycle. For elevated infrastructure repeater nodes, we take advantage of the 869.4–869.65 MHz sub-band, which permits up to 27 dBm (500 mW ERP) and a 10% duty cycle, ensuring superior signal penetration across valley terrain. - Open Meshtastic Protocol: Nodes run a decentralized ad-hoc mesh protocol using managed flood routing with configurable hop limits (typically 3 to 5 hops). Packets carry encrypted text messages, GPS coordinates, and sensor telemetry, with each node maintaining an in-memory deduplication hash table to suppress broadcast storms. - Cryptographic Channel Security: The protocol supports AES-256-CTR/GCM channel encryption for private responder groups (e.g., rescue squads and field teams), while broadcasting open distress beacons and public telemetry over the default primary channel.

The aloft airborne node acts as a dominant sky repeater (Router_Client or Client_Mute role), capturing weak micro-watt transmissions from handheld units down in urban street canyons and bridging them directly across mountain ridges to distant municipalities.

Aerodynamics: Kites and Tethered Balloons

To maintain the radio payload aloft in varying weather conditions, we employ two complementary flight platforms: 1. Aerodynamic Parafoil Lifter Kites (Flowform 16 & Rokkaku): Ideal for moderate to stiff breezes between 12 and 45 km/h (Beaufort 3–6). Flowform kites are completely frameless (no rigid carbon or fiberglass spars to snap upon ground contact); their aerodynamic wing profile and stability are maintained entirely by internal ram-air pressure cells with self-balancing vents. With an angle of attack trimmed between 15° and 20° and a high lift-to-drag ratio ($L/D \approx 4$), a Flowform 16 stably holds a 200–350g payload at altitudes between 80 and 120 metres for hours. The tether is braided Kevlar/Dyneema line (60–90 kg breaking strength)—ultralight, low-stretch, and non-conductive to eliminate static shock hazards. 2. Tethered Helium Sounding Balloons: Used during dead calm or light winds under 10 km/h. High-grade 100g–150g latex meteorological sounding balloons filled with 99.9% pure helium provide ~250–350g of gross lift, generating 100–150g of net free lift above the payload mass to keep the transceiver stationed directly above deployment ground zero.

The airborne payload is enclosed in a weather-sealed, 3D-printed PETG aeropod weighing under 115 grams, housing a Semtech SX1262 transceiver, an ESP32 dual-core MCU, a 3.2V 1500mAh LiFePO4 cell, a miniature MPPT solar trickle board (CN3791), and an omnidirectional half-wave coaxial sleeve dipole tuned to 868 MHz (VSWR < 1.25:1).

Line-of-Sight Calculations & Link Budget

Our field deployments are grounded in rigorous radio frequency physics and mathematical modeling:

  1. Radio Horizon & Geometric Line-of-Sight:
    Accounting for standard atmospheric refraction using the 4/3 effective Earth radius model ($k = 4/3$), the maximum line-of-sight radio horizon distance is calculated as: $$d_{\text{max}} \approx 4.12 \times \left(\sqrt{h_{\text{tx}}} + \sqrt{h_{\text{rx}}}\right) \text{ km}$$ For an airborne transceiver elevated to $h_{\text{tx}} = 100\text{ m}$ transmitting to a high-elevation ground node on Mount Vodno ($h_{\text{rx}} = 1060\text{ m}$): $$d_{\text{max}} \approx 4.12 \times (\sqrt{100} + \sqrt{1060}) = 4.12 \times (10 + 32.56) \approx 175.3\text{ km}$$ This geometry unlocks uninterrupted radio visibility across the entire Vardar valley, Kumanovo basin, and northern Pelagonia.

  2. Free-Space Path Loss (FSPL): $$\text{FSPL (dB)} = 20\log_{10}(d) + 20\log_{10}(f) + 32.44$$ For a link distance of $d = 100\text{ km}$ at $f = 868\text{ MHz}$: $$\text{FSPL} \approx 20(2) + 20(2.938) + 32.44 \approx 40 + 58.77 + 32.44 = 131.2\text{ dB}$$

  3. Link Budget & Margin:
    With transmitter power $P_{\text{tx}} = +22\text{ dBm}$ (160 mW), transmit antenna gain $G_{\text{tx}} = +2.15\text{ dBi}$, receive antenna gain $G_{\text{rx}} = +3.0\text{ dBi}$, and connector losses of $0.5\text{ dB}$: $$P_{\text{rx}} = 22 + 2.15 + 3.0 - 0.5 - 131.2 = -104.55\text{ dBm}$$ Because the Semtech SX1262 has a receiver sensitivity of $-130\text{ dBm}$ at Meshtastic's LongFast preset (SF11, BW 250 kHz) and down to $-137\text{ dBm}$ at SF12/BW 125 kHz, the resulting link margin is greater than 25 dB. This generous margin guarantees successful packet decoding even through atmospheric rain attenuation and urban RF noise.

  4. First Fresnel Zone Clearance:
    The radius of the 1st Fresnel zone at mid-path ($d_1 = d_2 = 50\text{ km}$) is $r_1 \approx 8.656 \sqrt{100 / (4 \times 0.868)} \approx 46.5\text{ m}$. Lofting the node to 100 m clears well over 60% of the first Fresnel zone above intermediate valley ridges and obstacles.

Disaster Resilience & Open Telemetry

During natural disasters (earthquakes, summer wildfires, or electrical blackout cascades), commercial cellular base stations typically fail within two hours once backup batteries exhaust. An airborne LoRa mesh repeater can be launched in under 10 minutes directly from a vehicle trunk or hiker's backpack.

Simultaneously, the airborne pod carries open micro-sensors for barometric pressure, temperature, relative humidity (BME280), and optical particulate smoke counters. Telemetry packets are broadcast unencrypted over open frequencies, logged by community ground stations, and archived into public datasets. All CAD models, PCB schematics, and firmware repositories are free and open source under CC-BY-SA and CERN-OHL licenses.