Maximizing Wireless Microphone Performance through RF Signal Path Analysis

When designing antenna systems for wireless microphones, it is important to understand how each component in the signal path contributes to loss or adds gain to the RF signal before it reaches the receiver. To really optimize this signal and get the best performance from our wireless systems, it is critical to understand the impacts of every component in the signal chain. Let’s examine the following and learn how to create a loss budget:

Frequency – How does the operating frequency impact the system’s behavior?
Receiving Antennas – Pattern, polarization, gain
RF Cable – Impedance and loss
Splitters and Combiners – Passive vs. Active

RF Budget

When planning your antenna distribution system, it is a good rule of thumb to plan for unity gain or 0 dBm of gain/loss at the input of the receiver. Each component can create loss or add gain to the signal chain. While 0 dBm of gain/loss in your system is ideal, knowing the tolerance of your RF receiver will give you the range you need to stay in for a stable system. For Sennheiser receivers, plan to keep within a range of +4 dBm and -6 dBm. When designing a multi-zone architecture in which multiple pairs of antennas combine to the same receiver, it is important to calculate the run from each antenna to the receiver to ensure that it is with/within the range for gain/loss.

Frequency

The operating frequency of a system has impacts on both the range of the transmitted signal, as well as how much signal is lost over the antenna system. Note in the first chart (below), it takes more RF power from a transmitter at a higher frequency to cover the same distance as a transmitter at a lower frequency. If you have a scenario in which a transmitter is going to operate at a longer distance from the receiving antenna, it may be beneficial to use a transmitter with a lower operating frequency.

In the second chart, we see various 50 Ohm cables and the RF Attenuation per 100 feet at both 400 MHz and 700 MHz. Note that as the frequency gets higher, the RF signal strength attenuates at a much higher rate. The operating frequency has an impact in your cable loss calculation. Use a cable calculator like the following to get a more accurate representation of loss at your operating frequency: www.qsl.net/co8tw/Coax_Calculator.htm

Finally, it is important to review the specifications of all devices in the signal chain to verify that they are designed for use with the operating frequency of your RF system.

Receiving Antennas

There are many remote antenna designs to consider when designing an antenna system. A good practice is to use antennas that have specifications listed for pattern, polarization, and gain (active, passive). The signal at the receiving antenna is the best place in the signal path to add gain, as this point in the signal chain has the highest carrier to noise ratio.

Pattern – Different antenna designs create different polar patterns. Directional patterns can help to define an area of pickup and also reject RF interference from other sources. Directional antenna designs often have the additional benefit of generating some passive gain which can be used to overcome loss elsewhere in the system. Omni-directional antenna designs tend to not generate extra passive gain, but they do benefit more from the reflected signals from transmitters. This makes them ideal for operating in less defined coverage areas.

Polarization – Linear polarized receiving antennas and transmitting antennas in free space, or applications with little reflection, can lose signal when the antennas are in different polarizations (i.e., transmitter (horizontal), receive antenna (vertical). When signals are reflected off of structure, the polarization is altered which helps to minimize losses due to different antenna orientations. Antennas with a circularly polarized design eliminate any impact from changes in transmitter orientation.

RF Cable

RF cabling is a common and predictable source of loss in an RF system. One way to make a system’s performance less predictable is to use 75 Ohm cable, which is designed for video signals. A 75 Ohm cable creates additional loss with reflections and impedance mismatches which can cause your system to operate poorly. Only use 50 Ohm coaxial cable which is designed for RF wireless systems.

Cable manufacturers will list the loss at a specific frequency to help you choose the cable that best fits the application and budget of your project. Lower loss cables will have a higher cost per foot and generally be less pliable for installation. You can consider the loss/gain tolerance of your receiver and the length of the cable run to determine the best cable for your system with manufacturer data and calculators listed below.

Cable Calculator:
www.qsl.net/co8tw/Coax_Calculator.htm

Splitters and Combiners

When sending the signal of an antenna to multiple receivers or combining multiple antenna zones, it is important to check the loss/gain specification of all of the distribution equipment in the signal path. Passive devices create loss that needs to be accounted for. Active devices use internal RF amplifiers to balance out the loss of the splitters, or in some cases create additional gain.

RF Amplifiers

RF Amplifiers can be inserted into the signal path to overcome loss. Some amplifiers have adjustable gain which allows you to fine tune your system and not overdrive the front end of your receiver. Other amplifiers have a set value for gain. RF amplifiers also come in both narrow band and wide band designs. It is important to note that the operating frequency is within the bandwidth of the RF Amplifier, or else the transmitter signal will not be amplified and will possibly be attenuated.

Sennheiser RF Amplifiers

When it comes to anything created by Sennheiser, always know that you can reach out to us for design review. Also, if you would like to dive deeper into this topic, you can schedule our Multi-Zone Antenna Systems Design course for a group at your office. The course is 90 minutes and has been approved for 1.5 AVIXA CTS Renewal Units.