Originally posted on Professional Wireless Systems
All radio frequency (RF) systems have a transmitter (Tx) and receiver (Rx). Transmitters and receivers can take many forms including handheld, beltpack, rack mount or desktop. Regardless of how they look, all audio RF systems take in an audio signal, attach it to an RF carrier wave at the transmitter, transmit the signal to the receiver where the RF carrier wave is detached from the audio allowing the audio to be sent to the audio output and into the audio system.
With RF microphones, the microphone (or beltpack the microphone connects to) is the transmitter and the receiver is, typically, a stationary unit with an audio output. With in-ear monitors (IEM), the transmitter is, typically, a stationary unit with an audio input and the beltpack the headphones connect to is the receiver. The notable exceptions to this rule are RF systems used in film and broadcast where, to save space and weight both the Rx and Tx units may be the size of a beltpack.
All RF systems require an antenna. Depending on the size of the device these may be obvious, or hidden. On hand-held microphones the antenna is normally integrated into the handle; on beltpacks, whether a microphone Tx or IEM Rx, the antenna is normally a small piece of wire protruding from the device.
On the normally stationary parts of the system, the antennas connect to the back or the front with BNC connectors. BNC connectors are keyed so that the ring uses the pins to locate the connection securely. BNC connectors require a 90 degree turn to lock them in place.
What Do All These Bits Do?
How do RF signals get from A to B? Let’s discuss some of the system components to better understand them.
Transmitters (Tx) – Combine the RF and audio signals and transmit the audio signal to a receiver.
Receivers (Rx) – Receive the combined RF and audio signal and remove the audio routing it to an audio output which, depending on the system, could be balanced (XLR or ¼” TRS jack), unbalanced (¼” TS jack) at line level or mic level & headphones in mono or stereo.
Antennas – Broadcast the RF signal. There are 3 different types normally used depending on the coverage and gain required. From lowest gain and widest coverage these are…
- Whip (99% of all RF systems ship with these)
- LPDA (aka Fin, Yagi)
Cable – Not all BNC cable is suitable for RF use. All RF cable to be used with an RF system should be a 50 Ohm cable. 50 Ohms is the resistance of the cable. 50 Ohms is the standard for all microphone, IEM and communications RF systems. All 50 Ohm BNC cable loses RF gain, more gain is lost at higher frequencies and at longer distances. Loss of RF gain due to cable length may be the difference between stable, useable RF and drop outs.
RF signal gain and loss – RF signal gain is raised or lowered by passive components on the system. For example…
- Whip Antenna – 0db
- LPDA – Around +6db on axis
- Helical – Between +10-12db on axis
- Passive filters – Between -1 and -6db depending on manufacturer
- Passive Splitter/Combiner – Between -1 and -7db depending on number of splits and manufacturer.
- Cable – Dependent on length and frequency. Even 25ft can lose 1-4db of gain (depending on cable used) which may be the difference between stable, useable RF and drop outs.
Every RF system should calculate the gain and loss of the RF signals across the whole system and compensate accordingly. For longer runs (over 100ft) RF signal can be converted to be sent down a fiber-optic cable eliminating signal loss due to distance.
Amplifiers – An RF amplifier raises the gain of RF signal to compensate for loss due to distance or using passive splitter/combiners.
Filters – These devices filter out unwanted RF signal, lowering the overall RF noise floor making it easier for a local RF system to find open frequencies to use.
Distro’s – Sometimes called splitters, these take the RF signal received by an antenna and split it into a number of different outputs allowing multiple Rx units to use the signal from a single set of antennas. Distro’s are only ever used for microphone systems.
Combiners – An RF combiner takes multiple outputs from a number of Tx units (IEM for example) and combines them together, allowing multiple Tx units to share a single antenna. Combiners are only ever used for IEM systems.
Passive splitter/combiner – These devices allow a number of RF signals to be split or combined passively. There is a loss in RF gain when used (see RF signal gain and loss).
Basic Mic System
As described above, all RF systems are composed of a transmitter and receiver. We will now look at how multiple wireless mics can work together in one RF system.
When using four or more wireless mic systems together, it is better to use one set of antennas, mounted in a position advantageous for reception, rather than trying to cram all the antennas supplied with the microphone systems in one place. When trying to place four or more sets of whip antennas in a rack or on a table top, it becomes difficult to make certain that each set is oriented correctly to ensure maximum reception.
A single set of antennas can be distributed to multiple receivers using an antenna distro. Using one set of antennas ensures that they can be oriented for maximum reception and each receiver is receiving the same signal strength, as if it was connected directly to the antenna. This system is scalable depending on the number of receivers and the outputs on the distributor. If another distro is needed, just use one set of outputs of the distro connected to the antennas to send signal to the input of the next distro.
Basic IEM System
We will now look at how multiple wireless IEM channels can work together in one RF system.
Just like wireless mic systems, when using four or more wireless IEM systems together, it is better to use one set of antennas, mounted in a position advantageous for transmission, rather than trying to cram all the antennas supplied with the IEM systems in one place.
The output of multiple IEM transmitters can be combined to send signal to a single set of antennas. Using one set of antennas ensures that they can be oriented for maximum transmission signal and each receiver is receiving the same signal strength, as if it was the only system being used. Best practice is that one combiner, with 4 or 8 inputs sends signal to one antenna. A passive combiner can be used to combine the antenna output of multiple combiners, but using a passive combiner introduces an RF signal loss which may negatively affect the stability and usability of the transmitted signal.
These are some of the basic components of RF systems. Next time, we’ll discuss more about positioning of antennas.