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Detailed Finding 10: mode advantage breakdown by band/frequency

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CW advantage scales dramatically with frequency: +29% at 10G, +48% at
47G, +221% at 75G. At 24 GHz the pattern reverses (PH wins by 8%)
due to rainscatter favoring wider-bandwidth modes. Above 122 GHz,
100% of contacts are CW — SSB can't close the path.
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# Finding 10: Mode Matters — Detailed Breakdown by Band and Frequency
Analysis of 57,488 tropospheric QSOs (distance < 3,000 km) from the ARRL Microwave Contest dataset (1992-2024).
## Summary
The mode advantage is **not constant across bands** — it scales dramatically with frequency. CW's 7 dB bandwidth advantage over SSB/PH translates to a modest 29% distance gain at 10 GHz but an extraordinary 221% gain at 75 GHz. At 24 GHz, the pattern reverses entirely: PH outperforms CW, likely due to rainscatter propagation that favors wider-bandwidth modes.
## Theoretical Bandwidth Advantage
The SNR advantage of a narrower-bandwidth mode is:
```
Advantage (dB) = 10 * log10(BW_reference / BW_mode)
```
| Mode | Typical BW | vs SSB (2700 Hz) | vs CW (500 Hz) |
|------|-----------|------------------|-----------------|
| CW | ~500 Hz | +7.3 dB | — |
| SSB/PH | ~2700 Hz | — | -7.3 dB |
| FM | ~16,000 Hz | -7.7 dB | -15.1 dB |
| DG (FT8/WSJT) | ~50 Hz effective | +17.3 dB | +10.0 dB |
In theory, every 6 dB of link margin buys roughly 2x path loss tolerance, which on a free-space path translates to ~41% more distance. On a ducted path the relationship is non-linear — excess margin exploits marginal ducts that can't support wider-bandwidth signals.
## CW vs PH (SSB) by Band
| Band | CW Avg (km) | PH Avg (km) | CW Advantage | CW P90 | PH P90 | CW n | PH n |
|------|------------|------------|-------------|--------|--------|------|------|
| **10 GHz** | 248.2 | 193.0 | **+29%** | 460.9 | 338.3 | 17,874 | 34,706 |
| **24 GHz** | 91.8 | 100.1 | **-8%** | 159.4 | 178.8 | 1,601 | 2,003 |
| **47 GHz** | 74.8 | 50.4 | **+48%** | 116.9 | 98.8 | 335 | 330 |
| **75 GHz** | 54.2 | 16.9 | **+221%** | 93.1 | 36.7 | 46 | 38 |
### 10 GHz: CW +29%
The classic result. CW operators average 248 km vs 193 km for SSB. The 7.3 dB bandwidth advantage translates well at 10 GHz where ducting is the primary propagation mechanism — CW can exploit weaker ducts that can't support the wider SSB signal. At P90, CW reaches 461 km vs 338 km (36% advantage).
The CW advantage here is likely slightly inflated by operator selection bias: more experienced operators disproportionately use CW, and experience correlates with better antenna systems and site selection.
### 24 GHz: PH -8% (PH wins)
**The anomaly.** At 24 GHz, SSB/PH contacts average 100 km vs 92 km for CW — PH is actually better. This reversal holds across all percentiles:
| Percentile | CW (km) | PH (km) |
|-----------|--------|--------|
| Median | 87.1 | 98.0 |
| P75 | 130.4 | 139.8 |
| P90 | 159.4 | 178.8 |
| P95 | 206.9 | 201.7 |
| Max | 323.4 | 358.8 |
PH also has 5.5x more contacts over 300 km (13 vs 1) and 33 contacts over 250 km vs CW's 6.
**Why?** The most likely explanation is **rainscatter**. At 24 GHz (near the 22.235 GHz water vapor absorption line), rain creates strong backscatter that enables contacts at 200-500+ km. Rainscatter signals are typically strong enough that the bandwidth advantage of CW is irrelevant — operators use SSB because they can, and it's faster for exchanging contest information. The dataset's longest 24 GHz contact (710 km) was CW rainscatter, but the bulk of long PH contacts (250-360 km) suggest widespread rainscatter activity on SSB.
This means 24 GHz propagation scoring should distinguish between ducting mode (where CW advantage would apply) and rainscatter mode (where mode is largely irrelevant).
### 47 GHz: CW +48%
CW advantage returns strongly at 47 GHz. Average CW distance is 75 km vs 50 km for PH. At P90, CW reaches 117 km vs 99 km. The 47 GHz band is a "window" frequency (between H2O and O2 absorption lines) where tropospheric ducting is the primary long-range mechanism, and the extra 7 dB of CW margin matters.
### 75 GHz: CW +221%
The most dramatic mode effect in the dataset. CW operators average 54 km vs just 17 km for PH — a 3.2x multiplier. Median distances: 57 km (CW) vs 13 km (PH).
At 75 GHz, atmospheric absorption is high enough that the path loss budget is razor-thin. The 7.3 dB CW advantage is the difference between making the contact and not. PH contacts at 75 GHz are essentially limited to short paths where the link closes with margin to spare. CW opens up the entire "marginal duct" regime that PH can't access.
P90 distances tell the story: CW reaches 93 km vs PH's 37 km. The CW max (126 km) is 47% higher than PH max (86 km).
## All Modes at 10 GHz
| Mode | n | Avg (km) | Median | P90 | Max |
|------|---|---------|--------|-----|-----|
| CW | 17,874 | 248.2 | 228.5 | 460.9 | 1,429 |
| DG (digital) | 109 | 207.4 | 151.3 | 421.2 | 701 |
| PH (SSB) | 34,706 | 193.0 | 177.6 | 338.3 | 999 |
| FM | 175 | 145.2 | 118.8 | 301.9 | 664 |
| RY (RTTY) | 14 | 86.2 | 34.2 | 262.9 | 364 |
**FM** at 10 GHz averages 145 km (25% less than PH). FM's ~16 kHz bandwidth is 6x wider than SSB, costing 7.7 dB of link margin. However, FM contacts still reach 664 km max and 302 km at P90, indicating that when ducting is strong, even wide-bandwidth modes work.
**Digital modes** (DG, primarily FT8/WSJT) average 207 km at 10 GHz with only 109 contacts. Despite the theoretical 17 dB advantage over SSB, digital modes don't dramatically outperform CW in practice. The likely reason: digital modes are used primarily for weak-signal work on VHF/UHF, and 10 GHz operators default to CW for weak signals. The DG contacts may be biased toward casual operators.
**RTTY** (RY) shows the worst performance (86 km avg) but with only 14 contacts, this is not statistically meaningful.
## CW Proportion Increases with Frequency
As bands get harder (higher absorption, tighter link budgets), operators shift to CW:
| Band | CW % | PH % |
|------|------|------|
| 10 GHz | 34% | 66% |
| 24 GHz | 44% | 56% |
| 47 GHz | 50% | 50% |
| 75 GHz | 55% | 45% |
| 122+ GHz | 100% | 0% |
Above 122 GHz, every contact in the dataset is CW. This is because the link budget is so tight that the bandwidth advantage of CW is essential — SSB simply cannot close the path.
## N/A Mode Contacts
The dataset contains 134 contacts at 10 GHz with mode "N/A" that show abnormally high average distances (870 km). These are from older contest logs (2007-2012) where mode was not recorded. Many involve 4C2WH (Mexico) working California stations at 1,300-1,600 km — exceptional transoceanic/trans-Gulf ducting events. These should not be included in mode comparisons.
## Implications for Scoring
1. **Range estimates should be mode-qualified.** The current tier ranges assume CW. For SSB, reduce by 20-30% at 10 GHz, but NOT at 24 GHz. For FM, reduce by 40%.
2. **The 24 GHz anomaly matters.** Rainscatter propagation at 24 GHz is a distinct mechanism where mode advantage disappears. If the algorithm or ML model is tuned for ducting, it should not penalize PH at 24 GHz.
3. **At 75+ GHz, CW is effectively required** for anything beyond short-range contacts. Range estimates for PH should be 1/3 of CW estimates at these frequencies.
4. **Mode advantage scales non-linearly with frequency.** The 7.3 dB bandwidth advantage produces increasing distance gains as the path gets harder:
- 10 GHz: +29% (ducting, moderate absorption)
- 47 GHz: +48% (ducting, higher absorption)
- 75 GHz: +221% (ducting, high absorption — every dB counts)
This suggests the relationship is: `distance_advantage ≈ bandwidth_dB * absorption_scaling`, where absorption_scaling increases with frequency.