Winter vs Summer on 40 Metres

 

Winter vs Summer on 40 Metres

Why season often matters more than the solar cycle

During the peak of Solar Cycle 25, many operators noticed something that at first seemed puzzling. Solar activity was high, higher HF bands were lively, and yet 40 metres often behaved very differently depending on the time of year. Winter sessions produced long runs of workable DX, while summer daytime operation could feel noisy and unrewarding despite excellent conditions elsewhere on the spectrum.

This contrast is not an anomaly, nor a failure of the solar cycle to deliver on 40m. It reflects a basic reality of HF propagation: on 40 metres, seasonal effects often dominate behaviour more strongly than the solar cycle itself.

Using CW Reverse Beacon spot data covering late 2024 through the whole of 2025, this article looks at how and why winter and summer produce such different 40m experiences, even under broadly similar solar conditions.

What operators commonly notice

Most experienced HF operators will recognise the pattern immediately.

In winter:

  • 40m often becomes productive earlier in the afternoon.

  • Signals are steadier, and long-haul paths are more reliable.

  • DX frequently persists well into the evening and night.

  • The noise floor is usually lower.

In summer:

  • Midday and early-afternoon operation can sound busy but yield few results.

  • Signals are weaker or more variable.

  • The band often does not come into its own until later in the day.

  • Static and local noise are more prominent.

These differences recur year after year and do not track sunspot numbers particularly closely. The explanation lies much closer to Earth.

The dominant role of the D-layer

For 40 metres, the most important seasonal influence is D-layer absorption.

The D-layer is created by solar radiation and exists only in daylight. Unlike the F-layer, it does not support propagation; instead, it absorbs RF energy, particularly at lower HF frequencies. How strong that absorption becomes depends on both solar activity and daylight geometry.

Season affects both:

  • Day length, which is much longer in summer and much shorter in winter.

  • Solar elevation angle, which is higher in summer and lower in winter.

As a result:

  • In summer, the D-layer forms earlier, becomes stronger, and persists longer into the afternoon.

  • In winter, the D-layer is weaker overall and decays earlier, allowing 40m to become usable sooner and remain so for longer.

Solar maximum strengthens the D-layer further, but it does not remove this seasonal contrast — it often accentuates it.

A simple winter vs summer comparison

Viewed through long-term Reverse Beacon data, the seasonal contrast on 40m can be summarised simply:

AspectWinter behaviourSummer behaviour
Daytime absorption      Lower     Higher
Midday performance      Often usable     Frequently poor
Afternoon improvement      Earlier     Later
Evening DX      Strong and reliable     Often strongest period
Night-time performance      Good, season-dependent     Good, but noisier
Sensitivity to solar max      Moderate     Often strongly negative

This table captures the key point: the same band, under similar solar activity, behaves very differently depending on season.

What the Reverse Beacon data reflects

Looking across many months of CW spots received on 40m, the same seasonal structure appears repeatedly:

  • Winter days tend to show rising 40m activity earlier in the afternoon.

  • Summer days often show a noticeable flattening or dip around local midday.

  • Evening behaviour converges, with winter and summer looking more alike once the D-layer has largely collapsed.

This pattern does not require contest weekends or exceptional events to appear,  it emerges naturally when many ordinary days are viewed together.

The data reflects exactly what operators hear on the band.

Same solar activity, very different outcomes

One of the most useful insights from this comparison is that two days with similar solar activity can produce very different results on 40m, purely because one occurs in January and the other in July.

Near solar maximum, this effect becomes especially noticeable:

  • Higher solar flux increases D-layer ionisation.

  • In summer, that increased absorption combines with long daylight and high solar angles.

  • In winter, the same increase in solar activity has a much smaller impact on 40m usability.

This is why some summers near solar maximum feel surprisingly difficult on 40m, even while higher bands are thriving.

Why solar maximum does not compensate for summer absorption

It is tempting to assume that increased ionisation at solar maximum should compensate for seasonal absorption. For 40m, that compensation is limited.

While F-layer ionisation does increase, the band sits low enough in frequency that absorption remains the controlling factor during daylight hours. As a result, solar maximum tends to:

  • dramatically improve higher HF bands,

  • leave 40m largely governed by season and time of day.


Practical implications for 40m operation

A few practical conclusions follow naturally:

  • Expect your best daytime 40m performance in winter, not necessarily at solar maximum.

  • In summer, focus expectations on late afternoon, evening, and night-time operation.

  • Avoid judging antennas, noise mitigation, or station performance based solely on summer midday results.

  • Plan experiments, skeds, and contests with season first and solar cycle second when 40m is involved.

Understanding this prevents a great deal of unnecessary frustration.

How this fits into the wider HF picture

Higher HF bands are primarily driven by the solar cycle. Lower bands are strongly influenced by season. Forty metres sits at the crossover between these two regimes.

That makes it an especially instructive band. It demonstrates clearly that propagation is governed not by a single parameter, but by the interaction of solar activity, absorption, geometry, and time.

Seen in this light, 40m is not a band that “fails” at solar maximum. It is a band that responds honestly to the physics that dominate its frequency range.

Closing thoughts

The contrasting behaviour of 40m in winter and summer is one of the clearest examples of why long-term amateur observations are valuable. They remind us that solar maximum is not a universal upgrade, but a redistribution of opportunity across the HF spectrum.

Once that is understood, 40 metres becomes far more predictable, and far more rewarding.



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.

Comments

Post a Comment

Popular posts from this blog

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 o...
Read more

Exploring 40MHz Propagation with WSPR

  Exploring 40MHz Propagation with WSPR  Over the past year, I have been running a 40MHz WSPR beacon under a UK Ofcom Trial and Innovation Licence. Operating at 1W into a dipole (08:00–20:00 UTC daily), this beacon has provided a unique opportunity to study propagation at a seldom-used part of the spectrum — sitting between the well-known 10m (28 MHz) and 6m (50MHz) amateur bands. WSPR (Weak Signal Propagation Reporter) proved invaluable here. By capturing reports far below the noise floor, and thanks to the WSPRnet permanent archive the dataset reveals detailed trends in signal-to-noise ratio, distance, and time of day across thousands of reception reports. Key Findings Sporadic-E dominates : Most spots fell in the 800–2,200 km range, especially during summer. F2-layer events — rare but remarkable : On 29 November 2024 , the beacon was received over 8,600 km away , a clear signature of F2-layer propagation. The F2 region, forming 250–400 km above Earth,...
Read more

Solar Maximum and Lower-Band Absorption

  Solar Maximum and Lower-Band Absorption Why 40 metres does not simply improve at solar maximum Each solar maximum brings a familiar expectation among HF operators: if the Sun is more active, surely all bands should benefit. When 10 metres begins producing regular and sometimes spectacular openings, it is natural to assume that 40 metres will also improve in some obvious and sustained way. In practice, that expectation is often disappointed. Many operators find that daytime performance on 40m remains unchanged, or even feels worse, during periods of high solar activity. Using spot data sent to the Reverse Beacon Network (RBN) spanning late 2024 through 2025, this article looks at why that experience is not only common, but entirely consistent with how the ionosphere behaves near solar maximum. The simple intuition - and its limitation The intuitive picture most of us carry is straightforward: more solar activity produces more ionisation, more ionisation should improve...
Read more