OctaLoop 1.2M: Prioritizing CMRR Over Noise Figure in Real-World Reception
The OctaLoop 1.2M is a high-performance shielded active magnetic loop antenna designed for HF reception from 1 MHz to 30 MHz. In contrast to conventional loop amplifiers that prioritize theoretical noise figure (NF), the OctaLoop takes a pragmatic approach to maximize real-world signal quality by emphasizing Common Mode Rejection Ratio (CMRR).
Design Philosophy
While low noise figure is often touted as a performance metric, in actual HF environments—particularly in suburban and urban installations—common-mode noise dominates the received signal. This makes a high CMRR far more valuable than a marginal improvement in NF.
The OctaLoop addresses this by using a fully shielded, coaxial loop geometry with a true differential amplifier front end. The shielded pickup element prevents electric field coupling, focusing entirely on the magnetic component of the wave. A push-pull amplifier topology ensures symmetry, and a 1:1.5 unun provides the required impedance transformation. The entire architecture is optimized for rejection of common-mode interference.
Electrical Configuration
- Receiving Loop: 1.2 meter diameter shielded loop
- Amplifier Topology: Differential push-pull using matched MMICs
- Transformer: 1:1.5 wideband RF unun
- Power Supply: 10–15 VDC with onboard regulation and decoupling (no switch-mode components)
- Output: 50-ohm single-ended (converted in the shack from a 75-ohm system via bias-T)
Performance Summary
| Parameter | Value |
|---|---|
| Frequency Range | 1 – 30 MHz |
| Loop Diameter | 1.2 meters |
| Common Mode Rejection | > 50 dB (typical) |
| Noise Figure | < 1.7 dB (typical) |
| Output IP3 (OIP3) | +41 dBm |
| P1dB | > +19.2 dBm (push-pull) |
| Supply Current | 160 mA |
| Output Impedance | 50 ohms (converted from 75) |
| Input Impedance | Differential, balanced |
| Shielding Effectiveness | > 60 dB E-field suppression |
CMRR as a Design Driver
The OctaLoop demonstrates that real-world reception is often limited by common-mode coupling into the feedline and frontend, rather than preamplifier NF. Its >50 dB CMRR across the HF range ensures minimal pickup of local switching noise, PLC interference, or household EMI.
Conventional unbalanced loops or single-ended input stages—even with a good NF—will underperform in electrically noisy environments. The OctaLoop's architecture directly addresses this by treating the antenna-feedline system as a differential transmission line, suppressing unwanted currents.
Use Cases
The OctaLoop is ideal for:
- Low-noise reception in suburban HF installations
- DXing and weak signal work where SNR matters more than absolute voltage gain
- Diversity or phased-array systems where predictable symmetry and rejection are required
- Good companion for our PolarFlip (LHCP and RHCP for HF)
Conclusion
By engineering for high CMRR rather than chasing lowest theoretical NF, the OctaLoop offers a superior signal-to-noise experience in practical installations. For real-world HF reception, particularly where interference is common, the OctaLoop 1.2M is the definitive choice.
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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.