Is the 4m Band Always Open? - Part 2

 

Is the 4m Band Always Open? - Part 2

A Data-Driven Look Using the GB3MBD Propagation Beacon

The previous article in this series covered an analysis of reception reports of the GB3MBD beacon from my perspective, ie the spots reported just by me. This follow up looks at all spots from everyone reporting reception of the beacon during the period February 2024 to November 2025. During this period there were 829 reports from 495 days reported to the Beaconspot.UK database from stations across the UK, EI and EU.

The results show that 4m is far more active, and more often open than most  realise.

1. Where the Data Comes From (and why it matters)

The entire analysis is based on the Beaconspot.uk archive maintained by the UK Microwave Group. This isn’t an automated monitoring system, it collects voluntary spots that are: 

  • Posted to the Cluster system by stations who hear or decode a beacon

  • Or uploaded directly to the Beaconspot.uk site

As a result:

  • If no one is listening, no spots appear — even if the beacon is loud and clear

  • Spot density depends on operator activity, not just propagation

  • The archive records show actual reception reports but obviously not spots from stations that hear the beacon but don't report it.

Despite these limitations, Beaconspot is the UK’s best long-term VHF propagation record, because it aggregates reports from a diverse set of stations with different antennas, and locations.

2. GB3MBD – A Useful Propagation Probe

GB3MBD operates from IO92ra (Bedfordshire):

  • 70.050 MHz, CW + FT8

  • ~9 W ERP

  • Horizontal halo at ~20 m AGL

Its modest power and simple antenna make it ideal for studying:

  • Troposcatter

  • Diffraction

  • Inversion-enhanced tropo

  • Marine ducts

  • Sporadic-E

  • Aircraft scatter

  • Night-time propagation

And because reports come from across the UK, EI and Europe, the dataset reveals a lot about how 4m really behaves in the wild.

3. What the 20 Month Dataset Shows

Across February 2024 → November 2025:

  • 829 total spots

  • 495 days with ≥1 report

  • Beacon reported on ~79% of all days

Propagation types (as submitted):

  • TR (Tropospheric): 798 spots

  • ES (Sporadic-E): 18 spots

  • AS (Aircraft scatter): 9 spots

Tropospheric propagation dominates, appearing on almost every active day.

4. Distances Covered

Because many stations contribute, there is a natural spread of path classes:

4.1 Local / Near-LOS (0–75 km)

G8GPO, G4RGK, G4HTZ and others hear the beacon reliably whenever they're active.

4.2 Domestic Mid-Range (140–180 km)

This is where the dataset becomes interesting.

Two stations contribute the bulk of reports:

  • G0API (172 km): 380 spots, 379 days

  • G0KTN (147 km): 353 spots, 311 days

As both paths are well beyond line-of-sight reception must use a combination of:

  • Terrain diffraction

  • Troposcatter

  • Tropospheric bending

  • Inversion layers

Yet these paths are open on most days — a key finding that contradicts the casual belief that 4m “is hardly ever open”.

4.3 Longer Tropo (300–600 km)

The dataset contains several long-haul TR receptions:

  • PA0O (503 km)

  • EI9KP (608 km): 19 spots, 18 days

These occur during warm, settled spells, especially in late spring and early summer — classic marine duct and high-pressure tropo conditions.

4.4 Sporadic-E DX (874–1627 km)

18 ES-tagged reports include:

  • DF6HT (874 km)

  • DK2EA (879 km)

  • EA4KM (1340 km)

  • 9A2SB (1571 km)

  • HA8MV (1627 km)

All occur in June–September, as expected for mid-latitude Es.

5. Night-Time Propagation – surprisingly strong

Across all stations:

  • 69 uniquely identified nights had GB3MBD reception

  • Almost all were tropospheric

  • Most on 140–180 km domestic paths

Night-time temperature inversions often improve propagation.
The dataset shows: 4m can be better at 1am than 1pm.

6. Weather Correlations

Spring 2025 – exceptional high-pressure tropo

The warmest, sunniest spring on UK record aligns with:

  • High numbers of mid-range TR spots

  • Many night-time spots

  • Some long tropo to EI and PA

Summer 2025 – warmest on record

Four heatwaves and warm seas produced:

  • Consistent mid-range tropo

  • Frequent marine-enhanced ducts

  • A healthy Es season

Spring–Summer 2024 – mild to warm early season

Long-haul tropo into EI began as early as April/May.

Overall:

High pressure, warm seas, and stable air dramatically improve 4m — the dataset shows this clearly.


7. CW vs FT8 – what the beacon log really shows

GB3MBD transmits both CW and FT8, and Beaconspot shows reception of each mode as follows:

  • CW: 434 reports, across 403 days

  • Digital (FT8): 390 reports, across 321 days

This alone demonstrates that CW remains widely used and that operators continue to report it — especially mid-range UK stations and long-distance Es DX.

However, the patterns of CW vs FT8 reception reveal much more.

7.1 SNR and sensitivity

  • FT8 decodes of the beacon fall between -15dB and -20dB, often just a couple of dB above its decoding limit.

  • CW reports are dominated by 529–559.

This infers that:

FT8 frequently catches openings that are too weak for comfortable human CW copying. In fact I have never seen (on screen) or actually heard the CW beacon.


7.2 Monitoring efficiency

This is crucial:

  • CW requires someone to be actually at the rig and listening.
    If no one is present, the band may be wide open but nothing will get spotted.

  • FT8 monitoring can run 24/7 unattended, providing the user with a list of successful decodes from which a sample can be sent manually to the Beaconspot archive. This is most useful for decodes that occur during the night.

In this dataset:

  • G0KTN effectively provides a continuous propagation monitor

  • G0API and others provide high-quality but operator-dependent CW reports

In short:

FT8 is not just more sensitive; it is much more efficient at capturing long-term propagation trends because it never sleeps.
CW is excellent for real-time operating, but poor for unattended monitoring.

7.3 Practical implication

The 4m band is open for CW far more often than the CW logs show — but unless someone is actually monitoring at the precise time the beacon becomes detectable those moments are lost.

8. So… is the 4m band always open?

For FM?
Probably not - FM requires a far higher signal margin.

For SSB?
Often, but not always.

For CW?
More often than most realise — but only if someone is listening.

For FT8?
Very close to “yes”.

Across 20 months:

  • The beacon was reported on 79% of all days

  • Mid-range 150–180 km tropo on most days

  • Long 300–600 km tropo (EI, PA)

  • Regular Es up to 1600 km during the season

  • 69 nights with reception

  • Very clear weather correlations




This article reflects a combination of traditional amateur radio experimentation and modern AI assisted analysis, where original data and direction are provided by the author and advanced tools enable deeper exploration and presentation of the results.


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