OctaLoop — Technical Overview
Updated: 2025-08-11 — Technical overview validated against OctaLoop schematic. Values with ~ are sweep-derived approximations.
The OctaLoop is a high-performance, shielded active magnetic loop antenna optimized for 1 MHz to 30 MHz HF reception. Built around a differential push-pull architecture, it achieves excellent Common Mode Rejection Ratio (CMRR) while maintaining low distortion and high dynamic range.
Design Highlights
- Loop Structure: 1.2 m diameter coaxial shielded loop rejects unwanted E-field pickup, focusing solely on magnetic (H-field) reception.
- Amplifier Topology: True differential push-pull front end using matched gain devices for symmetry, improved IP3, and low noise.
- Broadband Impedance Matching: Wideband transformers between loop, gain stage, and output ensure consistent performance from 1–30 MHz.
- Low-Pass Filtering: Integrated ~50 MHz LPF suppresses harmonics and prevents VHF instability.
- Protection: Multi-layer ESD and over-voltage clamping on both loop feeds protect the active devices against static and nearby RF bursts.
Key Performance Metrics
| Parameter | Value |
|---|---|
| Frequency Range | ~1 – 30 MHz |
| CMRR | > 50 dB typical |
| Noise Figure | ~< 1.7 dB |
| Output IP3 | ~+41 dBm |
| P1dB | ~+19 dBm |
| Output Impedance | 50 Ω (converted from internal 75 Ω) |
| Supply Voltage | 10–15 VDC |
| Current Draw | ~160 mA |
For all bolts, screws, and external coax connectors, apply AL-1100 aluminum paste during assembly to prevent galvanic corrosion and preserve low-loss contact over time.
How often? One careful application at installation is usually sufficient. In coastal or polluted areas, reapply every 2–3 years during inspection, or sooner if you disconnect/reconnect cables.
Output Low-Pass Filter (measured from schematic values)
| Metric | Value |
|---|---|
| Topology | Two-section LC low-pass (NP0 caps, ferrite inductors) |
| -3 dB cutoff | ~41–46 MHz |
| Attenuation @ 100 MHz | ~-22 dB |
| Attenuation @ 200 MHz | ~-40 dB |
| Passband ripple (1–30 MHz) | ~<0.5 dB |
Figures derived from the installed LC values; exact results vary slightly with PCB parasitics and part tolerances (1% resistors, NP0/C0G capacitors).
Why CMRR Matters More than NF
In real-world HF reception, particularly in urban and suburban environments, common-mode interference on coax shields dominates over thermal noise. High CMRR ensures that shield-borne noise is rejected before it reaches the receiver, making it far more important than shaving fractions of a dB from the noise figure.
Placement & Grounding
- Install at 1.2–5 m above ground for a balance of low noise pickup and good signal strength.
- Maintain ≥2 m clearance from conductive structures to maximize CMRR benefits.
- Bond the loop base or mount to a suitable RF ground or counterpoise to stabilize noise floor.
- Use a high-CM choke at the shack end of the feedline for additional noise suppression.
Enclosure & Installation
- Mounting: Rugged RVS316 stainless steel base hardware.
- Enclosure: Weather-sealed UV-stable housing.
- Feedline: Includes 10 m of 75 Ω coax; supports longer runs with optional inline RX CMR chokes.
Accessories & Options
- Bias-T Power Injector (F-type to SMA)
- SMA to BNC short patch cable
- Optional Ground Peg + RX Line Isolator for enhanced CMRR
Use Cases
- Weak-signal DXing on 160–40 m
- SWL and AM Broadcast listening
- Diversity and phased loop arrays
- SDR and spectrum monitoring
Perfect Companion for PolarFlip NVIS Arrays
For operators focusing on 160–40 m Near Vertical Incidence Skywave (NVIS) communication, the OctaLoop is an ideal match for the PolarFlip phasing system. When deployed as a pair in left-hand (LH) and right-hand (RH) loop orientations, the system can switch between circular reception modes to mitigate fading (QSB) and polarization shifts. This combination delivers stable, low-noise NVIS reception even under rapidly changing ionospheric conditions.
The OctaLoop’s high CMRR and magnetic-only pickup ensure maximum benefit when used in diversity or phased array configurations such as PolarFlip.
Mini FAQ
- Q: Why use a magnetic loop instead of an E-probe? — A: Magnetic loops reject more local E-field noise, making them quieter in high-RFI areas.
- Q: Can I place it indoors? — A: Possible, but outdoor placement offers much better noise performance.
- Q: Is the loop shield necessary? — A: Yes, it’s key to maintaining high CMRR and minimizing E-field pickup.
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