There are other blockage issues to consider when using wireless in security applications. For example, a microwave transmitter can be set up to send closed circuit television (CCTV) video from one location to another location many miles away. Signal blockage can occur with the construction of a new structure that is in the signal's path. This problem is exacerbated when there are changes of elevation along the wireless signal path. Any RF signal at high frequencies will require an unobscured line-of-sight. If there is a rise in elevation along the signal path, a structure can be built that is not very tall and still block the path. This type of interference is unintentional.
Another example of unintentional interference occurred in the early development of garage door opener technology. The early garage door openers would open the owner's garage door and, often times, the neighbor's garage door. Some people have experienced or have heard of someone whose garage door electronically opened by someone other than the owner. This is because a frequency was generated that was close enough to the garage door frequency that the opener accepted the signal. One way to minimize the problem is for the garage door opener manufacturer to incorporate a dipswitch in the transmitter and receiver for the garage door opener to set a “unique” code. Because the power level of the garage door opener is fairly low and a code is added, the amount of interference of garage door openers across a large metropolitan area is minimized.
Sometimes you can take advantage of interference. TV stations, for example, transmit microwave signals from vehicles with mobile microwave transmitters back to the TV station for remote broadcast. These vehicles often have one or more microwave dish/dishes mounted to an extendable arm or pole. Quite often, a TV station engineer faces RF interference issues, so the operator must direct the microwave antenna's line-of-sight signal directly at a source of interference, such as a metallic- or mirror-finished glass building to bounce the microwave signal back to the TV station-receiving antenna.
There are security concerns in reference to intentional interference, which blocks reception of the signal. Transmitting a high-power signal at the same frequency that is being used for the security wireless application can cause intentional signal blockage. (This problem is fairly unlikely for most commercial security applications.) Wireless security system manufacturers can incorporate different technologies to avoid the jamming or intentional interference problem by using what is referred to as “spread spectrum” technology, which enables the message or signal to be sent on one frequency and the next message to be sent on a different frequency.
The message can also be broken up into pieces of data referred to as a packet. One piece can be sent on one frequency and another packet on a different frequency, and so on. No matter how the data is spread over the frequency spectrum, the receiver and transmitter must be in synchronization to ensure proper reassembly of the data. Synchronization allows the total data stream to be recombined into a usable recognizable signal.
The power radiated by the transmitter will affect the maximum distance the receiver can be placed from the transmitter to receive a valid signal. The Federal Communication Commission (FCC) controls the power levels and frequencies used by all RF devices in the United States .
The requirement for an FCC license depends on the frequencies used and the transmitted power. Many security products and wireless LANs on the market fall into one of several frequencies. 5.8 billion Hertz (GHZ) is one of the common frequencies, while other frequencies include 2.4 GHZ and 433 million Hertz (MHZ). The 5.8 GHZ, 2.4 GHZ and 433 MHZ security RF wireless products can be legally used, in many cases, without an FCC license.
If a product's literature does not indicate whether an FCC license is required, it is important to verify that requirement before purchase. Deciding between products that require an FCC license and do not, primarily depends on the distance needed between the transmitter and receiver. Longer distances require more power and may require an FCC license.
Some caution is needed when applying the distances advertised for a given product vs. what can be expected in real-life. The wireless manufacturer provides the distance their product will transmit in their literature. This distance is under ideal conditions in unobstructed “clear air.” The assumption is that there are no obstructions, a clear line-of-sight, low humidity and no signal loss or interference due to reflections.