Another drawback of increasing transmitter power is the resulting increase in the level of inter-modulation (IM) products. Normally, when a larger-scale wireless system is set up, careful frequency coordination is done to minimize the impact of these IM products, and usually, a certain amount of them can be ignored because they’re so low in level.
If the level of these products increases, then so does the potential for interference and reduced range.
What causes these IM products to be created in the first place? Any time multiple RF signals (such as from wireless microphones, TV transmissions, etc.) are combined in a non-linear device (any active device such as a transmitter output stage, receiver front end, antenna combiner, etc.), these signals can interact and create new signals which, although lower in level, can act just like additional transmitters.
Unfortunately, a typical multi-channel wireless setup generates thousands and even likely millions of these IM products! So keeping a handle on the level of these unwanted signals is important.
If there’s need to coordinate a large number of channels in a fairly small geographic area, transmitter power should be carefully considered, and the antenna system should be designed to match.
This particular subject has been a matter of debate in some circles. The IM problem is usually at its worst when a lot of transmitters are physically close to each other (such as in a theater situation).
Some manufacturers, such as Lectrosonics, have classically oriented their wireless mic systems towards high power transmission and solve the problem with their product designs by using an “isolator” to prevent the mixing of signals in the transmitter.
Although this does nothing to mitigate these signals from mixing in the receivers or receiver antenna systems, generally simpler, even passive antenna systems can be used effectively, along with receivers incorporating a robust front end.
Other manufacturers have maintained that “low power is better” and have oriented their system designs around this concept, with more complex antenna systems and highly selective receivers.
Another factor is that most real-world situations involve products from a variety of manufacturers (for example, IEMs from one manufacturer, handhelds from another, and belt-packs from yet another), and it is important to understand how all of the system components (transmitters, antenna systems, splitters, receivers, etc.) will react to the different power levels of the various transmitters.
As you can see, this issue is not always a simple matter of “more is better,” nor for that matter, is it always “more is worse.” Manufacturers of wireless equipment have addressed this issue in different ways, and one of the best is to offer variable power settings on transmitters, which has been done by Shure, Sennheiser and Lectrosonics, among others, in the past few years.
Mike Wireless is the “nom de plume” of a long-time RF geek devoted to better entertainment wireless system practices the world over.