Intruder Watch: How the IARU Monitoring System Protects Ham Bands

Sooner or later every operator tunes across an amateur band and hears something that does not belong there: a foreign-language broadcast, a rhythmic pulsing signal that sounds like a machine, or a digital burst with no callsign at all. Most of us shrug and tune on. A small group of volunteers does not. They take part in the intruder watch, a worldwide service run by the International Amateur Radio Union that records and reports non-amateur stations on bands that belong to the amateur services. This guide explains what the service is for, which signals count as intruders, what you need to record if you want to help and why the volunteers insist that they are not the band police.

What the Service Is

The IARU Monitoring System, formerly known as Intruder Watch, is a worldwide service authorised by the IARU Administrative Council and staffed by volunteers. Its stated purpose is the search, classification, identification and initiation of steps leading to the removal from the amateur bands of radio signals of non-amateur stations that cause harmful interference to the amateur services, contrary to the Radio Regulations of the ITU and to national regulations. The service concentrates on the bands that are allocated exclusively to the amateur and amateur-satellite services. The Region 1 page of the IARU monitoring system says that the number of intruders of all kinds is growing rapidly, and stresses that monitoring is teamwork: national coordinators and volunteers in many countries collect observations and pass them on.

The structure is simple. Each IARU region has a coordinator, many national societies have their own monitoring coordinator, and individual volunteers send in their observations. The results are collected, compared and, where possible, taken up with the national telecommunications authority of the country where the intruder is located. Summaries are published regularly in newsletters of the monitoring system, which show which frequencies are affected and what types of signals have been identified.

Who Counts as an Intruder?

An intruder, in the sense of the service, is a station that is not licensed in the amateur service and that transmits on a band reserved for amateurs. Typical examples named by the monitoring organisations include broadcasters, over-the-horizon radars, illegal operators such as taxi companies and fishermen who use the bands for their own purposes, various military modes and many other unwanted signals. Some of these signals are easy to identify: an international broadcast in a clear voice announces itself. Others need a close look at the waterfall: a radar produces a regular series of sweeps or pulses, while a data link may have a characteristic pattern of tones and bursts.

The key phrase is the one about exclusive allocation. Some amateur bands are shared with other services on a primary or a secondary basis, so a non-amateur signal there may be perfectly legal. The 30 meter band, for example, is shared in many places and has special rules; knowing which segments are exclusive and which are shared is part of the skill. Read the band plan and the national allocation table before you decide that a signal is an intruder. Our guide to the IARU band plan is a starting point, and the table of allocations on the website of your national regulator gives the details.

What Is Not an Intruder

The volunteers draw a clear line. According to the IARU guidance, the service does not deal with harmful interference in amateur bands caused by stations that are identified as, or believed to be, amateur stations, and it does not monitor compliance with band plans or contest rules. A station that calls in the wrong segment, a contester who sits on a frequency or an operator who breaks the rules is a matter for the national society and the regulator, not for the intruder watch. Similarly, a jammer who deliberately disturbs amateur contacts is a different problem; our article on intentional interference explains what to do in that case.

The same applies to local noise. A switching power supply, a poorly shielded device or a solar inverter is not an intruder in this sense, even though it spoils the band; see our guide to finding and killing RF noise for those problems. The IARU writes plainly that monitors are not a band police: they can only report what they hear, and the action is taken by the authorities.

What to Record

A useful report is a complete one. The IARU Region 1 guidance on how to start an intruder watch lists the items to capture whenever you suspect that a signal does not belong to the amateur service. The table summarises them.

Audio recordings, preferably in WAV format, screenshots of the waterfall and short videos add a lot of value. A recording lets a specialist who knows a language or a protocol identify the signal later, and a waterfall image shows the shape of a radar or a data link at a glance. Keep the recordings short and label them with the date, the time and the frequency in the file name.

Equipment and Setup

You do not need special gear. Your own transceiver or receiver is enough, and the IARU notes that online receivers such as WebSDR or a network of KiwiSDR receivers can also be used for remote reception. A wideband display makes the job much easier, because you can watch a whole band at once and see patterns that you would never notice by tuning. Our articles on panadapters and waterfalls and on SDR for ham radio explain the tools. Software that records the audio or the IQ data on a schedule lets you capture signals that appear at set times of the day.

Two practical points help. First, use UTC for everything, including the file names, and keep your computer clock accurate. Second, be careful with the accuracy of your frequency readings: calibrate the receiver against a time signal or a known beacon, because a coordinator who is trying to match two reports needs a reliable number, not a rough one.

How to Report

Send your observations to the national coordinator of your IARU member society, or, if your society has none, to the regional coordinator of the intruder watch. Include all of the information above in a clear format, and one report per signal. Many societies collect the reports by email and compile them monthly. Do not post accusations on social media, do not try to contact the intruder on the air and do not transmit to disturb it. The aim is to document, not to confront.

The IARU also advises societies without a coordinator to recruit volunteers with different skills, such as CW operators, people who understand digital modes and speakers of foreign languages, to present the case to the board and to establish contacts with the national telecommunications authority before any complaint is filed. If you are a club member, you may be able to suggest that your club takes on the role, which is a modest commitment compared with its value to the whole community. Our article on bringing new members into a club offers ideas for finding people with the right interests.

Learning to Recognise Intruders

Recognition comes with experience, and there are some general habits that speed it up. Listen first without assumptions, and look at the waterfall: amateur signals have a typical width and structure for each mode, while many intruders look different. A broadcast signal is wide and steady, with the continuous energy of music or speech. A radar appears as a regular sequence of short sweeps or pulses that repeat at a fixed rate and cross a wide part of the band. A data link or a military signal may show a characteristic comb of tones or a pattern of rapid frequency changes. An unusual signal that repeats at the same time every day is more likely to be an operational service than a random transmission.

Language is a strong clue. A voice in a language that you do not know can be recorded and sent to someone who can place it, and the recording is much more useful than a note that says that it was foreign. Similar rules apply to Morse code and to digital modes: record the characters you can read, even if they make no sense to you, because a specialist may recognise a call sign, a schedule or a procedure. Do not guess the identity of a station in a report; write what you heard, and mark anything uncertain as such.

Good Practice and Ethics

Keep the work factual and calm. Never accuse a person or a company by name unless the identification is clear from the transmitted information, and leave any accusation to the authorities. Do not reveal the content of private communications that you may overhear while monitoring a band: the point is to record the existence and the character of the signal, and in many countries the law restricts what you may do with the content of transmissions that are not intended for you. Follow the guidance of your society. If a signal appears to be a distress call or an emergency, treat it as such and inform the proper authority at once, rather than filing it as an intruder.

Finally, accept that progress is slow. Intruders may stay on a frequency for years, and action by a regulator depends on international cooperation and on evidence that has been gathered patiently. Reports from many countries pointing at the same signal are more convincing than a single complaint. The satisfaction in this work comes from the long record rather than from a quick result, and from the knowledge that a clear stretch of a band is the outcome of many small observations by people who cared enough to write them down.

Why It Matters to Every Operator

Amateur allocations are a shared resource that exists because the hobby makes constant and visible use of it. Intruders can wipe out a section of a band, and a persistent one can make an exclusive allocation almost unusable. Regulators respond to evidence, and a documented record of an intruder over many months, from several countries, carries weight that a single complaint does not have. That is why the volunteers who sit with a receiver and a notebook are, in effect, defending your operating space. You do not need to join them to benefit, but you can help by reporting what you hear and by encouraging your society to take part.

The Bottom Line

Intruder watch is the IARU service that records non-amateur signals on bands allocated exclusively to the amateur services and passes the evidence to the authorities. Anyone with a receiver can take part: record the date and UTC time, frequency, mode, origin, call signs or identity and remarks, add an audio file or a screenshot if you can, and send the report to your national or regional coordinator. The service does not police band plans, contests or amateur behaviour, and its volunteers do not confront intruders. Check that the allocation is exclusive before you report, keep your records clean and let the process do its work.

Frequently Asked Questions

What is the IARU Monitoring System?

A worldwide volunteer service, formerly called Intruder Watch, that identifies and reports non-amateur stations on amateur bands so that the authorities can act on them.

Which signals count as intruders?

Stations outside the amateur service on bands reserved for amateurs, such as broadcasters, over-the-horizon radars, illegal operators and certain military signals.

Do I need special equipment?

No. Your own receiver is enough, and online receivers such as WebSDR or KiwiSDR can be used. A waterfall display helps a great deal.

What should I record?

Date, UTC time, frequency in kilohertz, mode, country if known, call signs or identity if received and remarks, plus an audio recording or screenshot if possible.

Should I contact the intruder on the air?

No. Monitors only report what they hear. Do not transmit to the intruder or try to jam it; send the report to the coordinator.

Is a contester on the wrong frequency an intruder?

No. Violations of band plans and contest rules by amateurs are outside the scope of the intruder watch and are matters for the national society and the regulator.