Wireless microphones in the frequency jungle

At first glance, it seems like David is fighting Goliath. Small wireless microphone transmitters with just a few milliwatts of output power are struggling for their place in the frequency spectrum occupied by TV transmitter giants with thousands of watts. As we all know, David did not beat Goliath with muscles and power, but with skill. Following this analogy, wireless microphone systems must use smart technologies and clever systems to survive in an environment becoming more and more hostile for microphones.

Whether in music or theater, performers no longer expect to trail a microphone cable. The wireless microphone gives talent complete freedom of movement. Unfortunately, demands on the wireless spectrum are increasing. In-ear monitoring for artists has become in vogue. It removes the need for foldback wedges at the front of the stage and allows individual control over level, but the systems are wireless. The UHF band is becoming crowded with DTV and with pending auctions of existing analog TV slots.

The DTT squeeze

Digital TV signals transmit in the same frequency band as their analog predecessors. Wireless audio systems are allowed to operate as secondary users in vacant analog TV channels. In Europe and Africa, analog PAL television occupies only 7MHz of the 8MHz-wide UHF channel. (See Figure 1.) Until now, the remaining 1MHz gap has been used for production communication, reporters' transmitters and, to some extent, wireless microphones.

Following the introduction of DTT transmissions alongside existing analog channels, the spectrum available for wireless microphones is severely limited in many countries. (See Figure 4 on page 76.) In practice, the frequency spectrum in major cities is crowded. Figure 5 on page 76 shows a spectrum sweep taken during planning for the 2004 Olympics in Athens.

In rural areas, there may be significantly fewer transmitters on-air, so the field strength average is less. However, TV productions, live concerts and musicals are usually staged in metropolitan areas.

As many governments plan to auction off the analog TV channels after digital switchover, the spectrum available for wireless microphones could be even less as demands for wireless communication increase. A musical production in a theater may require 60 wireless microphone channels. Broadcast productions and concerts often require a similar number of channels.

Frequency efficiency

In such a crowded spectrum, no kilohertz can be wasted. And it becomes more necessary to pack transmitting frequencies as close as possible in the spectrum while not reducing the transmission safety of a system. This is not an easy task, taking into account that in a wireless microphone system there are many more unwanted signals than wanted signals.

Figure 6 shows 16 wireless microphone channels between approximately 1600 unwanted intermodulation products. Different RF signals mix on the nonlinear curve of any amplifier to make new unwanted signals — intermodulation. The different carrier frequencies produce not only harmonics (integral multiples) but also a large number of (odd number) sums and differences of integral multiples of the input frequency components. These limit the number of usable frequencies within a certain band.

The better and cleaner the transmitted signal from the wireless microphone is, the higher the density of transmitting frequencies in a wireless system can be chosen. Figure 7 shows the spectrum of a Sennheiser SK 5212 bodypack transmitter compared to a transmitter with lower spectrum efficiency.

Operating a multichannel wireless microphone system in a crowded spectrum requires the right equipment, a high level of RF experience and detailed planning. A recent example was the 2006 Eurovision Song Contest, which used 54 wireless microphone channels and 16 wireless monitor systems.

The set-up

The first step to set up a multichannel system is to check the frequency spectrum for available frequencies for wireless microphones. Ideally, the spectrum is scanned with a spectrum analyzer.

An alternative is to use a microphone receiver system that automatically finds available frequency slots. The selected frequencies need to be free from intermodulation. Calculating these frequencies requires a software application. Even in a mid-size system, billions of calculations are made to find intermodulation-free frequencies. Once the gaps in the spectrum are identified, then the frequencies can be calculated.

Filters are key

The problems of intermodulation can be eased by using receivers that are restricted to part of the UHF spectrum with steep fall-off bandpass filters. The performance is mainly determined by the quality of RF filters that are used. The purpose is to get rid of as many unwanted signals before the active part of the system.

Not only will the receivers have to operate near TV transmitters, but walkie-talkies, cell phones and other local devices should not block the receiver. The use of wireless in-ear monitoring adds to the potential for intermodulation interference. Great care, therefore, is required to produce highly selective filters providing the best rejection of unwanted signals to the receivers.

By taking these precautions, running a wireless microphone system in today's frequency environment is eminently possible. But the performance of a system and proper planning requires more attention to detail then ever before.

The worldwide approach to reduce the spectrum for broadcast and grant licenses in the UHF spectrum to new services will severely effect the operation of wireless microphones. A common approach needs to be found to allow the future use of radio microphones in areas of RF background.

Every technical approach must be taken to make the systems as smart as possible and allow coexistence with other signals. In the end, the allocation of bandwidth by government regulators is an obvious prerequisite to run a radio microphone system.

Finally, the David and Goliath of UHF are not enemies but companions, because where would a TV program be without wireless microphones? And what is a wireless microphone without its users in TV productions? The deployment of wireless microphone systems shows how spectrum can be shared with other users by careful product and system design.

Sven Boetcher is product manager for professional wireless microphones at Sennheiser Electronic, Wedemark, Germany.

Rules for wireless microphone use

  • Place the antenna as high and near the scene as possible to get a direct line of sight between transmitter and receiver.
  • Directional antennas can be used to suppress unwanted signals.
  • Avoid any shielding of the transmitter antenna. For example, never bend the antenna between the body and the bodypack transmitter.
  • Use low-loss cable between the antenna and receiver.
  • Use a booster to compensate for cable losses.
  • In a receiving system, put the booster directly behind the antenna in the RF cable.
  • Do not overboost your system. For example, if you use only a short cable with 1dB or 2dB loss, a 10dB booster may overload the input stage of the receiver.
  • Use only intermodulation-free frequencies calculated with special SIFM software.
  • Never work in an occupied TV channel. This may reduce the working range and cause problems for TV reception.