ANTI-JAM
HARDENING

Electronic warfare (EW) is the enemy's attempt to deny you control of your drones by jamming, spoofing, or intercepting your radio signals. This page covers practical hardening measures for MANET 300 MHz (mil-band) links.

How The Enemy Jams You: JAMMING Approach

Military jamming countermeasures protect drone communications through five layered defenses. Radio jamming threatens every drone operation. Jamming is the primary electronic warfare threat to drone operations. A jammer is a radio transmitter that blasts noise on YOUR frequency. Think of it as someone screaming in a library — nobody can hear anyone else talk. The enemy points a jammer in your direction and floods the 300 MHz (mil-band) band with powerful noise. Your drone's receiver can't distinguish your control signal from the noise. Link lost.

There are two types. Spot jamming targets one specific channel — defeated by frequency hopping (FHSS). Barrage jamming floods the ENTIRE band — much harder to defeat, requires more enemy power, but blocks everything including FHSS. Against barrage jamming, your options are: directional antennas (focus your signal away from the jammer), more power (shout louder), or fiber-optic FPV (no radio at all, see §1.5).

Layer 1: Frequency Hopping (FHSS)

ELRS hops between ~80 channels up to 150 times per second. The enemy can't follow. By the time their jammer retunes to your channel, you've already moved to the next one. This defeats spot jammers automatically — ELRS does this out of the box, no configuration needed.

Layer 2: Directional Antennas

A stock omnidirectional antenna radiates signal in all directions — including toward the enemy's jammer. A directional antenna (Moxon, patch, or Yagi) focuses the signal toward your drone and reduces signal going sideways. This means your signal is stronger in the right direction and the jammer's noise (coming from a different direction) is weaker at your receiver. Net effect: 6-12 dB advantage, equivalent to 4-16× more effective power. RadioMaster sells a Moxon antenna for 300 MHz (mil-band) at €12.

Layer 3: Power Management

ELRS supports up to 1W (1000 mW) output on 300 MHz (mil-band). Most pilots fly at 100-250 mW. In a jamming environment, increase to maximum power. The stronger your signal, the harder the enemy must work to drown it out. Combined with a directional antenna: 1W + Moxon gives roughly 4W equivalent in the antenna's beam direction. That's enough to maintain link through moderate jamming at 5+ km range.

Implementation

# pip install numpy
# Anti-Jam Link Budget — How Much Jamming Can We Survive?
import numpy as np

def jam_resistance(tx_power_dbm, tx_gain_dbi, rx_gain_dbi,
                   distance_km, freq_mhz,
                   jammer_power_dbm, jammer_distance_km,
                   fhss_gain_db=0, crpa_null_db=0):
    """Calculate signal-to-jam ratio with countermeasures."""
    # Desired signal path loss
    fspl_sig = 20*np.log10(distance_km) + 20*np.log10(freq_mhz) + 32.44
    rx_signal = tx_power_dbm + tx_gain_dbi + rx_gain_dbi - fspl_sig
    
    # Jammer path loss (jammer has omni antenna, 0 dBi)
    fspl_jam = 20*np.log10(jammer_distance_km) + 20*np.log10(freq_mhz) + 32.44
    rx_jam = jammer_power_dbm + 0 - fspl_jam
    
    # Apply countermeasures
    rx_jam_effective = rx_jam - fhss_gain_db - crpa_null_db
    
    sjr = rx_signal - rx_jam_effective  # Signal-to-Jam ratio
    
    return {
        "rx_signal_dbm": rx_signal,
        "rx_jam_dbm": rx_jam,
        "rx_jam_after_cm_dbm": rx_jam_effective,
        "sjr_db": sjr,
        "link_survives": sjr > 10,  # Need 10 dB SJR minimum
        "countermeasures": f"FHSS:{fhss_gain_db}dB + CRPA:{crpa_null_db}dB"
    }

# Scenario: 100W jammer at 2km vs Silvus at 10km
no_cm = jam_resistance(33, 2, 2, 10, 300, 50, 2)
with_fhss = jam_resistance(33, 2, 2, 10, 300, 50, 2, fhss_gain_db=15)
with_all = jam_resistance(33, 2, 2, 10, 300, 50, 2, fhss_gain_db=15, crpa_null_db=25)

print(f"No countermeasures:   SJR={no_cm['sjr_db']:.0f}dB  {'✓' if no_cm['link_survives'] else '❌'}")
print(f"FHSS only:            SJR={with_fhss['sjr_db']:.0f}dB  {'✓' if with_fhss['link_survives'] else '❌'}")
print(f"FHSS + CRPA:          SJR={with_all['sjr_db']:.0f}dB  {'✓' if with_all['link_survives'] else '❌'}")

Sources

ELRS anti-jamming characteristics (expresslrs.org). "Electronic Warfare in Ukraine" — RUSI (2023). Moxon antenna design for 300 MHz (mil-band) (radiomasterrc.com). FOI Memo 8336 on drone EW protection (2024).

Five-Layer Defense Architecture

The five layers of jam resistance are cumulative — each layer doubles the system's ability to maintain communications under electronic attack. Layer 1 FHSS (free, built into Silvus): spreads the signal across 4,600 channels. A narrowband jammer on one channel misses 99.98 percent of hops. Layer 2 Adaptive FHSS (free, firmware feature): automatically excludes jammed channels from the hopping sequence. Maintains communication as long as 10 percent of channels remain clean. Layer 3 CRPA antennas (€500-2,000 per ground station): places a 25 dB null in the jammer's direction. Reduces effective jammer power by 316 times.

Layer 4 Increased transmit power (hardware upgrade): Silvus SL5200 at 33 dBm (2W) can be paired with an external amplifier to reach 40 dBm (10W) — adding 7 dB of jam resistance margin at the cost of increased RF signature and power consumption. Layer 5 Fiber-optic FPV (€85 per mission): eliminates radio entirely for the control link. Zero RF emission means zero vulnerability to any electronic attack. Each layer addresses a different class of jamming threat. Together, only a barrage jammer exceeding 200W at less than 200 meters range can defeat all five layers simultaneously — and such a jammer is trivially detectable by SDR and targetable by FPV or artillery.

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