Unlocking NVIS Performance with the PolarFlip: Circular Beats Linear

The TerraBooster active loop-on-ground system is already well-known for its low-noise design and excellent performance on the lower HF bands. But when it comes to NVIS (Near Vertical Incidence Skywave) communication, a hidden gem lies in our new phasing method: the PolarFlip.

PolarFlip NVIS Phasing

What Is the PolarFlip?

In its standard configuration, each TerraBooster provides a linear output from its shielded loop. This already delivers a clean, quiet signal with excellent common-mode rejection. However, when two loops are placed side-by-side and connected through the PolarFlip phasing hybrid, something special happens: you get true circular polarization, selectable as Left-Hand (LHCP) or Right-Hand Circular Polarization (RHCP).

The PolarFlip is strictly optimized for low-band work — 1 MHz to 8 MHz — covering 160 m, 80 m, 60 m, 40 m, and part of 30 m.

The PolarFlip provides four outputs:

  • ANT1 – Linear output of loop 1
  • ANT2 – Linear output of loop 2
  • LHCP – Left-hand circular
  • RHCP – Right-hand circular

ANT1 and ANT2 are mostly for reference channels or for debugging noise sources, such as locating a QRM source or verifying loop balance.

Why Circular Polarization Matters for NVIS

In NVIS propagation, signals leave the antenna almost vertically, reflect off the ionosphere, and return to Earth at steep angles. During this hop, Faraday rotation alters the signal’s polarization — often converting a transmitted linear wave into an arriving wave with a rotated, elliptical, or fully circular polarization state.

If you only receive in linear polarization, the mismatch can cause deep fades and significant SNR loss. The PolarFlip solves this by offering switchable LHCP/RHCP, ensuring optimal alignment with the received signal.

  • Improved SNR: Up to 6–12 dB gain simply by selecting the correct hand.
  • Deep Rejection: >10 dB suppression of the opposite polarization.
  • Consistent Copy: Less fading during rapid Faraday rotation shifts.
NVIS Optimization Tip: Ben Witvliet’s research shows that in the northern hemisphere, RHCP often dominates NVIS signals, especially from sunset to midnight (“Happy Hour”). In the southern hemisphere, LHCP is more common. The PolarFlip lets you flip instantly to match.

Rejecting Local Noise

Most man-made HF noise is linearly polarized. When the PolarFlip combines two loops in quadrature, it naturally rejects this linear interference while enhancing the circularly polarized component of skywave signals.

Real-World Results

  • 160 m and 80 m see the biggest SNR gains due to strong Faraday rotation.
  • Signals remain copyable through heavy electrical QRM.
  • NVIS stations 300–800 km away become dramatically easier to hear.

Clean NVIS reception with PolarFlip

Conclusion

The PolarFlip is more than a phase box — it’s a signal-to-noise weapon. Whether you’re using a TerraBooster, OctaLoop, or SkyTracer, pairing two units with a PolarFlip unlocks circular reception with powerful linear noise rejection on the low bands.

Mini-FAQ

  • Do I need two loops? — Yes, PolarFlip requires two matched loops for circular operation.
  • Will it work on all HF bands? — No, it is optimized for 1–8 MHz low-band operation (160 m–30 m).
  • Does it help for DX? — It’s designed for NVIS, but in high-angle DX openings, it can improve SNR.

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Joeri Van Dooren – RF, electronics & software engineer, complex platform and antenna designer. Founder of RF.Guru. Expert in active & passive antennas, high-power RF transformers, and custom RF solutions.