TerraBooster High-CMRR Active Shielded H-Field Loop Technical Overview

A High-CMRR Active Shielded H-Field Loop for Quiet HF Reception

Introduction

The TerraBooster series redefines the Loop-on-Ground (LoG) concept by abandoning brute-force impedance transformation and embracing the physics of near-ground magnetic coupling. Designed for environments plagued by urban noise, switching transients, and conducted EMI, the TerraBooster acts as a broadband magnetic field current sensor — not a resonator.

Core Design Philosophy

Conventional LoGs rely on 9:1 or 6:1 impedance transformers and hope for "quiet" reception by virtue of symmetry and proximity to lossy ground. But this approach ignores fundamental physics:

  • A wire near ground does not resonate
  • Performance is dominated by geometry, symmetry, and coupling — not impedance
  • The LoG is not a voltage source but a time-varying magnetic field integrator

The TerraBooster is designed as a linear H-field sensor with maximum control over:

  • Shielding and current return paths
  • Loop geometry and directional response
  • Common-mode rejection (CMRR)
  • Differential signal extraction and RF limiting

Amplifier and Signal Chain

The amplifier core is a dual MMIC in push-pull, offering:

  • +19.5 dBm P1dB
  • +41 dBm IOP3
  • ~1.7 dB noise figure
  • Differential input transformer: 1:1.5 (ADT1.5-1)
  • ESD protection and passive RF limiter on both inputs
  • High-CMRR bias tee with integrated line isolator

This design prevents overload, supports long coax runs, and keeps both CMC and differential noise under control.

Loop Element

Shielded Variants (Mini / Medi / Maxi)

All shielded versions use RG178B/U coax with:

  • The center conductor as the active loop
  • The braid as a Faraday shield, grounded via a high-impedance RC drain
  • Termination:  parallel RC to ground via RVS rod (high impedance) on one side only

This termination:

  • Prevents braid from floating at RF
  • Bleeds off CMC and induced noise
  • Preserves the differential field symmetry

Xtreme Version

The Xtreme uses FS2 stranded antenna wire, offering:

  • Maximum physical length (84 m)
  • Beverage-like gain on 10–40 m
  • Full omnidirectional coverage (unshielded)
  • Higher susceptibility to man-made noise

RF Physics: How It Really Works

The TerraBooster as a Magnetic Field Sensor

The induced voltage follows Faraday's Law:

V = -N · dΦ/dt,

where Φ is the magnetic flux through the loop.

  • No resonance: current distribution is linear, not sinusoidal
  • No impedance swing: the loop acts as a transmission line, not a tuned dipole
  • No ground reflection: image currents cancel near-field response
  • Signal voltage is proportional to the H-field component parallel to the loop plane

Not Just a Loop — A Traveling-Wave Pickup

The TerraBooster loop acts as a broadband traveling-wave receiver, with magnetic sensitivity tailored to the loop length and frequency. This is how it achieves wide coverage from 500 kHz to 50 MHz without tuning.

Models and Use Cases

Model Circumference Best Bands Key Use
Mini 16 m 160–40 m (NVIS) Urban NVIS, compact
Medi 32 m 80–20 m (NVIS + DX) All-round performer
Maxi 56 m 40–10 m (DX angles) Long-haul reception
Xtreme 84 m (wire) 40–10 m (+gain DX) Beverage-like behavior

Note: All shielded models integrate a CMC drain path and push-pull amplifier. Only the Xtreme omits the shield.

Technical Specs

Parameter Value
Frequency Range 500 kHz – 50 MHz
Amplifier IOP3 +41 dBm
Output Impedance 75 Ω
Power Supply 13.8 V @ ~180 mA
Output Connectors F-Type (RF), SMA (output)
Amplifier Topology Push-pull dual MMIC
Shield Drain Network 2.4 kΩ
ESD Protection Yes (TVS + fast diodes)
RF Limiter Yes (BAV99 + shunt limiter)
Bias-T Isolation Dual-stage, high-CMRR

Performance Highlights

Noise Floor Reduction

  • 60 dB shielding effectiveness from braid
  • ~20–25 dB lower noise floor compared to passive LoGs
  • Common-mode suppression across 500 kHz–30 MHz
  • Ideal for switching noise, PLC hash, SMPS trash rejection

Gain Behavior (Measured Total System Gain)

Band Mini Medi Maxi Xtreme
160 m -33 dBi -26 dBi -18 dBi -12 dBi
80 m -26 -18 -12 -6
40 m -18 -12 -6 +2
30 m -18 -6 -2 +6
20 m -12 -6 +2 +6
10–15 m -6 to -2 +2 +6 +6

Placement and Grounding Guidelines

  • Place loop on or slightly above ground for best magnetic coupling
  • Use RVS ground peg (included) to drain shield currents effectively
  • Maintain minimum 15–20 m separation from house grounding to prevent loops
  • Avoid routing coax near digital equipment or power cables

Conclusion

The TerraBooster is not just a "loop with a preamp" — it's a system-level design that treats the HF environment as a battlefield of noise, currents, and magnetic fields. Built from the ground up to function as a high-dynamic-range magnetic current probe, it beats passive loops in gain, beats active LoGs in noise rejection, and approaches Beverage-like performance at a fraction of the size.

Stop chasing resonance. Start designing with field physics.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.