LONG RANGE ELRS
MAXIMIZING 868 MHZ RANGE
MANET 300 MHz (mil-band) RANGE OPTIMIZATION
The Fresnel zone concept explains why radio links that appear to have clear line of sight still experience unexpected signal loss. The electromagnetic wave does not travel as a thin beam — it occupies an ellipsoidal volume between transmitter and receiver. Objects that intrude into the first Fresnel zone cause diffraction losses even if they do not physically block the direct path. At 300 MHz over 10 kilometers, this zone has a radius of 50 meters at the midpoint — a significant clearance requirement that only an elevated platform can reliably satisfy.
The Fresnel zone concept is fundamental to understanding why elevated relay nodes dramatically improve communication reliability. Ground-level obstacles that appear to leave clear line of sight still degrade signals by intruding into the electromagnetic wave propagation volume. The mathematical relationship between frequency, distance, and clearance determines the minimum height for reliable links — and this calculation drives the entire Fischer 26 deployment altitude strategy.
Fresnel Zone — The Invisible Obstacle
Radio waves do not travel as thin beams — they occupy an ellipsoidal volume between transmitter and receiver called the Fresnel zone. The first Fresnel zone radius at the midpoint between two antennas is calculated as F1 = sqrt(wavelength × distance / 4). At 300 MHz (wavelength 1.0 meter) over 10 km distance: F1 = 50 meters. Any obstacle within this 50-meter radius zone around the direct path causes diffraction losses even without physically blocking the line of sight. A tree line 30 meters below the direct path still degrades the signal by 3-6 dB.
This is why ground-to-ground radio links in forested or hilly terrain perform dramatically worse than free-space calculations predict. The theoretical range at 300 MHz with 33 dBm transmit power and omni antennas is 56 km. In Swedish terrain with forest cover averaging 20 meters height: practical range drops to 3-8 km because trees intrude into the Fresnel zone everywhere. The solution is elevation: raising one end of the link above the terrain clears the Fresnel zone and restores near-theoretical performance. Fischer 26 at 200 meters clears the Fresnel zone for links up to 50 km.
Link Budget for Mission Planning
Before every deployment, the signals officer calculates link budget for critical communication paths. The calculation is straightforward: received power (dBm) = transmit power + transmit antenna gain + receive antenna gain - free space path loss - cable losses - miscellaneous losses. The result must exceed the receiver sensitivity (typically -110 dBm for Silvus) plus the required SNR (10 dB for reliable MANET) by a margin of at least 10 dB for weather and vegetation variability. If the calculation shows negative margin — the link will not work without a relay or higher-gain antennas.
Example: Silvus SL5200 at 300 MHz, 33 dBm power, omni antennas (+2 dBi each), 10 km distance. FSPL = 20×log10(10) + 20×log10(300) + 32.44 = 102 dB. Received power: 33 + 2 + 2 - 102 = -65 dBm. Receiver sensitivity: -110 dBm. Required SNR: 10 dB. Margin: -65 - (-110) - 10 = 35 dB. This link works with 35 dB of margin — it would survive heavy rain (3-5 dB loss) and partial Fresnel zone obstruction (10-15 dB) comfortably. At 30 km: margin drops to 25 dB. At 50 km: 21 dB — still viable but sensitive to conditions.
Try the interactive Link Budget Calculator →
Open the interactive Mission Planner →
Open the interactive Link Budget Calculator →
MANET Range Optimization
With Silvus SL5200 at 300 MHz military band, distance optimization focuses on antenna selection and Fresnel zone management rather than packet rate (MANET handles bandwidth allocation automatically).
RANGE OPTIMIZATION (300 MHz)
Antenna length at 300 MHz: quarter-wave = c/(4×f) = 0.2498m ≈ 25 cm. This is physically larger than 868 MHz (8.6 cm) — consider mounting on Fischer 26's vertical stabilizer.
← Del av Component Architecture
Implementation
# Fresnel Zone Clearance Calculator
import math
def fresnel_radius(distance_km, freq_mhz, zone=1):
"""Calculate first Fresnel zone radius at midpoint."""
d = distance_km * 1000 # meters
wavelength = 300 / freq_mhz # meters
# F1 = sqrt(n * λ * d1 * d2 / (d1 + d2))
# At midpoint: d1 = d2 = d/2
radius = math.sqrt(zone * wavelength * (d/2) * (d/2) / d)
return radius
# Silvus at 300 MHz
for dist in [1, 5, 10, 20]:
r = fresnel_radius(dist, 300)
print(f"{dist:2d} km: Fresnel radius = {r:.1f}m (antenna must clear this)")
# 1 km: 15.8m
# 5 km: 35.4m
# 10 km: 50.0m — Fischer 26 at 200m clears easily
# 20 km: 70.7m — ground-to-ground needs hilltop or mastRelated Chapters
Sources
See the categorized source sections earlier on this page for specific citations supporting each claim. Cross-referenced technical baselines: ArduPilot developer documentation; ExpressLRS hardware documentation; NATO STANAG 4609 Ed. 4 (motion imagery metadata), 4671 (UAV airworthiness), 2022 (intelligence evaluation); Watling & Reynolds, "Meatgrinder: Russian Tactics in the Second Year of Its Invasion of Ukraine", RUSI (2023); ISW daily campaign assessments at understandingwar.org (archive). FSG-A has no own operational experience.