The same goes for other transmitters, especially those of TV stations.
And because these transmitters send out very powerful signals, they are a common cause of interference for wireless systems.
Even though a wireless system needs a clear frequency for the area where it’s going to be used, every frequency is used again and again across the nation.
Again, this is because the power of the output signal of wireless systems is very low.
Keep in mind, however, that there is no absolute guarantee that a clear frequency in one area will be clear elsewhere, even just across town.
This is an aspect about wireless systems that sometimes puzzles users; the government takes care of the problem for the high-power signals of commercial broadcasting, but wireless system users are responsible for avoiding this problem on their own.
Fortunately, most modern wireless systems (developed in the past 15 years or so) offer some degree of frequency agility (also called frequency synthesis). This means that the user is able to select an operating frequency from a number of possible choices, ranging from as few as four frequencies to 1,400 or more, depending upon the model.
The more frequencies offered by a wireless system, the better the chance of finding a clear frequency that is not being used by someone else in the area. Further, in larger cities, where there are more frequencies occupied by numerous users, the ability to choose from a larger number of frequencies is especially important.
Having plenty of open frequencies also helps wireless system users get around another potential problem: intermodulation (or intermod for short). This can occur where the frequencies of two transmitters (of any type) “combine” in a wireless system receiver, resulting in noise and interference.
Most often, intermod is caused by a combination of the frequencies from two TV transmitters, or by the frequency of a TV transmitter combined with the frequency of a wireless system transmitter.
Because the source of intermod is usually not under the control of the wireless user, there is usually little choice except to change the frequency of the wireless system. This is yet another reason for choosing a wireless system outfitted with a wide range of frequency selections.
By law in the U.S., wireless systems are supposed to operate only on TV channels not in local use. If a wireless system happens to cause interference to TV viewers in the area of its use (and this can happen even with their lower output level), the interference is likely to be reported, resulting in the user drawing unwanted attention from law enforcement.
Thus it’s vital for the wireless system user to keep handy a list of local TV frequencies in use (available online at www.antennaweb.org/aw/Address.aspx), and to avoid those frequencies.
Although many wireless systems can “automatically” select frequencies or scan to see local RF activity, it is still possible to select the frequency of a local TV channel and get the innocent user into trouble.
Wireless systems are available for “VHF” and “UHF” frequency ranges (also called bands), roughly corresponding to VHF TV channels 7 though 13 and the UHF TV channels 14 through 69.
The question as to which range is “best” has pretty much been settled by the wireless manufacturers, who generally only offer systems with numerous frequency choices in the UHF band.