A scanner is a type of radio receiver that allows you to receive transmissions from police departments, fire departments, ambulance services, government agencies, aircraft, marine traffic (boats), amateur radio, CB radios, and much more. These radios sometimes come pre-programmed with banks of freguencies that cover things like police, fire, medical, etc., to allow you to search through the bands to find frequencies that are active in your area. Others come completely empty with up to 1000 channels for you to program with specific frequencies of your choosing.
There are three types of scanners; Handheld, Mobile, and Base Station. Each of these three types of radios are designed for specific uses and each have unique advantages and disadvantages.
Handheld scanners are designed to be portable and handy. They are commonly used by individuals and off-duty emergency personnel. They can be powered by alkaline or rechargeable batteries, or they can run on AC or DC current with the appropriate adaptors. Car Lighter Plug adaptors will allow you to power your scanner in the car will also recharging you batteries (if you are using rechargeables). House (AC) adaptors allow the same thing ensuring that you will always have a fully charged set of batteries. There is also a wide selection of antennas available for handheld scanners. Most come standard with the small rubber type antenna but do allow themselves to be connected to mobile and base station type antennas. The biggest drawback to portable scanners is their little rubber antenna. It doesn't allow for pulling in weaking or distant signals.
Mobile Scanners are just that, designed to be mounted and used in motor vehicles. They run off DC power either hard wired or through a lighter adapter plug. These units require some type of external antenna, usually a magnetic mount or gutter mount antenna. These antennas come with all wires and connectors pre-attached. They are designed for mobile use only and are not portable and require an AC-DC converter to be used as a desktop unit. They are not portable but they will receive better than a handheld scanner.
Base Station scanners are for use at home or work. They run off AC power and usually sit on a desk or table. Most base station scanners come with either a screw-in telescoping antenna or a back-of-the set antenna. These units come equipped with jacks to accept plugs run from external antennas. These antennas can be mounted on a mast attached to the exterior of your building and will allow you the best reception possible. These units are not portable, they require household (AC) current to operate and are designed for home/office use only.
All scanners currently available today are programmable. They have circuits allowing you to program in a wide range of frequencies. Early scanners were crystal controlled and many only allowed for receiving in one or two bands. Many of todays scanners receive ten or more bands. Recently, Digital Scanners have been introduced. these units are still very expensive and require digital cards to be installed in them to make them function. As many government and police departments switch over to digital transmissions digital scanners will grow more popular, but for now they aren't really needed for the majority of departments that you will want to listen to.
Reception with any scanner is dependent on the antenna used. Base-station antennas will give you the best reception and allow you to pick up individual units at a greater distance, with less static, than smaller antennas. Both handheld and desktop scanners can be connected to base-station antennas. There are a wide variety of base-station antennas available and the type you choose should be determined by the type of listening that you do and the bands that you listen to.
Mobile scanners require some type of vehicle mounted antenna. These come in varying sizes and styles. Most mobile antennas available today are magnetic mount with a few being glass-mount (cellular style). These antennas will work equally well with handheld scanners when they are being used in a vehicle.