All About Wireless: Transmission Lines Part II
Welcome to the seventh installment of All About Wireless. In this issue, we will continue our focus on transmission lines, examining the importance of impedance matching, the effect of standing waves, and the implications of transmitting RF over fiber optic cables.
Getting the Best Performance from IEMs
To achieve the best possible performance from an IEM system, it is important to ensure that the signal power output from a transmitter or active combiner is transferred to the transmit antenna with minimal loss. It is equally important to ensure that the low power signals detected by receive antennas in wireless microphone systems are transferred to the receiver or distribution amplifier without suffering further attenuation. Maximum power transfer occurs when the load impedance is equal to the source impedance.
If the source and load impedances do not match, for example if a 75-ohm TV antenna were used in a wireless microphone or IEM system, a portion of the signal relative to the magnitude of the impedance difference will be reflected at the point of impedance change. These reflections mix with the incident signal to produce fixed voltage and current waveforms in the transmission line, diminishing the power transferred. In practice, this typically manifests as a reduction in system operating range.
From the point of impedance change, the reflected voltage waveform will propagate out of phase with the incident voltage waveform. Voltage nodes occur at the 0 and 180-degree points of the incident carrier waveform. Voltage antinodes occur at the 90 and 270-degree points. In the case of a short circuit, where 100 percent of the incident signal will be reflected, the increased voltage present at the antinodes may permanently damage the dielectric and destroy the cable.
In wireless microphone and IEM systems, we generally do not need to consider the impedance specification of the hardware used as equipment is manufactured to the 50-ohm standard. However, we do need to carefully consider the specification of the coaxial cables used to connect system components. Coaxial cables themselves present a load to the source, so they too must be matched in impedance to avoid the generation of standing waves. Therefore, it is a best practice to use 50-ohm coaxial cables in wireless microphone and IEM systems.
Standing waves can also be caused by damaged cables, even if the specified cable impedance matches that of the connected system components. The dielectric material may be compressed if the cable is bent in too tight a radius, caught in a road case lid, or driven over by a forklift, for example. The outer sheath of the cable may recover from the damage, making the compression of the underlying dielectric difficult to spot. The compressed dielectric will alter the impedance of the cable, causing a portion of the signal to be reflected at the point of damage.
Poor system range is often attributed to faulty transmitters or receivers, but in most cases the hardware is not to blame. Standing waves in damaged 50-ohm cables are an overlooked but common cause of reduced operating range in wireless microphone and IEM systems.
Despite the recommendation to use only 50-ohm coaxial cables in wireless microphone and IEM systems, 75-ohm cables can perform adequately in certain situations. In practice, the length of a coaxial cable influences its electrical characteristics.
When a source is connected to a load via a short coaxial cable, the specified cable impedance is of little consequence as the load impedance dominates the circuit. The cable itself is, effectively, electrically transparent. As a rule-of-thumb, any cable less than a quarter wavelength in length may be considered short.
If using a 75-ohm coaxial cable to transmit a 600MHz signal, the cable would need to be less than 12cm long to remain electrically transparent in a 50-ohm circuit. Therefore, it is possible to use short 75-ohm patch cables to cascade RF between receivers without severely degrading system performance. Whilst it is not recommended, the electrical transparency of short coaxial cables allows the engineer to make an informed choice regarding the use of 75-ohm cables in wireless microphone and IEM systems. Long 75-ohm coaxial cables are often used accidentally, and do work to a degree, but the standing waves generated within the cable degrade system performance somewhat.
Fiber Optic Cable
The use of fiber optic instead of coaxial cable can be advantageous in applications where very long transmission lines are required. The lower cost of fiber optic cable and the avoidance of excessive signal loss make fiber optic an attractive alternative. There are, however, a few potential disadvantages to be aware of. Coax to fiber converters are required at both ends of the transmission line. These converters are active devices, so power is required for their operation.
Fiber optic cable does not carry DC, so mains power is required at each converter location. This may be inconvenient, or impossible, to provide. The inability of fiber optic to carry DC also means that active antennas cannot be powered by a receiver or distribution amplifier, necessitating the use of a Bias-Tee to inject DC at the active antenna location. Depending on the fiber optic system used, there may also be some considerations regarding RF power handling and noise ingress via the converter connection points. Field termination of fiber optic cables is also more challenging than coaxial cable termination.
This concludes our examination of transmission lines. Next month, we will begin looking at sources of noise and the effect of noise on system performance. We will also identify a few key techniques for optimizing the signal-to-noise ratio in challenging environments.
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