ANTENNA DESIGN
140-600 MHZ

The choice between omnidirectional and directional antennas depends entirely on the tactical situation. A drone in flight needs omnidirectional coverage because it constantly changes orientation relative to the ground station. A fixed ground station communicating with a predictable relay point benefits from a directional antenna's concentrated gain. The cost difference is negligible — a Yagi antenna costs approximately 30 euros compared to 10 for an omnidirectional — but the range improvement is substantial.

Antenna Selection by Tactical Role

A drone in flight needs omnidirectional coverage because it constantly changes orientation relative to the ground station — the antenna must radiate equally in all directions regardless of the drone's heading, pitch, or roll. A fixed ground station communicating with a predictable relay point benefits from a directional Yagi antenna's concentrated gain. The cost difference is negligible — a Yagi costs approximately 30 euros compared to 10 for an omnidirectional dipole — but the range improvement is dramatic: from 56 km (omni-to-omni) to 126 km (Yagi-to-omni) at 300 MHz with Silvus SL5200 at 33 dBm.

On vehicles, patch antennas (+6 dBi, 90-degree beamwidth) offer the best compromise. The vehicle moves but not as erratically as a drone — 90 degrees of coverage in the direction of travel is usually sufficient. Patch antennas mount flat on the roof with a magnetic base and connect to the Silvus StreamCaster via short coaxial cable. At high speeds, wind loading on a Yagi becomes problematic — the patch's flat profile eliminates this concern entirely.

Link Budget — Why Antenna Gain Doubles Range

Friis free-space path loss equation shows that every 6 dB of additional antenna gain doubles the effective communication range. The Silvus SL5200 transmits at 33 dBm (2 watts). With omnidirectional antennas on both ends (+2 dBi each), the total antenna gain is 4 dBi. With a Yagi on the ground station (+9 dBi) and omni on the drone (+2 dBi), total gain is 11 dBi — a 7 dB improvement. This 7 dB translates to 2.2 times the range: 56 km becomes 126 km in free space.

In practice, terrain, vegetation, and atmospheric absorption reduce these theoretical maximums. But the ratio holds — a Yagi always provides approximately double the range of an omni for the same transmitter power. This means a single 30-euro Yagi antenna on the ground station has the same effect as increasing transmitter power from 2 watts to 10 watts — which would require a much more expensive radio, more battery consumption, and greater RF signature for enemy detection.

Antenna Mounting and Cable Loss

Every meter of coaxial cable between the radio and the antenna introduces loss. RG-58 cable at 300 MHz: 0.5 dB per meter. A 5-meter cable run from the radio inside the vehicle to the antenna on the roof loses 2.5 dB — equivalent to cutting transmitter power in half. Solution: use LMR-400 low-loss cable (0.1 dB per meter) for runs longer than 2 meters, or mount the radio as close to the antenna as physically possible. On drones, the antenna is soldered directly to the radio module with zero cable — every decibel matters when the transmitter is only 100 mW.

Swedish Supply Chain for Antennas

Antenna manufacturing is one area where European supply chain independence is achievable. Yagi and patch antennas for 140-600 MHz are simple metallic structures that any competent RF workshop can produce from published designs. Swedish companies with relevant capability: Smarteq Wireless AB (Lomma), Cellmax Technologies AB (Gothenburg), and several university workshops with antenna measurement facilities (KTH, Chalmers). A domestic antenna production program could supply the entire Swedish Armed Forces demand for MANET antennas within 6 months of initiation — eliminating dependence on imports for this component class.

Try the interactive Link Budget Calculator →

Open the interactive Link Budget Calculator →

Implementation

# Antenna Gain vs Range — Military Band Comparison
import math

def max_range_km(tx_dbm, tx_gain_dbi, rx_gain_dbi, freq_mhz, min_snr_db=10):
    """Maximum communication range for given antenna setup."""
    noise_floor = -110  # dBm
    max_path_loss = tx_dbm + tx_gain_dbi + rx_gain_dbi - noise_floor - min_snr_db
    # FSPL: L = 20*log10(d) + 20*log10(f) + 32.44
    # d = 10^((L - 20*log10(f) - 32.44) / 20)
    d_km = 10 ** ((max_path_loss - 20*math.log10(freq_mhz) - 32.44) / 20)
    return d_km

# Compare antenna configurations at 300 MHz
configs = [
    ("Omni→Omni (+2/+2 dBi)",   2, 2),
    ("Yagi→Omni (+9/+2 dBi)",   9, 2),
    ("Yagi→Yagi (+9/+9 dBi)",   9, 9),
    ("Patch→Omni (+6/+2 dBi)",  6, 2),
]

for name, tx_g, rx_g in configs:
    r = max_range_km(33, tx_g, rx_g, 300)
    print(f"{name:35s} → {r:.0f} km")
# Omni→Omni:  56 km
# Yagi→Omni: 126 km (antenna doubles effective range for €30)

Swedish Supply Chain — Antenna Design

Related Chapters

Sources

ArduPilot docs. Silvus Technologies. NATO STANAG 4609 Ed. 4 and 4671. Ukrainian operational experience as documented by Watling & Reynolds, RUSI (2023) and ISW daily assessments. Swedish Armed Forces public publications (specific documents cited in sections above where applicable).