Shure Wireless Microphone Antenna Distribution
Our college's theater group has a large quantity of Shure ULX microphones, which is fantastic. What's not so great is that we currently have to individually set up every one every time we want to use them (power and antennas) and I'm looking to rack mount them.
Wondering about the feasibility of doing a DIY antenna distribution. There are directional antennas available with built in linear amplifiers, which I could then feasibly run to an 8 way antenna splitter. The problem is the impedance of the system is 50 ohms and nearly every splitter/amplifier combo available is CATV compatible at 75 ohms. Anyone tried to do this or can point me in the right direction? Anybody know any good suppliers for 8-12 way antenna splitters and maybe some good 12 dB linear amplifiers functional in the 500 mHz range?
Yes, Shure is willing to provide a solution at $500 for a pair of linear amps and 4 way splitters and a glorified power supply. That means to rack mount 10 units, I'd have to pay $1000 or more for some pixy dust.
Doesn't sound like pixie dust to me. If you can't figure out how to convert impedance than I think you are in over your head. The price of a reliable commercial solution is probably worthwhile. b.
"RF gotta go somewhere!"
Look, I'm tired and cranky this morning, so I apologize.
I'm sure what you want can be done. But it is not trivial. I don't think you will find a tinker toy solution where you just buy stuff off the shelf and plug it in. Just routing the RF at high frequencies can be difficult with errors of tenths of inches resulting in signal losses. How is your experience level with EM compatibility and shielding design?
If it was inexpensive to make, Radio Shack would sell these systems. If it was easy to build Vectronics/MFJ would have scalable kits. The market is huge for wireless mikes what with churches, theatres, musicians, clubs et al.
If you amortize the cost of the system over 8 or ten years of use, the cost must start to look more reasonable. It is unfortunate your theatre group doesn't have the means, but that is not valid reason to imply the manufacturer is ripping off their customers. Reliability has an incalculable value. Ask any technician and director agonizing over troubleshooting five minutes before curtain.
Good luck with your project. p.s. Your best value may be in searching the used market. bill.
Last edited by KB4QAA; 12-02-2012 at 07:39 PM.
"RF gotta go somewhere!"
A Possible Solution
OK, you got my interest up. After scanning the Shure literature, I believe there IS a cheaper, OTS solution for you.
From what I see, the frequency range of the mics is around 400-600Mhz, or older TV frequencies. I believe a typical household TV preamp or booster would serve your purposes. They range in price from around $50=$74. You can get cheapies at Radio Shack, Walmart etc, or buy higher quality ones made by Winegard and others.
Signal Levels: They tend to come in different amplification levels ranging from the low tens of dB to about 20dB, depending on your need. This can be compensated for depending on the type and gain of your antennas and the quality/length of your coax cable runs. For audio purposes, even these low cost alternatives can provide satisfactory performance, because the ear is much less discerning than the eye is to 'snow' and other video quality problems.
Ports: The preamps come with anywhere between one and four ports. This leaves the choice of using more than one preamp or using a passive splitter, which again are available for perhaps less than $10 in any consumer store. The quality of splitters can reportedly vary so it may need experimenting or simply purchase from more reliable vendors. Note that signal splitters necessarily cause a loss in signal strength, and are not a substitute for preamps.
Impedance Matching: As you mention, TV RF lines are typically 75 Ohm and the Shure system uses 50 ohm standards. Ideally this should match throughout a system to avoid signal loss. Second best is to have signal paths go from low impedance into high impedance, i.e. 50 ohm into 75 oms. Audio engineers are familiar with this principle. However, if signal strength is sufficient to overcome loss, and the impedance difference is relatively small, as in this case it can often be ignored. Experimentation is the only way to determine adequate performance. Further searches may find 50 ohm to 75 ohm consumer grade matchers/splitters.
Measuring Success: Empiric testing by ear is the final test. Ensuring the correct signal level can best be determined by using a Signal Analyzer or Service Monitor to test each RF frequency level at each receiver input. A scanner or wide band ham FM receiver may be an acceptable substitute.
 Keys To Success: Keep Antenna to Preamp distance as short as possible. Use shortest coax lengths possible. Buy lowest loss coax. Buy Preamp with lowest noise figure (dB). Use low loss splitters.
Here is one vendor that offers an example for home television distribution that is essentially the same as your needs.
Ok, this project could be fun! Experimenting could be done using readily available equipment; perhaps you have a preamp and splitters at home. Bill
Last edited by KB4QAA; 12-02-2012 at 09:17 PM.
"RF gotta go somewhere!"
The Shure ULX system uses UHF TV channels, A standard TV distrubution amplifier and directional taps ought to get the job done. Think of it as a distributed RF system like a whole-house TV antenna system in a large apartment building. Tap isolations can be calculated using a TV antenna signal strength meter. Blonder Tongue and others market such equipment; however, it can also be done with a spectrun analyzer.
That said, make sure that your microphones are compatible with current communications law. There are many which aren't, and while there hasn't been any notoces of apparant liability that I have heard of (yet), doesn't mean that the commission won't start to crack down on wireless mic users should complaints of interference to licensed users (TV Stations) become a problem.
73 DE KAØGKT/7
You need a rack with front and rear rails. The simplest solution is to use the individual rear rack mount panels, cable jumper and antennas that came with the recievers. Then you won't need a distro amp. You need a rack anyway. Also I recommend buying a couple of large AA recharging stations from Costco and 1700 mh AA rechargable batteries. If you do the math on what your going to spend on batteries the initial investement is well worth it.