In the world of amateur radio, phenomena occur that act as moody fluctuations of the cosmic orchestra. The most prominent of these is Dellinger effect, also known as radio blackout type R or fadeout type I. It was discovered by American physicist John Howard Dellinger and for almost a hundred years it has maintained a reputation as a solar 'off switch' that can silence shortwave communication within seconds.
What is Dellinger effect actually?
Essentially it is about sudden, sharp weakening to complete cessation of shortwave radio signal propagation. It is not a subtle deterioration of conditions that radio amateurs know from solar cycles. The Dellinger effect is more like a sudden curtain of light falling on the stage: signals disappear, contest pile-ups fall silent, and the entire ionospheric bridge, which usually reliably reflects HF signals back to Earth, suddenly stops behaving as expected.
Most affected are frequencies between 3 and 30 MHz, which is precisely the shortwave band used by radio amateurs, maritime services, or aviation communication. Higher frequencies (for example in the VHF band) are already out of reach of this phenomenon and survive it without major scratches.
How does it arise: a dramatic scene on the solar stage
The trigger is strong solar eruption, typically of class M or X, which sends out a massive dose of extreme ultraviolet and soft X-ray radiation. With this radiation, nothing slow happens: since light needs nothing but time to travel from the Sun to Earth, we will pay the price for this speed in 8 minutes, so almost instantly.
When this intense radiation reaches Earth, it shines on ionosphere, specifically on D layer. Under normal circumstances, the D layer is like an inconspicuous service at the door: it absorbs some low frequencies, but mostly lets short waves pass through to the E and F layers, where signal reflection occurs.
However, after the arrival of a solar eruption, the D layer turns into a hyperactivated guard that pulls up the nets and slams shut all passages. X-ray and UV radiation dramatically increase the ionization of the D layer, turning it into an extremely efficient absorber of HF signals. Instead of reflecting back to Earth, the signal is 'eaten up' right in the lowest part of the ionosphere.
Result: total damping of short waves.
Sometimes there is a deafening silence.
How does Dellinger effect manifest in practice?
Radio amateurs know it intimately well. Typical are:
The spectrum of HF signals in SDR# sudden band silence, without warning and often within a few seconds,
disappearance of data from ionosond on HF frequencies,
absolute reduction of propagating frequencies, MUF drops sharply down,
malfunction of NVIS connections on lower bands (1.8, 3.5, 7 MHz),
aviation and maritime emergency frequencies on HF suddenly feel deserted.
The geographic extent typically mirrors the illuminated half of the Earth. On the night side of the planet, silence is still silent, but of a different kind: there is no D layer because it is kept alive only by solar radiation.
How long does it last?
This is one of the most exciting parts of the phenomenon. The Dellinger effect typically lasts from a few minutes to tens of minutes. Most common blackouts fit within the frame 10 to 40 minutes, although with extreme solar eruptions of higher X classes, the duration may exceed the one-hour limit.
The length is directly proportional to:
the intensity of the eruption,
the amount of X-ray radiation,
the duration of the solar flare itself.
When the eruption burns out, ionization in the D layer begins to decay as quickly as it formed, and communication gradually returns to normal. The return is usually magnificent: HF bands come back to life abruptly and often chaotically, as if all signals decided to arrive at once.
Why is this phenomenon so crucial?
The Dellinger effect belongs to the family of so-called R-sizes (Radio Blackout Classes), monitored by NOAA. It is one of those phenomena that affect not only amateur communications, but also:
aviation HF circuits in polar regions,
maritime communication,
military and diplomatic networks operating outside satellites,
time and navigation services operated on HF.
For radio amateurs, it is a reliable indicator that the Sun is currently in a noisy period and that the DX window may be much smaller than usual in the next few minutes.
How to recognize that it is Dellinger effect and not another issue?
In the middle of a contest, anything can happen, but Dellinger has a very clear signature:
The effect occurs immediately.
It is not a gradual deterioration of conditions, but a sudden shutdown.The illuminated side of the Earth suffers the most.
If it is daytime for you, but all bands have dropped, it is very suspicious.Even higher shortwave bands are declining, not just 80m or 40m.
After a few tens of minutes, the situation usually returns, often just as suddenly.
NOAA reports increased X-ray activity, often in class M or X.
Summary
The Dellinger effect is a rapid, dramatic, and uncompromising impact of solar energy on our shortwave world. It occurs after a powerful flash of X-ray and UV radiation from the Sun, which turbo-charges the D layer of the ionosphere and causes massive absorption of HF signals. It lasts from a few minutes to an hour and affects the entire illuminated hemisphere. For radio amateurs, it is clear evidence that the Sun occasionally plays its own improvisation, which is beautiful but unforgiving for HF DX.