Where do wireless system interferences come from?

Interference can come from both the external RF environment and the wireless system itself. Therefore, before the event, it is necessary to check not only what is "transmitting in the area" but also the mutual relationships between your own frequencies.

Potential sources of problems may include:

• other wireless microphones,
• IEM systems,
• transmitters located in adjacent rooms or on other stages,
• local radio and television broadcasts,
• other devices operating in the same band,
• intermodulation products generated by your own system,
• overloading the RF path with too strong a signal.

The symptom is not always a complete dropout. The problem may also appear as short crackles, an increase in noise floor, a temporary loss of quality in transmission, or an unstable RF level.

Therefore the selection of wireless microphones for a concert, conference, or event should take into account the ability to manage frequencies and antennas, not just the quality of the capsule or the nominal range of the device. 

Why is a "free frequency" not enough?

The frequency may appear to be free during the scan, yet it can be a poor choice for a specific set of channels. The reason is primarily mutual compatibility of frequencies.

Let's assume we have several transmitters. Each of them individually operates on a frequency that has no external emission. However, this does not mean that all of them can safely operate together.

RF signals can interact in nonlinear stages of transmitters or receivers and generate new frequencies. If any of these products is close to the channel used by the receiver, interference may occur.

Therefore, professional coordination performs two tasks simultaneously:

• avoids externally occupied spectrum,
• checks mutual compatibility of all its own channels.

This is a fundamental difference between simply searching for a "hole" in the spectrum and preparing a larger RF system.

What is intermodulation?

Intermodulation is the generation of additional frequencies as a result of the simultaneous interaction of several RF signals in a nonlinear system.

In practice, this means that transmitters A and B can indirectly create a problem on frequency C, even though neither of them is directly tuned to it.

Where do intermodulation products come from?

Intermodulation products can arise, among others, in nonlinear stages:

• transmitters,
• receivers,
• RF amplifiers.

Their frequencies are mathematically related to the frequencies of the fundamental signals. Shure indicates that IMD can occur both at the outputs of transmitters and at the inputs of receivers, especially when several strong signals are close to each other or the RF path is overloaded.

That is why in a large system, frequencies should not be assigned randomly.

Why does the problem grow with the number of transmitters?

Each additional transmitter increases the number of possible combinations of signals that can interact with each other.

With two channels, the situation is relatively simple. With several microphones and a few IEM systems, the number of potential relationships increases very quickly.

That is why large systems require tools that can automatically analyze channel compatibility.

This is also the reason why increasing the power of all transmitters "for safety" can have the opposite effect. Very strong signals can increase the risk of generating unwanted products in nonlinear RF stages.

How to perform an RF environment scan?

An RF scan involves examining the level of radio signals in the frequency range in which we plan to operate.

It is best to perform it on-site at the event, under conditions as similar as possible to those in which the system will later operate.

The scan allows identifying:

• occupied spectrum segments,
• strong local emissions,
• areas with elevated noise floor,
• potentially useful parts of the band.

However, it is not a complete coordination in itself.

Sennheiser WSM in Professional Setup mode allows for performing a frequency scan, spectrum analysis, marking useful and useless ranges, and then using this data in coordination.

A similar workflow is used by Wireless Workbench: scan → analysis of frequencies → calculation of compatible channels → assignment and deployment to devices.

It is important that during the scan:

• the antennas are in a configuration close to the target,
• the system covers the actual work area,
• to consider other active systems in the facility.

If we perform the scan with the rack hidden behind a metal structure, and later move the antennas to the stage, the result may not represent the actual environment as well as we expect.

What does professional frequency coordination involve?

Frequency coordination involves selecting a set of channels that are simultaneously as free from external interference and mutually compatible.

The process should consider:

• data from the scan,
• all wireless microphones,
• all IEM,
• operating ranges of devices,
• intermodulation requirements,
• backup channels.

Sennheiser emphasizes that coordination is important for both microphones and in-ear monitors, as it helps to reduce frequency conflicts and dropouts in a crowded RF environment.

Wireless Workbench

Wireless Workbench is a Shure tool used for managing devices, frequency coordination, and RF monitoring.

A typical workflow includes:

• execution of the scan,
• adding all devices to inventory,
• analysis of current frequencies,
• calculation of a compatible set,
• assignment of frequencies,
• sending them to devices.

The program can utilize scan data and information about specific device profiles.

This is important because a different RF profile may have a digital microphone, another an analog one, and yet another an IEM transmitter.

Wireless Systems Manager

Sennheiser Wireless Systems Manager also allows planning, scanning, and coordinating larger systems.

The Professional Setup mode allows:

• to add devices,
• to perform a scan,
• to mark interfered ranges,
• to create system regions,
• to designate backup frequency groups,
• to calculate a set of compatible channels.

It is particularly important to include microphones and IEMs in one project.

Backup Frequencies

In a large system, it is advisable to prepare backup frequencies before the event starts.

It is not about selecting a random "free channel" at the moment a problem occurs.

Backup should be:

• included in coordination,
• compatible with the rest of the system,
• quickly assignable.

WSM allows defining separate groups of backup frequencies as part of the coordination.

Thanks to this, in an emergency situation, there is no need to perform the entire analysis from the beginning.

How to position the receiving antennas?

Antennas should be located in a place that provides as good a radio path as possible between them and the transmitters.

The most important rule is as good as possible line-of-sight.

Shure recommends minimizing obstacles such as:

• large metal elements,
• walls,
• a large number of people between the antenna and the transmitter.

In many situations, it is beneficial to set the antennas high, above the audience or other obstacles.

However, this does not mean that antennas should always be placed as far and as high as possible.

One must also consider:

• the length of the cables,
• the area of movement of the performers,
• the directionality of the antennas,
• the positions of other transmitters.

A directional antenna can increase sensitivity in a specific direction and limit signals from other directions. However, if we point it at a strong source of interference, we will also increase the level of that interference.

Therefore, a "more directional" antenna is not automatically better.

Why does "more antenna gain" not always mean better?

The gain of the antenna or active RF amplifier should compensate for the actual losses in the path, not be used to achieve the maximum high RF indication.

A signal that is too strong at the receiver's input can overload its front-end.

The result can be:

• an increase in intermodulation,
• a decrease in selectivity,
• deterioration of the system's real resistance.

Shure warns that excessive antenna gain can overload the receiver, increase intermodulation issues, and in practice reduce its effectiveness.

This is an important principle:

the goal is not to achieve the highest RF level, but a stable level within the proper operating range of the devices.

If the path has 6 dB of loss, a reasonable compensation is about the loss, not an automatic setting of the amplifier to maximum.

How does RF cable attenuation affect the system?

Every coaxial cable attenuates the RF signal. Losses increase with the length of the cable and also depend on the frequency and construction of the cable.

This means that a very good antenna set up on stage may not provide the expected benefit if connected to a receiver with a very long cable that has high attenuation.

Therefore, when designing, the entire path must be considered:

antenna → RF cable → possible distribution / amplifier → receiver

Practically, this means that sometimes it is better:

• to place receivers closer to the antennas,
• to use a cable with lower losses,
• to transmit the audio or network signal further,

than to run a very long section of improper antenna cable.

One should not compensate for every problem with a large booster. If the cable generates significant losses, the first question should be whether the path and type of cable are appropriate.

When to use antenna distribution?

Antenna distribution makes sense when a larger number of receivers should use a common, well-designed antenna system.

Instead of building a "forest of antennas" at each receiver, one can use:

• two common diversity antennas,
• a distribution system,
• splitting the signal to multiple receiving channels.

Shure points out that antennas placed very close to each other can interact with each other, and a large number of antennas at the rack can lead to unfavorable effects. In the manufacturer's materials, distribution is recommended especially for multi-channel systems; one of Shure's guides indicates its use from about three channels upwards as a practical recommendation.

Distribution offers several benefits:

• fewer antennas,
• the ability to place them in a better location,
• a more organized rack,
• a consistent RF infrastructure.

However, it is necessary to maintain a proper balance of levels. Each split introduces losses, and active distribution should compensate for them without overloading the receivers.

How do wireless microphones work with IEM?

Microphones and IEM should be coordinated as one system, because all RF devices operate in the same space and can mutually affect the radio conditions.

In microphones, the transmitter moves with the performer, and the receiver is usually located in the rack.

In IEM, the situation is reversed:

• the IEM transmitter is located at the monitor world or rack,
• the performer’s bodypack is the receiver.

This means that at the stage there can be simultaneously:

• very sensitive microphone receiver antennas,
• transmitters IEM,
• transmitting antennas IEM.

If they are set too close to each other, a strong signal from the IEM transmitter can worsen the operating conditions of the microphone's receiving path.

Therefore, it is necessary to control:

• the physical separation of antennas,
• the power of transmitters,
• the frequency ranges,
• the common coordination.

The topic of the monitors themselves is described more broadly in the material floor monitors or IEM in-ear systems – what to choose for the stage?

The most important thing is not to coordinate the microphones in the morning, and then independently activate the next several IEM transmitters without reanalyzing the entire system.

How does the power of transmitters affect the RF environment?

Higher transmitter power can increase the signal margin in a demanding path, but it should not be automatically set to maximum.

Too high power has several potential consequences.

First, stronger signals increase RF energy in the entire environment.

Second, several transmitters operating at high power and located close to each other can increase the risk of intermodulation problems.

Third, a very strong transmitter located close to the receiving antenna can overload its further path.

Shure describes situations where closely positioned transmitters generate IM products in their output stages, as well as cases where strong signals near receiving antennas cause the formation of intermodulation products in the receiver's input stage.

Therefore, power should be selected for:

• required range,
• geometry of the system,
• required reliability,
• density of channels.

More RF power does not automatically mean greater reliability.

How to prepare RF before the event and what to check on site?

The best result comes from combining pre-event planning with on-site re-verification.

Before the event, it is worth preparing:

• a list of all RF devices,
• their frequency ranges,
• the number of microphones,
• the number of IEMs,
• a plan for antennas,
• preliminary coordination,
• backup frequencies.

If facility plans are available, probable antenna locations and cable run lengths can be determined in advance. Shure notes that plans and information about the physical space help in designing antenna coverage and identifying potential obstacles.

Upon arrival on site, you need to:

• perform a current scan,
• consider other teams' systems,
• recalculate coordination,
• send frequencies to devices,
• synchronize transmitters,
• check the entire work area.

A walk test is particularly important.

The person with the transmitter should walk through all the places, where they may be during the event:

• the stage,
• runways,
• backstage,
• the audience, if the microphone will be used for Q&A,
• other important areas.

Simply indicating the RF correctly at the rack does not guarantee that the system will be stable across the entire area.

In larger implementations professional RF coordination of the event should be treated as a separate stage of technical preparation, similar to tuning the PA or preparing the patch.

What to monitor during the concert?

During the event, both RF parameters and the state of the transmitters themselves need to be observed.

Depending on the system, you can monitor:

• the level of RF,
• the quality of the signal,
• the activity of diversity antennas,
• the level of audio,
• the state of the batteries,
• interference warnings,
• the status of devices.

Wireless Workbench allows not only for frequency coordination but also for later monitoring of connected systems. Sennheiser WSM also presents device parameters and system data at one station.

Monitoring allows you to react before the problem becomes audible to the audience.

For example, a gradually worsening RF margin may indicate:

• the appearance of a new source of interference,
• a change in the position of the antenna,
• a damaged cable,
• a problem with the connection.

Similarly, a dropping battery level can be addressed before the artist goes on stage.

The biggest mistake is assuming that since the system passed soundcheck, it no longer requires attention during the concert.

Summary

A stable wireless microphone system requires simultaneous control of the spectrum, frequency, RF levels, and physical antenna infrastructure. Simply choosing a frequency where no external transmitter is visible does not ensure the proper operation of multiple channels.

In a larger system, intermodulation, microphones, and IEM must be considered in one coordination, cable attenuation, antenna placement, and the appropriate transmitter power. A signal that is too strong can be just as problematic as a signal that is too weak.

The safest workflow is scan → coordination → antennas configuration → test the entire area → monitoring during the event. This way, RF is designed and controlled as one system, rather than being a collection of independently set receivers and transmitters.

Frequently Asked Questions