VHF (Very High Frequency) is defined as frequencies within the electro-magnetic spectrum of radio waves found in the range of 30 MHz to 300 MHz. We are all most familiar with our car’s FM band making use of VHF from 88 to 108 MHz. Also, the lower channels on television broadcast are located in the VHF bands.

For those of you who are pilots you know that route navigation uses VOR, short for Very High Frequency Omni Range and radio communications with air traffic control is also in the VHF bands.

Certain types of RFID devices are well suited to be operated in the VHF space. Typically they are self-powered, Active or Semi-Active tags. VHF frequency propagation characteristics are ideal for short-distance terrestrial communication. Another characteristic that makes VHF desirable for RFID application is that atmospheric noise and interference from electrical equipment less affect it than lower frequencies.

Dynasys offers AXCESS Active and Semi-Active RFID tags that transmit at 315 MHz. These tags can operate in “Beacon Mode” where they transmit a short identification burst at pre-programmed intervals. They can also operate in “Activator Mode” where they are asleep most of the time to conserve battery power and wake up only when they are subjected to a low frequency, 126 kHz magnetic field. When awakened they transmit their short identification burst and go back to sleep to save their energy.

There are many different configurations of these AXCESS Active Tags that can be utilized to monitor the movement of assets. The active tags can work in 2 main modes.

1.) BEACON MODE
The tag is factory programmed to chirp out its ID every so often. You can choose over a wide range, example once per hour down to once per minute. Transmission is at 315 MHz. Battery life of the tag is claimed to be about 1 year at a rate of one chirp per minute. Dynasys has some varieties that have replaceable batteries but these tags are in the key fob configuration, not the flat card type. Flat cards have permanent batteries that can not be replaced. If a "beacon" tag is placed in an "activator" field is responds regardless of beacon time interval.

2.) ACTIVATOR MODE
An ACTIVATOR UNIT and antenna system is set up in an area. One configuration is to mount the antenna over each doorway. Another configuration is to put a flat coil mat under the rug on the floor. The ACTIVATOR puts out a 125 kHz signal that wakes up the sleeping tag and causes it to identify. The tag(s) remain asleep until they are "activated" by this magnetic field.

Receivers in each area listen for a beacon ID from each tag. As long as the networked receivers can hear a reply from each ID every asset is accounted for. If an asset is removed from the reception area its reply is not heard and the application program can alarm the event of a missing asset. Putting a metal cover over the tag interrupts the radio reception. To detect a missing asset you must continually monitor for ID signals and not just try to detect a tag going out the area.

There is an option available for a “Tamper Detect”. If the tag is pried off the surface of the asset a switch can be triggered. This sets off an alarm signal that can be detected by the local receiver. Remember, this alarm signal can be blocked by placing a metal box over the tag, thus preventing detection.

A software program must be created to filter these multiple replies and keep a database of all of the tags and be able to alarm and report as required.

These AXCESS “Active Tags” come in multiple varieties; each designed for a specific application. Contact your Dynasys Sales Engineer, an expert in selecting RFID devices, to assist you with your selection.