Unlocking NVIS Performance with the PolarFlip: Why 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.

What Is the PolarFlip?

In the standard configuration, each TerraBooster provides a linear output from its shielded loop. These alone already deliver 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 provides four individual outputs:

  • ANT1: Linear output of loop 1
  • ANT2: Linear output of loop 2
  • LHCP: Left-hand circularly polarized output
  • RHCP: Right-hand circularly polarized output

The ANT1 and ANT2 outputs are not intended for regular listening but are available as reference channels or for debugging local noise problems, such as identifying noise direction or isolating common-mode interference paths.

This isn’t a gimmick. It has a measurable and audible effect, especially for NVIS.

Why Circular Polarization Matters for NVIS

Skywave propagation in NVIS involves signals going almost straight up, getting refracted by the ionosphere, and returning nearly vertically. During this hop, Faraday rotation alters the polarization — often by 90 to 180 degrees over a single path. This means that a signal transmitted as linear polarization may arrive at the receiver with arbitrary polarization.

If your antenna only receives linear polarization (e.g., one loop alone), it may completely miss the rotated signal or suffer from poor SNR.

With circular polarization from the PolarFlip:

  • The antenna remains insensitive to polarization rotation.
  • You maintain constant gain throughout the Faraday-rotated cycle.
  • Your SNR is higher, and signals sound more stable.

Rejection of Local Noise

Most manmade noise is linearly polarized. By combining two loops in quadrature, the PolarFlip rejects this linear interference — which enters both loops similarly — and favors the circularly polarized component of the actual skywave.

This is especially powerful for:

  • Low-band HF (160m, 80m, 40m)
  • Indoor or suburban operation
  • Times of high electrical noise

You’ll hear signals you couldn’t before. Not because they weren’t there — but because the noise floor was masking them until PolarFlip pushed it down.

Why Not Just Use One Loop?

Using a single TerraBooster loop gives you excellent results, but:

  • You get no polarization agility.
  • You receive all linear QRM directly.
  • You may lose Faraday-rotated signals.

Using two loops without PolarFlip, even if spaced, doesn’t help much either — both pick up the same local noise, just at slightly different levels.

The magic lies in the phasing.

Real-World Results

In tests with a 56-meter perimeter and two TerraBoosters:

  • SNR improvements of 10–20 dB were observed on NVIS signals.
  • Signals remained readable through heavy electrical noise.
  • 160m and 80m saw the biggest gains due to stronger Faraday rotation and higher noise levels.

Polarization Behavior and Hemisphere Effects

Research by Ben Witvliet and others has shown that in the northern hemisphere, NVIS signals are often more pronounced in Right-Hand Circular Polarization (RHCP) — a result of the geomagnetic field’s influence on ionospheric propagation and Faraday rotation. In contrast, LHCP tends to dominate in the southern hemisphere. During Witvliet’s so-called “Happy Hour” — typically from local sunset through to midnight — circular polarization effects are strongest. Field measurements have demonstrated peak signal strength differences exceeding 15 dB between matching and mismatched polarization modes. That means a signal can go from buried to booming simply by flipping the phase. The PolarFlip’s ability to select LHCP or RHCP dynamically allows operators to align with the dominant polarization mode for their region and time of day — a decisive advantage when chasing weak NVIS signals or operating through dense QRM on the low bands.

Conclusion

The PolarFlip is more than a phase box — it's an SNR weapon.

Whether you're using a TerraBooster, OctaLoop, or SkyTracer — always deploy them in cardinal pairs and add a PolarFlip to unlock circular reception with powerful linear noise rejection.

If you operate on 160–30m and want reliable NVIS performance, don’t settle for linear reception. Flip the phase — and boost the signal.

Your ears (and your waterfall) will thank you.

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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.