QRN on the Low Bands: Thunderstorm Static and How to Cope

Every low-band operator knows the sound: a steady frying, with sharp crashes on top, that rises on a summer evening until even strong stations disappear. This is QRN, the Q-code that means “are you troubled by static?” and that hams use for natural atmospheric noise. Unlike the hash from a bad charger or a faulty street lamp, QRN cannot be found and switched off, because its sources are thunderstorms that may be thousands of kilometres away. You can, however, understand it, predict it and reduce its effect on your contacts. This guide explains where the noise comes from, why it hits 160, 80 and 40 metres hardest, and which tools and habits help.

What QRN Really Is

A lightning stroke is a huge spark, and like every spark it radiates radio energy over a very wide range of frequencies. A single flash produces a short, powerful burst of noise, called a sferic, that travels away from the storm. Around the world there are roughly forty lightning flashes every second, and a large share of them occur in the tropical land areas. Each one sends out its burst, and the low bands carry these bursts over long distances by ground wave and by reflection from the ionosphere. The sum of many distant flashes is the steady rumble or frying sound, while a near storm produces individual loud crashes. The general science is summarised in the articles on atmospheric noise and on lightning.

The key point for radio amateurs is that QRN is an impulsive, broadband noise whose strength falls as the frequency rises. On 160 metres it can be the loudest thing in the receiver. On 40 metres it is often still strong in summer. By 20 metres, atmospheric noise is usually below other sources, such as the noise of the galaxy or the electrical noise of the neighbourhood, and on the VHF bands it plays almost no role.

Where and When It Is Worst

The world has three main thunderstorm regions: Central Africa, the Amazon basin with the northern part of South America, and the maritime area of South-East Asia. In each, storms build in the afternoon and are most active in the late afternoon and evening local time. The noise from these regions reaches Europe and North America mostly after dark, when the D layer of the ionosphere has disappeared and low-band signals travel far. This is the reason why QRN seems to get worse just as the best DX conditions begin.

The season matters as well. In the northern temperate zone, thunderstorms are most common in summer, and the local contribution adds to the distant noise. Many operators find the 160 and 80 metre bands almost unusable on summer evenings, while the same bands are quiet in winter. The tropics have storms all year, so the noise in a southern or equatorial location is higher on average, and operators there often use higher frequencies or digital modes for the low bands. The table below summarises the typical picture.

How to Recognise QRN and Tell It From Man-Made Noise

Natural static has a clear character. It is random: the crashes come without a pattern, and they do not repeat at regular intervals. It is also broadband, so you hear it all over the band, and it usually gets weaker as you tune upwards through the HF bands. If the noise is the same on every band, or if it has a regular pattern, such as a buzz at 100 or 120 times a second, it is probably man-made. Power line noise and household electronics have different signatures and different cures, as described in our article on finding and killing RF noise at home.

A simple test uses time and direction. If the noise changes with the weather forecast, rises in the evening and falls toward the morning, it is likely to be natural. If the signal of the noise disappears when you switch off the main breaker of the house, it is local and can be cured. A small battery-powered portable receiver away from the house will show a similar level of QRN in the same direction, but without the local hash. This gives you a baseline of what natural noise sounds like at your location.

Receive Antennas: the Biggest Improvement

You cannot remove static from the signal, but you can choose which direction you listen to. A transmitting antenna on 160 or 80 metres is often a vertical or a low wire, both of which collect noise from all directions. Because storm noise often arrives from a particular direction, a receive antenna with a directional pattern and a deep null can give a much better signal-to-noise ratio, even if its signal is weaker in absolute terms. What counts is the ratio, not the loudness.

The traditional solutions for the low bands are the Beverage antenna, a long wire close to the ground and terminated in a resistor, and small receive loops with phasing control, such as the K9AY loop. They are described in our article on low-band receive antennas. If you have little space, a small loop or a compact active antenna can help you to place a null on a storm. In a crowded suburb, a small loop on a pole in the garden, away from the house, often does more than a big antenna near the wiring.

Use the null on purpose

When the storm is local or in one direction, turn the pattern so that the null points to it. Even a simple rotation of a small loop by a quarter turn can change the noise by several S units. Make notes of which direction helped during which months, because the main storm areas are the same from year to year, and you will soon see a pattern.

Receiver Controls That Help

The most useful control against impulsive static is the noise blanker. It detects the short, strong pulses of a crash and mutes the receiver for the instant they last, so that the pulse is cut out of the audio. It works best on isolated, strong crashes with clear silence between them, and less well on the dense rumble of many distant storms, where the pulses overlap. We compare the two main tools in our guide to the noise blanker and noise reduction. Digital noise reduction smooths the steady part of the noise, but it can also add a watery sound to speech and trim weak CW, so use it with moderation.

A narrow filter helps, because the noise power in the receiver is proportional to the bandwidth. Switching from a 2.4 kHz filter to a 500 Hz filter for CW reduces the noise power by about seven decibels. For SSB, a bandwidth of 2.1 kHz, if your signal is clear, gives a smaller gain, but a gain nevertheless. The AGC settings matter also: a long hang time can make the receiver deaf after each crash, so use a faster setting when the crashes are strong. If your receiver has a preamplifier, switch it off on the low bands. A preamplifier amplifies noise as well as the signal, and at 160 and 80 metres the atmospheric noise is higher than the noise of the receiver anyway.

Operating Choices

Your timing can be as effective as any equipment. During the summer, the quietest period on the low bands is usually just before dawn, when storm activity is at its lowest in the world. Another choice is to move up in frequency: when the 80 metre band is noisy, 40 metres may be less so, and the signal that you want is often there too. Winter gives the best conditions for 160 metres, which is the reason why most of the top-band DXing and the top-band contests are in the cold season.

Weak-signal digital modes also tolerate the steady part of the static well, because they are built to decode below the noise. Impulsive crashes can still spoil individual transmissions, but a mode with a long integration time and forward error correction is much less sensitive than a voice contact in the same conditions. If you have a choice, a CW or FT8 contact is more likely to succeed than a phone contact on a stormy night. Read more about the night bands in our guides to 80 metres and 160 metres.

Safety First When the Storm Is Near

When thunder is audible, QRN stops being an inconvenience and becomes a warning. A nearby strike can induce very high voltages in antennas and feed lines, enough to damage equipment, and it is a danger to the operator. Disconnect the coax from the radio, and move the cables outside the shack or to a ground bar, before the storm arrives. Do not touch the equipment and the cables during the storm. A proper entry panel, a ground rod system and surge protection reduce the damage from nearby strikes, but nothing protects against a direct hit, and the safest antenna during a storm is a disconnected one. The details are covered in our guide to station grounding and lightning protection.

A practical rule is to count the seconds between flash and thunder: every three seconds is about a kilometre. If the interval is shorter than 30 seconds, stop operating and disconnect. Many experienced hams also use a lightning detector, either a small commercial unit or an online lightning map, to see storms before they are audible. This also warns you when static on the band is local and about to become dangerous.

Keeping a Noise Log

A short written log makes all of this easier. Note the date, the time, the band, the S-meter reading of the noise with the receive antenna and without it, and the weather in your area. After a few weeks you will see which hours are quiet, which band is the best compromise in each month and how much the receive antenna really gives you. This turns a general feeling about a noisy band into numbers that you can use when you plan a contest weekend or a DX night.

Keeping Perspective

QRN is part of what makes low-band radio what it is. The same ionosphere that carries your signal across the ocean carries the noise from the other side of the world, and the same long nights that give you DX bring the storms of the summer hemisphere. The best low-band stations are not those that have no noise, but those that have learned to hear signals within it. A good receive antenna, a calm hand on the filter and blanker, and a flexible plan for the evening give you more than an expensive transceiver on a noisy night.

The Bottom Line

QRN is atmospheric static from lightning anywhere in the world. It is strongest on 160, 80 and 40 metres, in summer and in the evening, and it cannot be removed at the source. Tell it from man-made noise by its random character and its fall with frequency, then fight it with a directional receive antenna, the noise blanker for strong crashes, a narrow filter and a switched-off preamplifier. Choose your hours and bands wisely, prefer CW and digital modes on bad nights, and disconnect the antenna as soon as a storm comes close.

Frequently Asked Questions

What does QRN mean in ham radio?

QRN is the Q-code for natural atmospheric noise, in practice the static caused by thunderstorms. As a question it asks whether the other station is troubled by static.

Why is QRN worse on 160 and 80 metres?

Atmospheric noise is strongest at low frequencies and weakens as the frequency rises. The low bands also carry noise from distant storms over long distances at night.

Can a noise blanker remove thunderstorm static?

It helps with strong, isolated crashes, because it mutes the receiver for the short time of each pulse. It is less effective against the dense rumble of many distant storms.

Does a preamplifier help against QRN?

Not on the low bands. Atmospheric noise is usually higher than the noise of the receiver, so a preamplifier raises signal and noise together and may add overload.

When is the best time to operate the low bands in summer?

Often in the hours just before dawn, when storm activity around the world is at its lowest. Winter is generally the best season for 160 metres.

Should I disconnect my antenna during a thunderstorm?

Yes. Disconnect the coax from the radio and move the cables away when thunder is audible, and stop operating. Surge protection and grounding reduce damage but do not protect against a direct strike.