The system itself is incompatible
When using wireless microphones, there are always interference problems between the systems themselves. Although each system has its own frequency or interval of several megahertz, intermodulation distortion (IMD) can still cause interference between microphones. If there is not enough megahertz space between the intermodulation signal and the operating frequency of the device, it is difficult for the receiver to pick up the signal emitted by the transmitter. Typical phenomena are crosstalk between systems, frequent signal loss or excessive noise and distortion. The smaller interval between frequencies depends on the design of the system receiver, and the entry-level receiver may need to be 1 MHz apart from the nearest adjacent system. The more expensive the receiver is, the narrower the "window" of adjustment, allowing smaller intermodulation frequency spacing between each system.
Solution: To avoid intermodulation distortion, choose compatible frequencies that have been calculated. This requires a wealth of knowledge of transmitter and receiver design, and wireless system manufacturers often have calculated these frequencies. For example, when only eight wireless microphones are used together, thousands of calculations are performed to ensure compatibility between microphones. As a result, most manufacturers have published lists of their system compatibility frequencies. In addition, software can be used to help users identify compatibility frequencies in some cases.

Insufficient compatibility of the whole system
Frequencies have different compatibility. If you know the system well, you can adopt more systems more boldly. But the key is how to balance the compatibility of the whole system. Most frequency compatibility software is designed with an important assumption that all receivers are always open or silent (even if some transmitters are occasionally turned off), thus ensuring that all receivers do not pick up intermodulation signals that may cause noise. Therefore, the software needs to leave enough space for the intermodulation signal and wireless microphone in the design. If you assume that the audio system operator will play a more active role in the activity, you need a broader compatibility of the system. In this case, assuming that the operator will silence all receivers, all transmitters will remain during the performance. The distance between the transmitter and the receiving antenna is similar. These assumptions are perfectly feasible in Broadway theatre performances, but in the school auditorium, the system is operated by untrained personnel. It is unlikely to achieve the same expected performance effect. When the transmitter is positioned very close to the receiving antenna, or the high-power transmitter is running, the interference will be more serious. That's why it's much harder to have 40 wireless systems working at the same time in a movie theater than in school (many transmitters are very close to receivers). There's a system in every classroom of a school. The transmitters are completely independent but close to their own receivers.
Solution: To balance a large number of system devices with high performance, ensure that the compatibility level between frequencies is appropriate for the system expected to be used. Keep the transmitter at least 10 feet away from the receiving antenna. If the transmitter's RF output power is adjustable, lower transmission power is used to cover the desired distance between the transmitter and the receiver.

Interference from other signal sources such as TV stations
Wireless microphones are also interfered by other sources of signals from the same spectrum transmission. Commonly used are television stations. FCC rules require users of wireless microphones to avoid using the frequencies occupied by radio and television stations in the same geographical area.
Solution: Avoid 40-50 mile TV channel interference indoors. When working outdoors, it should be maintained within a radius of 50-60 miles. Because the frequencies of each city are different, the appropriate frequencies of wireless microphones depend on where they are located. Equipment manufacturers usually provide guidelines to inform users of different frequencies in different cities. The FCC stipulates that all analog television stations will cease operation in February 2009. At the same time, the spectrum above 51 channel will be used for other purposes. Wireless microphone frequencies above 698 MHz should be adjusted to lower frequencies to avoid interfering with new services. As the conversion continues, the TV channel at a specific location may change, so users are better able to check official information regularly.

Interference of other digital devices
Other wireless audio devices such as ear monitors, intercom systems, and non-wireless devices can also cause interference problems. Digital devices (CD players, computers and digital audio processors) often emit strong radio frequency noise and may cause interference if they are installed close to wireless microphone receivers. For transmitters, the common sources of interference are GSM mobile phones and PDAs worn by hosts.
Solution: When choosing the frequency of wireless microphone, we should know other wireless audio devices clearly. The distance between the digital device and the wireless microphone receiver is at least a few feet.

Selection and Layout of Receiving Antenna

The receiving antenna of wireless microphone is one of the most misunderstood areas. Errors in antenna selection, layout and wiring may lead to short performance coverage areas and low signal intensity, leading to frequent dropouts. The performance of modern diversity receivers is much better than that of single antenna types, but in order to optimize the system performance and reliability, the antenna selection and layout must be correct.

Solution: To ensure good diversity of the system, the antenna space should be at least one and a half wavelength (about 9 inches 700 MHz). The angle of receiving antenna should be "V" configuration, which can provide better signal pickup performance when the transmitter moves or is placed at different angles. If the receiver is to be installed away from the performance area (e.g. in a closet or enclosed rack), a half-wave antenna or a directional antenna (preferably above the audience) should be installed remotely so as to have a clear line of sight between transmitters. Do not install HF antennas remotely because they use receiver chassis as ground antenna. Redundant distances between antennas will not significantly improve the diversity of the system, but may better cover a larger area of stage, church or conference room. If the antenna is installed far away from the stage, the directional antenna can be used to improve the signal reception by picking up more signals in this direction and reducing other angle signals picking up. If the coaxial cable is used to connect the antenna to the receiver, it may be necessary to use the antenna amplifier to solve the problem of signal loss in cable transmission. The loss of signal quantity depends on the specific length of cable and cable type. Therefore, please calculate according to the manufacturer's suggestion. The total net loss of signal should be controlled within 5dB.

Artificial radio signal blocked
The human body may also interfere with wireless signals. The human body is mainly composed of a large amount of water, which can absorb radio frequency energy. In addition, if the user surrounds the hand on the external antenna of the portable transmitter, its effective output can be reduced by more than 50%. Similarly, if the flexible antenna on the transmitter is curled or folded, the signal will be affected.
Solution: Keep the transmitter antenna fully deployed and unobstructed, so as to achieve better performance in a wide range of signal transmission.

Insufficient voltage of emitter battery
The battery life of the transmitter is the primary concern of the wireless microphone. Users always try to reduce the cost of equipment by using low-cost batteries. Most wireless manufacturers specify alkaline batteries or disposable lithium batteries because their output voltages are stable throughout the battery life cycle. This is very important because most transmitters suffer from acoustic distortion or signal loss at low voltage. Rechargeable batteries often seem to be the ideal solution, but most rechargeable batteries provide 20% lower voltage than disposable batteries even if they are fully charged.
Solution: In order to solve the problem of batteries, we should carefully compare the voltage output requirements of emitter batteries at any time to ensure the sustainability of batteries in the whole working process. Lithium-ion batteries and rechargeable alkaline batteries can usually work continuously, while nickel-hydrogen and nickel-cadmium batteries may last only a few hours. The performance of AA rechargeable battery is similar to that of disposable AA battery.

Unadjustable transmitter
The inherent noise and limited dynamic range of FM transmission make analog wireless audio transmission limited. To overcome this, most wireless microphone systems usually use two audio processing methods to improve the sound quality. Pre-weighting device is added to transmitter and de-weighting device is added to receiver to improve signal-to-noise ratio. The compressor and receiver expander in the transmitter can increase the dynamic range to over 100 dB. This makes volume setting very important. If the audio level is too low, it will produce a hissing sound; if it is too high, it may cause distortion.
Solution: In order to obtain better sound quality, the input gain of the transmitter should be adjusted so that the overall modulation occurs at higher volume without distortion.

The Setting of Wireless System
The most troubling problem in wireless systems is the constant change of radio waves themselves. Since the beginning of digital TV conversion, analog and digital TV channel radio waves have been changing. The FCC is trying to find a way to make consumer products (personal PDAs, smart phones or home devices) use empty television channels for wireless Internet access.
Solution: It was easy to know that the VHF channel in the user's city was odd or even. However, when people install and use wireless microphones (as well as ear-in monitors and intercom systems, etc.), they have to check the local spectrum regularly even when they work in familiar locations.

Error setting of receiver output level
With so much discussion on frequency, wavelength and antenna, it is easy to overlook the basic requirements of wireless microphone systems: in order to replace the connection cables between signal sources and audio systems, receivers are usually equipped with output level control, while most wired microphones do not. This provides a better opportunity for finer matching between receiver output and input.
Solution: Whether the microphone level or the line level, the output level should be set to a practical higher level, and at the same time not exceed the input limit of the audio system. This may be indicated in the input channel of the mixer, or can be judged by the distortion of the audio.