early bat detectors used ex-Navy low frequency radio sets, simply replacing the aerial with a microphone and pre-amplifier. It is also possible to modify a portable Long Wave radio to be a bat detector by adjusting the tuning frequencies and replacing the ferrite rod aerial with a microphone and pre-amplifier. How it is used The operator guesses the likely species to be present and tunes the frequency accordingly. Many users will start listening around 45 kHz. If a bat is seen or a bat-like call is heard, the frequency is tuned up and down until the clearest sound is heard. Species like Pipistrelles which end their call with a "hockey stick" CF component can be recognised according to the lowest frequency which gives the clearest "plop" sound. Horseshoe bats give a peeping sound at a frequency depending on their species. FM calls all tend to sound like clicks, but the start and end frequencies and the call repetition pattern can give clues as to the species. Pros and cons The advantages of a heterodyne bat detector is that it works in real time, exaggerates the frequency changes of a bat call, is easy to use, and is the least expensive. It is easy to recognise a doppler shift in CF calls of flying bats due to their speed of flight. Stereo listening and recording is possible with models such as the CSE stereo heterodyne detector, and this can help to track bats when visibility is poor. The disadvantages of a heterodyne bat detector are that it can only convert a narrow band of frequencies, typically 5 kHz, and has to be continually retuned, and can easily miss species out of its current tuned range. Frequency division Frequency division (FD) bat detectors synthesise a sound which is a fraction of the bat call frequencies, typically 1/10. This is done by converting the call into a square wave, otherwise called a zero crossing signal. This square wave is then divided using an electronic counter by 10 to provide another square wave. Square waves sound harsh and contain harmonics which can cause problems in analysis so these are filtered out where possible. Some recent all-digital detectors can synthesise a sine wave instead of a square wave. One example of a detector which synthesises a sine-wave FD output is the Griffin. Some FD detectors output this constant level signal which renders background noise and