In Chapter 1 of this series, we looked at issues that can impact wireless systems used in professional audio applications. In this latest chapter, let’s discuss antennas and reception, which can be just as important in attaining optimized performance.

Clear RF Path. Wireless systems require a clear path between the receiver and the transmitter. This is sometimes called a clear line-of-sight, but the visual analogy isn’t always accurate: light will go through a small hole but radio waves won’t.

Similar to the free space needed around an antenna, radio waves require a sizeable space in which to travel. The amount of space necessary depends upon frequency; the lower the frequency, the more space needed.

There should an imaginary tunnel of open air between the transmitter and the receiver antennas. For UHF, a diameter of 3 feet (1 m) or so is usually adequate, but for VHF it should be at least twice as large. Further, this space should be free of metallic objects such as scaffolding, iron beams, cables, cabinets, pipes and the like, which can cause interference.

If possible, large flat metal objects such as large ducts, rows of cabinets and truck bodies that are parallel to the path should also be avoided. Even though these might not be in the direct path, they will act similar to a mirror and reflect RF energy away from the direct path. With systems offering diversity reception, this probably won’t cause dropouts, but may considerably reduce range.

Long Antenna Cables. Sometimes it is necessary, and even desirable, to locate antennas some distance from the wireless receiver(s). In these cases, RF coaxial cables can be used to connect remote antennas to the receiver inputs. However, keep in mind that coaxial cables present signal loss problems that in turn reduce operating range. The amount of loss depends upon the size, construction and quality of the cable, and upon the system’s operating frequency.

The amount of loss and reduction in range is often much more than might be expected. The average loss of higher quality RG-58 cable is about 8 dB per 100 feet (30 m) at 200 MHz, and about 17 dB at 700 MHz. Since every 6 dB of loss cuts range by half, the working range with 100 ft of this cable will be only 40 percent of normal at 200 MHz and a mere 14 percent of normal at 700 MHz. With lower quality RG-58, losses can often be much higher, reducing useful range by another 50 percent or more.