The microphone is an essential tool to an audio engineer, being able to reproduce a sound wave into an electrical current created the ability to use an audio reinforcement system. In 1953, Shure Brothers designed and marketed a “Vagabond” system which dates to be the first wireless microphone system for performers. This system claims to have a range of approximately 700 sq ft, which equates to 15 feet from the receiver. Shure Brothers is now called Shure Incorporated, which many of you use in your venue or house of worship.
Wireless microphones in a house of worship make services and productions very clean with no wires onstage. Added to the cleanliness, you also have the ability to move around stage without getting tangled wires. Some disadvantages with wireless microphones, appear when they start receiving radio frequency interference. Some things like Radio Frequency (RF) signal gain, intermodulation distortion, and receiver desensitization can wreak havoc on the audio coming from a wireless microphone.
Today, I am documenting a long experiment of mine, designing a wireless microphone RF distribution system at low cost. When a venue has multiple wireless receivers, one will deploy an antenna distribution system to use one antenna for multiple receivers. By doing this, it will reduce the clutter in an audio rack. Also, the use of a high gain directional antenna can be employed to reduce the possibility of interference from other RF sources.
As you may know, most of the wireless microphones share the same frequency spectrum as the over the air television stations. This includes VHF, Very High Frequency (30-300 MHz), and UHF, Ultra High Frequency (300-1000 MHz) as the main two. To add to this, the digital TV transition that started in June of 2009 has reduced the available frequencies in that spectrum. You can read more about this is my earlier blog posting which explains the differences between digital TV and analog TV and how they are laid out in the 6MHz allotment inside their channel.
With wireless systems, you need to first select a good frequency for your system to operate on. At my church, North Ridge Community Church, we use the Shure SLX microphones with the H5 Frequency Band (518-542 MHz). This occupies over the air television channels 21-27. One of the main areas of problem in that frequency range lay in channel 24 (530-536 MHz). Channel 24 in Phoenix, Arizona is home to KTVK-DT aka 3TV which uses a 1000kW transmitter. Because this station is so powerful I have programmed our wireless channels to be outside of this spectrum range.
Here is why, when channel 24 is pulled up on the spectrum analyzer inside our building where our receivers are located, the measured signal strength is -69 dBm. Outside our building it is about -48 dBm. The signal of a SLX handheld up onstage measured to our receivers current location is -68 dBm. If a wireless pack was to use the same frequency as the 1000kW TV station, the TV station would overpower the wireless pack. Which would make your receiver have a lot of issues passing clean audio.
The main problem churches/venues will run into is interference from situations just like the KTVK-DT transmitter. So, before going and spending money on higher gain antennas, wireless antenna distribution, coax, or new wireless systems, please check your local TV stations and how they match up with your wireless frequencies. A good place to start if you don’t have a spectrum analyzer is www.tvfool.com. You can input your address and see what television stations you will receive. Also check http://en.wikipedia.org/wiki/North_American_broadcast_television_frequencies to translate the television channel into the frequencies in MHz.
The best tool to have is a Radio Frequency Spectrum Analyzer, I purchased my church a RFExplorer handheld spectrum analyzer which views the frequencies 240-960 MHz. There are add on boards which allow you to view 15-2700 MHz which includes WiFi frequencies. Using this tool will aid in finding specifically if your church/venue has any interference issues with RF. You can purchase this tool at: http://www.seeedstudio.com/depot/rf-explorer-model-wsub1g-p-922.html?cPath=174.
Place your wireless systems on frequencies that do not currently have anything transmitting. If you find yourself with lots of interference, place your wireless systems where there is the least amount of signal. Placing the wireless system over the lowest power television station is a valid option if you have no other open spaces. The Federal Communications Commission, FCC, is who allocates the radio frequency spectrum and allows the television stations to occupy those frequencies. It is said that they allow at least one 6 MHz chunk in every city free for alternate use such as wireless microphones.
Now that you have your wireless systems on correct frequencies, we can now take a look at the receiver end. It is best to place the receivers as close to the wireless transmitter as possible. This will allow the strongest signal possible. I did a bit of testing, and with the SLX bodypack at 10 feet from the receiver it measured -30.5 dBm. When moved to 25 feet, the signal dropped to -41.5 dBm. When moved to 50 feet, the signal dropped to -52 dBm. As you can see, the closer the receiver is to the transmitter the better signal received.
Line of sight from the antenna to the receiver is also important. Walls, people, audio racks, even metal near the antenna can affect the receive signal strength. At North Ridge Community Church, NRCC, we placed our receivers in an audio rack at the front of house, FOH, mixing position. We felt that this location served better for troubleshooting reasons incase we lost audio from the pastor. The antennas for the receivers are behind a small 4 inch thick wood framed wall, and mounted in an audio rack to the stock brackets that come from Shure. By taking the antenna from free space (nothing around it) and moving it down into that rack the signal dropped from -56 dBm (open space) to -68 dBm (rack mounted). That is a 12 dB loss just by having the antennas mounted in the rack which the receivers are in. By keeping a clear line of sight from antenna to transmitter, you can keep your losses to a minimum.
Here lays my problem which I wanted to fix. I have seven wireless receivers, all with two antennas each, having 14 antennas mounted in free space is going to be an eyesore! I wanted to find a system that would allow me to use two antennas and split them off into the 7 different receivers. Shure came to the rescue with the UA844SWB which is a 4-Way Active Antenna Splitter. This in combination with two Shure UA874 Directional Log Periodic Antenna you can distribute your two antennas to 4 wireless receivers. At a total cost of around $1,200 for only 4 receivers, totaling in at $2,400, my church did not have the budget for that!
In the next few weeks, I will be explaining how I solved this issue on a budget using some creative thinking and knowledge of radio frequencies.Read the rest of the series!
Read the rest of the series!