What Ben Witvliet’s NVIS Research Shows About Circular Polarisation
What Ben Witvliet’s NVIS Research Shows About Circular Polarisation
The important result is not the slogan “circular beats linear.” It is that the ionosphere can support two distinguishable characteristic-wave channels—and a receiver that observes both can turn polarisation physics into diversity.
Ben A. Witvliet and his co-authors did the useful thing: they stopped treating NVIS polarisation as a slogan and built calibrated experiments that could separate the ordinary and extraordinary waves, measure their isolation and test what diversity reception actually gained.
The Ionosphere Supports Two Characteristic Waves
Earth's ionised upper atmosphere sits in a magnetic field. That makes it anisotropic: the propagation constant depends on field orientation and polarisation. An HF wave entering this magneto-ionic medium can split into ordinary and extraordinary characteristic waves.
For the near-vertical paths studied by Witvliet and colleagues, those waves returned with approximately opposite circular senses. They did not necessarily follow identical paths or experience identical delay, absorption and fading. That difference is precisely what makes them interesting.
A linearly polarised transmitting antenna excites a combination of the two modes. At the receiver, their vector sum can be elliptical and can vary quickly with time and frequency. Describing the returned field simply as “random” hides the underlying structure.
The Experiment Was Designed to Separate the Modes
The research team used a stable beacon and a horizontal transmitting antenna to launch energy upward. At the receiving site, an antenna system could switch between complementary circular senses and feed a calibrated receiver. The selected 7 MHz path across the Netherlands was roughly 105–110 km—long enough for the intended NVIS signal to dominate the local ground-wave contribution.
Ionograms and propagation timing were part of the experiment. This matters because the ordinary and extraordinary waves do not begin and end their usable propagation intervals at exactly the same time.
“Happy Hour” Revealed Strong Isolation
Near the opening or closing of the path, one characteristic wave can still be returned while the other passes through the ionosphere. Witvliet and co-authors called this transition “Happy Hour.” During it, they measured characteristic-wave isolation exceeding 25 dB.
That figure belongs to the stated 7 MHz experiment, geometry, equipment and interval. It demonstrates that the two modes can be physically separated; it is not a permanent isolation specification for 40 m.
Diversity Reduced the Required Fading Margin
In related work, the same author group received both circularly polarised characteristic-wave channels and reported an 8–11 dB reduction in fading margin relative to using one circularly polarised receive channel. The benefit came from observing two signal streams whose fades were sufficiently independent, then using diversity reception.
Selection combining can choose the stronger branch. Other systems can combine weighted signals or use the separate paths for MIMO. The appropriate architecture depends on correlation, noise, receiver calibration and the communications goal.
What This Means for 40 Metres
The experiments provide direct evidence near the 40 m amateur band. When 7 MHz supports NVIS on a suitable path, complementary circular receive channels can expose the characteristic waves and reduce fading in a properly designed diversity system.
That does not mean 40 m always supports NVIS. The near-vertical usable frequency changes with electron density, time, season, latitude and solar conditions. When the path is closed, antenna polarisation cannot open it.
What We Can—and Cannot—Say About 80 Metres
Witvliet's thesis cites Netherlands observations of circular NVIS signals down to 3.5 MHz from linearly polarised transmitters. That makes 80 m a legitimate band for circular-polarisation and diversity experiments, and 80 m often supports regional NVIS when 40 m is above the usable near-vertical frequency.
But the headline isolation and fading-margin figures above came from the defined 7 MHz work. An 80 m system must be evaluated on its own path. Local noise, antenna size, ground interaction, receiver correlation, modal balance and ionospheric conditions can produce a different result.
Transmit Polarisation and Receive Diversity Are Different Decisions
A transmitter can launch linear, elliptical or circular polarisation. A receiver can observe one polarisation or multiple orthogonal channels. These choices should not be collapsed into one “best polarisation” claim.
For robust reception, the diversity question is whether the available branches provide useful signal-to-noise ratio and sufficiently independent fading after antenna, feed-network and receiver losses. A beautifully circular antenna is not automatically a successful diversity system if both outputs are highly correlated or one is noise-limited.
How to Translate the Research into an Amateur Experiment
- Confirm the path. Use ionograms or vertical critical-frequency data to establish whether 80 or 40 m can support NVIS.
- Define two calibrated channels. Measure gain, phase, axial ratio, isolation and receiver gain balance.
- Record both streams simultaneously. Sequential listening can confuse temporal fading with polarisation difference.
- Measure correlation and fade statistics. S-meter impressions are not enough for a diversity claim.
- Include noise. The strongest signal branch may not provide the best signal-to-noise ratio.
- Repeat across time and frequency. Dawn, daylight, dusk, night and seasonal conditions can behave differently.
Witvliet's work makes circular polarisation on lower HF more interesting, not less. It replaces a vague promise with a measurable mechanism: two ionospheric characteristic waves, two receive channels and a diversity result whose boundaries are clearly stated.
Primary research and authoritative references
- Witvliet et al. — Measuring the isolation of the circularly polarized characteristic waves in NVIS propagation
- Witvliet et al. — Characteristic Wave Diversity in NVIS propagation
- Witvliet — Near Vertical Incidence Skywave: Interaction of Antenna and Propagation Mechanism
- Witvliet et al. — NVIS magneto-ionic probes and sensors
- ITU-R P.531-13 — ionospheric propagation effects
Mini-FAQ
- What did Witvliet's team separate? The ordinary and extraordinary magneto-ionic characteristic waves using complementary circular receive polarisations.
- What was measured at 7 MHz? Isolation above 25 dB during a defined opening or closing interval, plus an 8–11 dB fading-margin reduction in related dual-channel diversity work.
- Does one circular sense always win? No. Mode availability and fading change; diversity obtains resilience by observing both channels.
- Does the 40 m result automatically apply to 80 m? No. Circular NVIS behaviour has been reported at 3.5 MHz, but each 80 m path and system needs its own measurements.