🌞 HF Absorption Explained: Inside DXLook’s D-RAP View

When HF bands suddenly go quiet in the middle of the day, the cause is often D-region absorption triggered by solar activity. To help operators understand why this happens — and which bands are still usable — DXLook includes a dedicated D-RAP (D-Region Absorption Prediction) view.

This article explains what D-RAP is, where the data comes from, and how DXLook calculates and visualizes HF absorption in real time.

What Is D-Region Absorption?

The D-region is the lowest layer of the ionosphere, roughly 60–90 km above Earth. Under normal conditions, it has little effect on HF propagation.

During solar X-ray flares, however, this region becomes heavily ionized, dramatically increasing HF signal absorption.

Key impacts on HF operation:

• Signal attenuation — signals may be weakened or completely blocked
• Strong frequency dependence — lower HF bands suffer far more than higher ones
• Daylight dominance — absorption is strongest on the sunlit side of Earth
• Short-lived but intense — effects may last from minutes to hours

This is the physical cause behind short-wave fadeouts (SWF) and sudden daytime blackouts.

Where DXLook’s D-RAP Data Comes From

Primary Source: NOAA SWPC D-RAP Model

DXLook primarily relies on official data from the NOAA Space Weather Prediction Center (SWPC).

Specifically, we use the global “frequency for 1 dB absorption” grid, which:

• Covers the entire globe on a ~2° × 4° latitude/longitude grid
• Reports, for each point, the highest HF frequency (1–30 MHz) suffering at least 1 dB of vertical-incidence absorption
• Is driven by real-time GOES satellite X-ray flux measurements

From this grid, DXLook builds smoothed absorption zones, allowing operators to instantly see up to which band HF signals are likely degraded.

Data Freshness Indicators

In the UI you’ll see clear status messages such as:

• “NOAA D-RAP data from X hours ago”
• “D-RAP data may be stale – NOAA update pending”

Transparency matters — you always know whether you’re seeing fresh or cached data.

When NOAA Data Isn’t Available: DXLook’s Fallback Model

Space-weather services occasionally fail, lag, or publish stale data. When that happens, DXLook automatically switches to a calculated D-RAP model.

You’ll see indicators like:

• “Using calculated D-RAP based on current solar position”
• “Using calculated D-RAP (NOAA data Xh old)”

This fallback ensures you still get a meaningful absorption map instead of a blank screen.

How DXLook Calculates D-RAP Internally

1️⃣ Solar Position Calculation

DXLook computes the exact solar position for the current UTC time across a global grid, determining:

• Sub-solar point
• Solar declination
• Hour angle for each grid cell

This mirrors NOAA’s spatial resolution.

2️⃣ Solar Zenith Angle (SZA)

For each grid point we calculate the solar zenith angle:

• Low SZA (sun high) → strong D-region ionization
• ~90° → twilight conditions
• >95° → night; D-region absorption effectively zero

SZA is the primary driver of absorption intensity.

3️⃣ Chapman Layer Modeling

DXLook applies a modified Chapman layer function to approximate D-region electron density.

This reflects real ionospheric physics:

• Exponential attenuation of solar X-rays
• Peak ionization where atmospheric density and photon flux balance
• Absorption proportional to electron density and collision rate

4️⃣ Frequency-Dependent Absorption Estimation

Instead of computing raw dB loss, DXLook estimates:

The highest HF frequency likely to experience at least ~1 dB of absorption

Assumptions:

• Typical mid-cycle solar flux (F10.7 ≈ 150)
• Absorption increases rapidly as the sun climbs
• At night, affected frequency is forced to 0 MHz

From an operator’s perspective, this answers a very practical question:
“Up to which band is HF absorption likely to be a problem right now?”

5️⃣ Geomagnetic & High-Latitude Effects

The model also accounts for:

• Enhanced absorption near the geomagnetic equator during extreme events
• High-latitude absorption related to auroral regions
• Magnetic field geometry influencing electron distribution

6️⃣ Terminator & Twilight Zones

Near sunrise and sunset, absorption gradients change rapidly:

• Mixed day/night propagation paths
• Sharp transitions in signal strength
• Complex gray-line behavior

DXLook preserves these gradients instead of smoothing them away.

How D-RAP Is Visualized in DXLook

🎨 Band-Based Color Coding

Absorption zones use the same color scheme as DXLook’s MUF view:

• 1.8–7 MHz → warm colors (red/orange)
• 10–14 MHz → yellow/green
• 18–28 MHz → cool colors (cyan/blue)

This makes it easy to correlate absorption with usable bands at a glance.

🌊 Smooth Zone Rendering

Although source data is coarse, DXLook applies:

• Catmull-Rom spline interpolation
• Chaikin smoothing

This improves readability while never inventing new data — smoothing is visual only.

🔍 Opacity Control

Using the Zone Opacity slider, you can:

• Overlay D-RAP with MUF, spots, or paths
• Reduce clutter
• Customize visibility for different screens and lighting conditions

How to Use D-RAP in Practice

During High Absorption Events

• Move to higher HF bands
• Expect NVIS failures during strong flares
• Increase power or antenna efficiency where possible
• Remember: most events last 30–120 minutes

Path-Specific Insight

• Fully daylight paths → severe loss (10–30 dB possible)
• Partial daylight paths → moderate degradation
• Nighttime paths → minimal D-region impact

What D-RAP Does Not Show

D-RAP focuses strictly on D-region absorption. It does not represent:

• Auroral absorption
• Sporadic-E
• Tropospheric propagation
• Local noise or interference
• Your station’s antenna performance

Think of it as an absorption floor, not a full propagation solution.

Why D-RAP Matters in DXLook

D-RAP complements other DXLook views by defining the lower boundary of HF usability, while MUF defines the upper one. Together, they reveal the true operating window between absorption and refraction.

Whether you’re contesting, chasing DX, or supporting emergency communications, understanding D-region absorption helps explain why the bands behave the way they do — not just what they’re doing.

DXLook is built by a ham, for hams — combining real-time data with physics-based modeling to make propagation easier to understand and operate.