feat(spots): add band-specific DX distance thresholds

src/main/frontend/components/cards/band-activity/BandActivityHelp.tsx

@@ -0,0 +1,90 @@
+/**
+ * Band Activity Help Content - Help registration for DX distance thresholds.
+ *
+ * Shows how DX scoring varies by band based on propagation characteristics.
+ * For example, 3,000 km is exceptional on 160m but routine on 20m.
+ *
+ * KEEP IN SYNC with FrequencyBand.java DxThresholds
+ * @see src/main/java/io/nextskip/common/model/FrequencyBand.java
+ */
+
+import React from 'react';
+import { Radio } from 'lucide-react';
+import { registerHelp } from '../../help/HelpRegistry';
+import type { HelpDefinition } from '../../help/types';
+import { bandDxThresholds } from '../../../utils/bandDxThresholds';
+
+/**
+ * Band DX Thresholds Help Content
+ */
+function BandDxThresholdsHelpContent() {
+  return (
+    <div className="help-content">
+      <p>
+        Different bands have vastly different propagation characteristics. What counts as &ldquo;excellent DX&rdquo;
+        varies by band:
+      </p>
+
+      <ul>
+        <li>
+          <strong>160m</strong> &mdash; 3,000 km is exceptional (requires night skip)
+        </li>
+        <li>
+          <strong>20m</strong> &mdash; 15,000 km is needed for top score (workhorse DX band)
+        </li>
+        <li>
+          <strong>6m</strong> &mdash; 5,000 km is legendary F2 propagation
+        </li>
+      </ul>
+
+      <p>
+        <strong>Scoring:</strong> DX distance contributes 20% to the overall band activity score. Scores scale from 0
+        (no DX) to 100 (excellent threshold reached).
+      </p>
+
+      <details>
+        <summary>
+          <strong>Distance Thresholds by Band</strong>
+        </summary>
+        <table className="help-section__table" role="table" aria-label="Band DX distance thresholds">
+          <thead>
+            <tr>
+              <th scope="col">Band</th>
+              <th scope="col">Excellent</th>
+              <th scope="col">Good</th>
+              <th scope="col">Propagation</th>
+            </tr>
+          </thead>
+          <tbody>
+            {bandDxThresholds.map((threshold) => (
+              <tr key={threshold.band}>
+                <td>{threshold.band}</td>
+                <td>{threshold.excellentKm.toLocaleString()}+ km</td>
+                <td>{threshold.goodKm.toLocaleString()}+ km</td>
+                <td>{threshold.description}</td>
+              </tr>
+            ))}
+          </tbody>
+        </table>
+      </details>
+
+      <p className="data-source">
+        Data source:{' '}
+        <a href="https://pskreporter.info/" target="_blank" rel="noopener noreferrer">
+          PSKReporter
+        </a>
+      </p>
+    </div>
+  );
+}
+
+// Register help definition
+const bandDxThresholdsHelp: HelpDefinition = {
+  id: 'band-dx-thresholds',
+  title: 'DX Distance Scoring',
+  icon: <Radio size={16} />,
+  order: 25, // After band conditions (20), before activations (30)
+  Content: BandDxThresholdsHelpContent,
+};
+
+registerHelp(bandDxThresholdsHelp);

src/main/frontend/components/help/HelpModal.tsx

@@ -23,6 +23,7 @@ import '../cards/propagation/PropagationHelp';
 import '../cards/activations/ActivationsHelp';
 import '../cards/contests/ContestsHelp';
 import '../cards/meteor-showers/MeteorShowersHelp';
+import '../cards/band-activity/BandActivityHelp';
 
 export function HelpModal({ isOpen, onClose }: HelpModalProps) {
   const dialogRef = useRef<HTMLDialogElement>(null);

src/main/frontend/components/help/HelpSection.css

@@ -102,6 +102,44 @@
   padding: calc(var(--spacing-unit) * 2);
 }
 
+/* Table styles for data displays */
+.help-section__table-wrapper {
+  overflow-x: auto;
+  margin: calc(var(--spacing-unit) * 2) 0;
+}
+
+.help-section__table {
+  width: 100%;
+  border-collapse: collapse;
+  font-size: 0.875rem;
+}
+
+.help-section__table th,
+.help-section__table td {
+  padding: calc(var(--spacing-unit) * 1) calc(var(--spacing-unit) * 1.5);
+  text-align: left;
+  border-bottom: 1px solid var(--border-color);
+}
+
+.help-section__table th {
+  font-weight: 600;
+  color: var(--text-primary);
+  background: rgba(0, 0, 0, 0.02);
+  white-space: nowrap;
+}
+
+.help-section__table td {
+  color: var(--text-secondary);
+}
+
+.help-section__table tbody tr:hover {
+  background: rgba(0, 0, 0, 0.02);
+}
+
+.help-section__table tbody tr:last-child td {
+  border-bottom: none;
+}
+
 /* Data source attribution - subtle but accessible */
 .help-section__content .data-source {
   font-size: 0.75rem;
@@ -130,3 +168,22 @@
 :root[data-theme='dark'] .help-content__definitions {
   background: rgba(255, 255, 255, 0.03);
 }
+
+/* Dark mode table styles */
+@media (prefers-color-scheme: dark) {
+  :root:not([data-theme='light']) .help-section__table th {
+    background: rgba(255, 255, 255, 0.03);
+  }
+
+  :root:not([data-theme='light']) .help-section__table tbody tr:hover {
+    background: rgba(255, 255, 255, 0.03);
+  }
+}
+
+:root[data-theme='dark'] .help-section__table th {
+  background: rgba(255, 255, 255, 0.03);
+}
+
+:root[data-theme='dark'] .help-section__table tbody tr:hover {
+  background: rgba(255, 255, 255, 0.03);
+}

src/main/frontend/components/help/types.ts

@@ -16,6 +16,7 @@ export type HelpSectionId =
   | 'solar-indices'
   | 'band-conditions'
   | 'band-condition'
+  | 'band-dx-thresholds'
   | 'pota-activations'
   | 'sota-activations'
   | 'contests'

src/main/frontend/utils/bandDxThresholds.ts

@@ -0,0 +1,70 @@
+/**
+ * Band-specific DX distance thresholds for scoring and help text.
+ *
+ * Different bands have vastly different propagation characteristics:
+ * - 160m: 3,000 km is exceptional (requires night skip, low noise)
+ * - 20m: 15,000 km is needed for excellent score (workhorse DX band)
+ * - 6m: 5,000 km is legendary F2 propagation; 2,000 km sporadic-E is exciting
+ *
+ * KEEP IN SYNC with FrequencyBand.java DxThresholds
+ * @see src/main/java/io/nextskip/common/model/FrequencyBand.java
+ */
+export const bandDxThresholds = [
+  { band: '160m', excellentKm: 3_000, goodKm: 1_500, moderateKm: 500, description: 'Difficult; night skip required' },
+  { band: '80m', excellentKm: 5_000, goodKm: 2_500, moderateKm: 1_000, description: 'Regional; some DX at night' },
+  {
+    band: '60m',
+    excellentKm: 6_000,
+    goodKm: 3_000,
+    moderateKm: 1_500,
+    description: 'Secondary allocation; variable propagation',
+  },
+  { band: '40m', excellentKm: 7_000, goodKm: 4_000, moderateKm: 2_000, description: 'Day/night transitions' },
+  { band: '30m', excellentKm: 8_000, goodKm: 5_000, moderateKm: 2_500, description: 'WARC; reliable propagation' },
+  { band: '20m', excellentKm: 15_000, goodKm: 10_000, moderateKm: 5_000, description: 'Workhorse DX band' },
+  { band: '17m', excellentKm: 12_000, goodKm: 8_000, moderateKm: 4_000, description: 'WARC; solar-dependent' },
+  {
+    band: '15m',
+    excellentKm: 14_000,
+    goodKm: 9_000,
+    moderateKm: 4_500,
+    description: 'Solar-dependent; excellent when open',
+  },
+  { band: '12m', excellentKm: 13_000, goodKm: 8_500, moderateKm: 4_000, description: 'WARC; similar to 10m/15m' },
+  { band: '10m', excellentKm: 12_000, goodKm: 7_000, moderateKm: 3_000, description: 'Magic when open' },
+  { band: '6m', excellentKm: 5_000, goodKm: 2_000, moderateKm: 500, description: 'Sporadic-E; rare F2' },
+  { band: '2m', excellentKm: 2_000, goodKm: 500, moderateKm: 100, description: 'Tropo/EME' },
+] as const;
+
+export type BandDxThreshold = (typeof bandDxThresholds)[number];
+
+/**
+ * Base threshold shape without band-specific literal types.
+ */
+export interface DxThresholdBase {
+  excellentKm: number;
+  goodKm: number;
+  moderateKm: number;
+  description: string;
+}
+
+/**
+ * Get DX thresholds for a specific band.
+ *
+ * @param band - Band name (e.g., '20m', '40m')
+ * @returns Threshold data for the band, or undefined if not found
+ */
+export function getThresholdsForBand(band: string): BandDxThreshold | undefined {
+  return bandDxThresholds.find((t) => t.band === band);
+}
+
+/**
+ * Default thresholds for unknown bands.
+ * Uses conservative values similar to 40m.
+ */
+export const defaultDxThresholds: DxThresholdBase = {
+  excellentKm: 7_000,
+  goodKm: 4_000,
+  moderateKm: 2_000,
+  description: 'Moderate propagation',
+};

src/main/java/io/nextskip/common/model/FrequencyBand.java

@@ -5,31 +5,69 @@
 /**
  * Amateur radio frequency bands.
  *
- * Represents the common HF and VHF amateur radio bands with their
- * frequency ranges in kHz.
+ * <p>Represents the common HF and VHF amateur radio bands with their
+ * frequency ranges in kHz and band-specific DX distance thresholds.
+ *
+ * <p>DX thresholds vary by band because propagation characteristics differ:
+ * <ul>
+ *   <li>160m: Difficult band - 3,000 km is exceptional (night skip required)</li>
+ *   <li>20m: "Workhorse" DX band - 15,000 km is needed for excellent score</li>
+ *   <li>6m: Sporadic-E at 2,000 km is exciting; F2 at 5,000+ km is legendary</li>
+ * </ul>
+ *
+ * @see DxThresholds
  */
 public enum FrequencyBand {
-    BAND_160M("160m", 1800, 2000),
-    BAND_80M("80m", 3500, 4000),
-    BAND_60M("60m", 5330, 5405),
-    BAND_40M("40m", 7000, 7300),
-    BAND_30M("30m", 10100, 10150),
-    BAND_20M("20m", 14000, 14350),
-    BAND_17M("17m", 18068, 18168),
-    BAND_15M("15m", 21000, 21450),
-    BAND_12M("12m", 24890, 24990),
-    BAND_10M("10m", 28000, 29700),
-    BAND_6M("6m", 50000, 54000),
-    BAND_2M("2m", 144000, 148000);
+    // Low bands - difficult DX, lower thresholds
+    BAND_160M("160m", 1800, 2000, new DxThresholds(3_000, 1_500, 500, "Difficult; night skip required")),
+    BAND_80M("80m", 3500, 4000, new DxThresholds(5_000, 2_500, 1_000, "Regional; some DX at night")),
+    BAND_60M("60m", 5330, 5405, new DxThresholds(6_000, 3_000, 1_500, "Secondary allocation; variable propagation")),
+
+    // Transition bands
+    BAND_40M("40m", 7000, 7300, new DxThresholds(7_000, 4_000, 2_000, "Day/night transitions")),
+    BAND_30M("30m", 10100, 10150, new DxThresholds(8_000, 5_000, 2_500, "WARC; reliable propagation")),
+
+    // High bands - workhorse DX bands, higher thresholds
+    BAND_20M("20m", 14000, 14350, new DxThresholds(15_000, 10_000, 5_000, "Workhorse DX band")),
+    BAND_17M("17m", 18068, 18168, new DxThresholds(12_000, 8_000, 4_000, "WARC; solar-dependent")),
+    BAND_15M("15m", 21000, 21450, new DxThresholds(14_000, 9_000, 4_500, "Solar-dependent; excellent when open")),
+    BAND_12M("12m", 24890, 24990, new DxThresholds(13_000, 8_500, 4_000, "WARC; similar to 10m/15m")),
+    BAND_10M("10m", 28000, 29700, new DxThresholds(12_000, 7_000, 3_000, "Magic when open")),
+
+    // VHF bands - rare propagation, low thresholds
+    BAND_6M("6m", 50000, 54000, new DxThresholds(5_000, 2_000, 500, "Sporadic-E; rare F2")),
+    BAND_2M("2m", 144000, 148000, new DxThresholds(2_000, 500, 100, "Tropo/EME"));
 
     private final String name;
     private final int startKhz;
     private final int endKhz;
+    private final DxThresholds dxThresholds;
 
-    FrequencyBand(String name, int startKhz, int endKhz) {
+    FrequencyBand(String name, int startKhz, int endKhz, DxThresholds dxThresholds) {
         this.name = name;
         this.startKhz = startKhz;
         this.endKhz = endKhz;
+        this.dxThresholds = dxThresholds;
+    }
+
+    /**
+     * DX distance thresholds for scoring band activity.
+     *
+     * <p>Different bands have vastly different propagation characteristics.
+     * These thresholds define what constitutes excellent, good, and moderate
+     * DX distances for each band.
+     *
+     * <p>KEEP IN SYNC with src/main/frontend/utils/bandDxThresholds.ts
+     *
+     * @param excellentKm distance for 100 points (exceptional DX for this band)
+     * @param goodKm distance for 70-100 point range
+     * @param moderateKm distance for 40-70 point range
+     * @param description human-readable description of band propagation
+     */
+    public record DxThresholds(int excellentKm, int goodKm, int moderateKm, String description) {
+
+        /** Default thresholds for unknown bands (conservative, similar to 40m). */
+        public static final DxThresholds DEFAULT = new DxThresholds(7_000, 4_000, 2_000, "Moderate propagation");
     }
 
     public String getName() {
@@ -44,6 +82,15 @@ public int getEndKhz() {
         return endKhz;
     }
 
+    /**
+     * Get the DX distance thresholds for this band.
+     *
+     * @return band-specific DX thresholds for scoring
+     */
+    public DxThresholds getDxThresholds() {
+        return dxThresholds;
+    }
+
     /**
      * Get the center frequency of the band in kHz.
      */