Listening Devices and Bugs

Introduction

Covert listening devices have been around for years in a variety of forms. Listening devices are otherwise known as listening bugs, or just ‘bugs’. Bugs are designed for covertly eavesdropping on other people’s conversations in order to gain useful information, evidence or intelligence. Originally bugs were just designed to relay sound, but now tiny wireless spy cameras are commonplace.

The most basic form of a bug is a sensitive microphone connected to an audio recording or listening device. A hidden voice recorder or dictaphone can be even considered as a crude bug. The common perception of a bug is that it’s a tiny microphone that transmits an audio signal to a nearby receiver. There are also a few other types of covert listening devices that come under the description of “bug” too. All of of these types of bug are covered in this section.

The 3 Main Types of Bugs

Hardwired bugs

Hardwired listening devices are physically wired into a room or object for longer-term surveillance. In this situation, the other end of the wires are likely to be connected to recording equipment, as well as someone listening with a pair of headphones. This is often cliched in the movies with some government agency tucked away in a hotel room!

Hardwired listening bugs are very simple. Usually a highly sensitive microphone connected to a powerful audio amplifier, with the the audio signal being sent over a pair of wires.

Recording bugs

Recording bugs are very much related to hardwired bugs. The microphone is connected to some kind of tape recorder. These days, most audio recorders are digital, where the audio is stored on an internal memory card. Memory cards typically hold far more audio than their cassette tape counterparts, which is why tape recorders are so rare these days.

It’s very common for an audio recorder to be voice activated to conserve battery life and memory/tape space. The recording is only triggered when sound is detected above a particular level, usually set by a control on the recorder itself. To optimise the quality of sound recorded, it’s common for the recorder to be fitted with an external microphone. This is so that the microphone can be optimally positioned to pick up the designed conversations.

Transmitting bugs

Transmitting bugs are small radio transmitters combined with a simple audio amplifier, which transmit conversations to a nearby radio receiver. Room bugs are fitted with a tiny electret microphone, which transmits the voices heard in close proximity to the microphone. Telephone bugs are wired to a telephone line, which transmits the audio carried along the telephone line.

To hear the conversations from a transmitting bug, you need a radio received tuned into the frequency of that bug. Transmitting listening bugs typically transmit using Frequency Modulation (FM) rather than Amplitude Modulation (AM) as FM offers more robustness against signal interference.

VHF bugs

The most basic FM transmitter listening devices transmit around 80MHz to 130MHz (also known as Very High Frequency (VHF) bugs). The full VHF band designates a range of frequencies between 30MHz to 300MHz.

A transmitting bug in this frequency range is going to cause problems of interference with domestic radio stations, which means there’s a high chance that other people can also hear your target conversations. With that in mind, VHF bugs are usually the cheapest bugs to buy.

UHF bugs

For added privacy, it’s common to use a transmitting listening bug that transmits on a higher frequency, often somewhere between 200MHz to 450MHz. UHF designates a band of electromagnetic waves with frequencies between 300MHz and 3,000MHz (3GHz), so bugs in this range are also referred to as UHF bugs.

Professional UHF bugs will have components such as crystals to fix the transmitting signal at a precise frequency. UHF bugs are typically made using surface mounted technology (SMT) to create very tiny devices. These slightly more expensive transmitting bugs use more sophisticated electronics. Where a transistor-based bug might suffer from frequency drift (where the actual transmitting frequency changes as the circuit warms up), a crystal-based FM transmitter will be locked to its transmitting frequency regardless of environmental changes.

Listening Bugs – Transmitter Range

It’s a myth that bugs have unlimited transmitting range. According to the laws of physics, the radio waves generated by the bug have infinite range, but the strength of the radio wave decreases exponentially. Therefore in practice, when using a standard receiver without any high gain or directional aerial, you are going to have a limited distance from the transmitter that you can receive a signal.

Bug Receivers – Scanners

To receive the radio signal from an FM bug, we’ve found that professional portable radio scanners are considerably versatile and cost effective (compared to a standard FM radio). A radio scanner is more precise and usually has a frequency memory, so that you can store the commonly used frequencies from your portfolio of transmitting listening devices.

Mobile phone-based bugs

Since about 2003, a new type of ‘transmitting’ bug has been easily available. These are bugs based on mobile phone technology, allowing you to dial a mobile phone to hear conversations going on nearby.

Read more about Spy Phones.

A limiting factor – Batteries

Pretty much any listening device needs a source of power, usually in the form of batteries. (Well, perhaps not a glass tumbler to a door, but hey!). Batteries run out, fast. A PP3 9V battery typically lasts a few hours, maybe a day. A lithium coin cell might last just a couple of hours. AA batteries might last a few days. The larger the battery, the more obvious a bug will be. Clearly this will affect how you conceal a bug.

Some devices can run directly from a mains source (with lots of filtering to prevent mains-hum interference), which addresses the power source problem. However, you’ll find you have a limitation on where you can place the bug.

So when you purchase a bug, think about the power source as one of your primary considerations.