The popularity of wireless microphone systems continues to grow on a daily basis. This growth is unfortunately paralleled by many other forms of wireless technology, including cellular phones, paging systems, and additional TV stations. As the available radio frequency spectrum continues to get increasingly crowded, the potential for interference in wireless microphone systems also increases, causing headaches for both the end user, and the sound system installer, and the audio consultant. Given the increasingly unpredictable nature of wireless technology, an RF (Radio Frequency) scanner can be an extremely useful tool for any installation employing wireless microphones.
An RF scanner is an electronic device designed to pick up radio activity over a certain range of frequencies. A car radio is a type of RF scanner focused on a very specific band of frequencies, commonly known as AM and FM radio. The practical application of an RF scanner to wireless microphone systems is two-fold: 1) To search for clear frequencies in a new installation, or 2) to troubleshoot interference problems at an existing installation site. If the use of traveling frequencies (169-172 MHz) is planned, it is essential to scan for potential problems ahead of time due to the ever-changing nature of that frequency range. [For more information on the problems with traveling frequencies, contact Shure for the Applications Bulletin, Traveling Frequencies: No Longer a Trouble-Free Solution. ]
Most RF scanners are simple to operate, and offer a variety of methods for frequency scanning. The simplest method to check for interference is to enter the frequency of the wireless microphone system in question and listen for any audible signals. Remember, anything that is picked up by the scanner when the wireless transmitter is turned off is a potential source of interference for the wireless system. Certain types of interference will produce distinctive sounds. Paging systems produce a series of beeps and crackling noises. The video signal carrier of a television station is identified by a steady buzz, and the color signal is usually heard as a high frequency whine. Audio information is, of course, easily recognizable. Be sure to check adjacent frequencies as they can also cause interference even though they are not on the exact frequency of the wireless system.
This method is useful for checking one or two problem frequencies, but can be cumbersome when searching for clear frequencies at a new site. In these situations it is best to use the scanner’s search or scan function. This function will automatically scan through a preset or programmed range of frequencies, stopping wherever there is significant RF. Avoid these frequencies.
Some scanners offer the ability to search repeatedly over a certain range. This feature allows the scanner to catch intermittent transmissions missed on initial scans. To further refine this technique, a scanner can be connected to a voice-activated tape recorder and operated over an extended period of time, repeatedly scanning a particular frequency range. The tape recorder will activate whenever the scanner catches an active frequency. This is useful for catching intermittent interference on-site, instead of spending an entire day waiting for the problem to occur. Most scanners are equipped with a headphone output that can drive the record (or line) input of a recording device. Voice-activated (VOX) recorders are available from Radio Shack. See the below diagram illustrating the connection of a scanner to a tape recorder:
To make effective use of an RF scanner, be familiar with the concept of squelch. Most quality scanners are equipped with a squelch control which allows the user to eliminate unwanted background noise on normally inactive frequencies. Care must be taken when setting the squelch control. If squelch is set too high, significant sources of RF interference may be overlooked. If squelch is set too low, the scanner will frequently stop on background noise when using scan or search modes. To set the squelch control, select a known clear frequency, e.g., an unused local TV channel, and rotate the squelch control until the background noise disappears, then add just a bit more squelch for extra insurance.
When scanning for potential sources of interference, it is only necessary to search the frequency ranges where wireless microphone systems are assigned. The most common (and most crowded) frequency range for wireless microphone use is high-band VHF, which extends from 174 to 216 MHz. The “traveling” frequency range is 169 to 172 MHz. The spread of UHF frequencies is much wider, from 470 to 806 MHz, and can take a long time to scan. If you are considering a UHF system, first determine the operating range of the wireless system in question and scan only those frequencies. The UHF band is much less congested than VHF, making it easier to locate unused frequencies. Frequencies outside of those used by wireless microphones do not normally cause problems. Commercial AM and FM radio bands, for example, are well below the VHF band and are unlikely to cause interference. However, occasional interference can occur in cases of extreme proximity (less than a mile) to a high-power radio transmitter.
RF scanners are not difficult to find. Several electronics catalogs carry scanners, as well as Radio Shack. For information on where to purchase scanners, contact the manufacturers directly. In addition to Radio Shack, contact AOR (www.aorja.com) through their US distributor, Electronics Distributors Corp. at 703-938-8105, or for Bearcat scanners (www.uniden.com) call the Uniden Answer Line at 900-225-4822. Expect to spend between $300 to $400 for a quality scanner. The single most important feature in determining which scanner to purchase is what frequency ranges it covers. Many scanners exclude the high-band VHF range, which makes them useless for VHF wireless applications. Be certain that the wireless microphone frequencies mentioned in this bulletin are covered by your choice of RF scanner. Other useful scanner features include a good squelch control, the ability to limit the scan range, and programmability. This last feature allows a variety of frequency ranges to be saved in memory and recalled at any time. Finally, a headphone output or line output is useful for aural recording of RF interference. An investment in a scanner with these basic features will prove invaluable for all wireless microphone installations.