If you have never owned a bat detector before you should purchase a detector that has a heterodyne system.

This type of detector is the best for identifying bats in the field. You hear the calls in real time and you can differentiate between different tonal qualities and repetition rates of bat calls. You can also obtain the 'peak' frequency of the call that will be decisive in making an identification.

Heterodyne detectors are simple to operate requiring only the adjustment of a volume control and a frequency control. The frequency control is similar to the tuning control on a radio; it just tunes to different frequencies.

The microphone picks up the ultrasonic bat calls, so the type and quality of the microphone determines in part how sensitive the bat detector is.

Piezo microphones are most sensitive at mid frequencies (45kHz). Electret are best at lower frequencies (30kHz) and capacitance are very good for detecting high frequency calls (80kHz). This does not mean you cannot, for example, pick up high frequency calls with a piezo microphone - it simply means that the distance you can detect them at will be less than with a capacitance microphone.

The Pettersson D-100 has long been a popular model as it combines microphone types to give an excellent sensitivity but comes at a higher price tag.

This is a measure of how many kHz either side of your tuned frequency you can hear calls on. For example, if the bandwidth is 8kHz, then you will hear calls 4kHz above and 4kHz below your tuned frequency.

A wider bandwidth will give you more chance of picking something up if it is echolocating on a slightly different frequency to which you are tuned to. But having a very wide bandwidth will mean once you hear something you could have to do a lot more fine tuning to work out the peak frequency and identify the species.

Today's heterodyne detectors tend to have a bandwidth of 8 to 16kHz which is suitable for most purposes. One of the best beginners' detectors, the Batbox 3D, has one of the widest bandwidths, which combined with the latest digital technology makes it a popular choice even for the more experienced user.

Most bat detectors are tuneable within the range 20-120kHz, appropriate for detecting all European species.

Some detectors tune down to 10kHz and can also be used for picking up lower frequencies as produced by some grasshoppers and crickets. The Magenta 5 is one such model and in addition to crickets it can be used as an aid to listen to some bird and rodent species.

If the controls are on the side of the detector rather than the top it is easier to operate with one hand. You may also wish to consider the weight of the detector, particularly if you are carrying a tape recorder, torch and other survey equipment. The Pettersson D-200 is extremely compact, lightweight and can be easily operated in one hand.

An accurate LCD or LED display makes the determination of frequencies easier. The frequency dials on most lower cost detectors have a small backlight to help read the dial in the dark. The Magenta Bat5, Batbox III D and Pettersson D200 all have digital displays.

Most detectors have a built-in loudspeaker, though you probably want to use headphones at least some of the time. Using headphones means you can listen with less background noise and without disturbance.

Most detectors have headphone sockets and an output socket for digital, tape, minidisc or other sound recorders.

One of the best ways to choose your first detector is to try out some models and get advice from experienced users.

Joining in on a local bat walk organised by a bat group is an excellent chance to try out a wide range of bat detectors.

For more information on how to get involved, and how to find your local group in the UK, contact The Bat Conservation Trust.