The issue is not always a simple matter of “more is better," nor for that matter, is it always “more is worse"
December 19, 2013, by Mike Wireless
One of the topics that I’ve seen poorly understood, and even deliberately used to mislead people, is the issue of wireless microphone transmitter power and the effects said power has on system performance.
Let’s start with the basics: all things being equal, more transmitter power = more range for the system, but not in a linear way.
In broad terms, when discussing analog wireless systems, the receiver wants to see a signal from a transmitter (carrier) that is at least about 4-6 dB greater than the noise floor before it can make use of that signal - this is known as the receiver CNR or Carrier to Noise Ratio. The better the receiver front end, the lower this number can be.
And we all remember the inverse square law, right? In other words, every time you double the distance between transmitter and receiver (in free space), you cut the amount of RF power received by four times. Thus distance is a greater factor in losing your signal than the level of power from the transmitter.
Another way to look at it is that you need to quadruple the RF power in order to have the same reception at only double the distance. This is why in the real world, twice the power at the transmitter yields only about a 20 percent increase in range.
But the other problem is that things are never equal. There are many other factors affecting the potential range of the system, from the type of surfaces nearby (or lack thereof), weather conditions, how many people are around, and of course; potential interference from other sources. Antenna system design and positioning can also significantly affect range.
In other words, transmitter power is one factor, and relatively speaking, can be a minor one. This is not to say that more power doesn’t help, because it very well may. Background noise is nearly always present, and has lately been on the increase due to DTV and other sources such as digital data services, so greater transmitter power can help to “cut through” and provide a solid signal with no dropouts.
In some cases, however, it can also be advantageous to use the lowest power you can manage with your wireless transmitters. For one thing, there is less battery draw, which may be an important factor depending upon the type of application.
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.