The Truth About Low Noise Figures: Why MMICs Beat Low-NF Op-Amps in Real-World Antennas
Design Choices Matter: Amplifier Selection for H-Field and E-Field RX Antennas
Engineers often obsess over Noise Figure (NF). In practice, especially on HF and with H-field loops, the environmental noise floor dominates. That means NF is not always the bottleneck — front-end linearity, stability, and CMRR usually decide what you actually hear.
Summary
E-field probes: keep NF low but never at the expense of linearity and stability.
H-field loops: prioritize IP3, CMRR, and symmetry; the band noise hides tiny NF wins.
Push-pull stages: deliver clean linearity, better CMRR, and stable phasing for arrays.
System truth: lowering amp NF below the environmental noise floor doesn’t improve SNR — but poor IP3/CMRR will definitely wreck it.
Mini-FAQ
- Why isn’t Noise Figure the main spec? — Because on HF, atmospheric and man-made noise dominate. Beyond ~1 dB, NF rarely changes what you hear.
- When does NF matter most? — For E-field probes with very small output voltages, NF directly impacts SNR and must be kept low.
- What’s more important for H-field loops? — Linearity (IP3), CMRR, and overload resilience. These loops already deliver enough voltage; noise pickup and distortion are the threats.
- Why push-pull? — Push-pull topologies cancel even-order distortion, boost IP3, and improve balance/CMRR, making them ideal for HF RX loops and balanced dipoles.
- Can RF.Guru RX antennas survive near transmitters? — Yes. With protection networks, they tolerate ~100 W at 5 m and ~1 kW at 8 m. Closer installs may need TX-sense muting relays.
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