Wireless Microphone Antenna Distribution on a Budget – Continued

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  1. Wireless Microphone Dead Zones?!
  2. DIY Wireless Microphone Antenna Distribution – Antennas
  3. DIY Wireless Microphone Antenna Distribution – Introduction
  4. DIY Wireless Microphone Antenna Distribution – Part 2

In continuation to my previous post, I will start discussing how I solved the wireless distribution at my church in an economical way.

One of my many hobbies include amateur radio, which is the personal & experimental use of specific radio frequencies established by the FCC.  My callsign is KD7QCU.  Basically, I am a big nerd of radio stuff.  I get to talk to people all around the world with my radio and antennas outside of my house.  It is quite a fun hobby and our technological age with regards to RF has a lot to thank the amateur community for experimenting with radio.

This is me at the top of my ham radio tower that I purchased.

This is me at the top of my ham radio tower that I purchased.

I figured in my mind that because TV and the wireless microphones share the same frequency spectrum, some of the tv splitters might work the same with wireless microphones as they do with TV.  Thus we find ourselves looking at a two-way coaxial splitter.  This is something that you would connect your TV antenna to on one side.  Then the outputs would go off to your different TV’s allowing multiple TV’s to get the same signal.

The coaxial splitter uses transformers or different combinations of resistors to split a signal to its separate outputs.  Being that TV’s don’t transmit themselves, only receive signal, you can hook as many TV’s up as you want without causing harm to a television.  Wireless microphone receivers do not transmit either, they only receive.  So we can treat the wireless microphone receivers the same as TV’s.

I conducted a series of tests using the RFExplorer handheld RF spectrun analyzer.  This analyzer has a 50 ohm SMA antenna port on it, so to connect the antennas up to it, I needed some adaptors.  There was a SMA to BNC adaptor placed on the unit.  Then from there I used a length of LMR 200 coax to connect into the different devices in test.  A transmitter was placed onstage with a sine wave being played into it.  This assured even signal transmission levels as the level did not deviate in amplitude.

Screen Shot of the RFExplorer Spectrum Analyzer

First, I need to talk about an important unit, the dBm.  This is the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW).  dBm is a common unit for displaying signal strength.  -73 dBm is an “S9” signal strength on an S-Meter on a common ham radio.  Most 801.11 wireless network receive strength is between -70 dBm and -90 dBm with a maximum signal of -10 dBm to -30 dBm.

My first test was to test the signal strenght of a SLX body pack at different distances to see the dBm level.  I plugged in a Shure 1/4 wave vertical antenna into the RFExplorer for the test.  This is the stock antenna that Shure ships with their wireless systems.  I then measured the following levels at 3 different distances.

  • 10 feet = -30.5 dBm
  • 25 feet = -41.5 dBm
  • 50 feet = -52.0 dBm

The purpose of this initial test is to see how much signal should be going into the receiver at a max setting.  I would presume Shure would make their receivers work with the transmitter right next to it, without overloading the receiver.  Taking this presumption, I can now say that -30.5 dBm would be a good maximum receive strength to have at the receiver without causing any overload of the receiver.  So in any of my testing I am wanting to make sure the receiver does not receive any more signal than -30.5 dBm.

My next test would involve connecting the spectrum analyzer up to the stock antenna and Shure UA221 splitters which we use at North Ridge Community Church.  The purpose of this test is to see the effective signal that the SLX receivers are getting from a microphone on stage.  The arrangement of the setup you can see in this photograph below, it is part of the Shure dual side by side mounting kit, which uses two UA221’s to split two antennas off to the two receivers.  Note where the antenna is located, next to metal.  The signal strength read -68 dBm.

This is how the dual mount SLX receivers look on the front. Note where the antennas are mounted.

This is how the dual mount SLX receivers look on the front. Note where the antennas are mounted.

After this test I decided to test the signal strength in the same configuration, but moving the 1/4 wave vertical antenna into open space, meaning nothing around the antenna.  The measured signal was now -56 dBm, a +12 dB boost.  The reason for this is that the metal from the rack and mounting hardware of the SLX receivers was affecting the antennas ability to receive.  Much like a tuning fork, antennas will resonate at certain frequencies, when metal gets near a tuned antenna, it will start to change the frequency that the antenna resonates at or change the impedance, both of which will affect signal quality, normally in a degrading way.

I was able to source a ham radio operator who manufactures antennas out of printed circuit boards.  His callsign is WA5VJB.  One of the antennas that caught my eye is a 400-1000MHz Log Periodic antenna.  This type of antenna is directional, high gain and has a wide bandwidth.  Also the 400-1000MHz has our wireless frequencies falling in that range.  Shure sells a similar and more polished version of this antenna, they paint theirs black, add a mount to it and sell it for $200-$300.  It just so happens that the WA5VJB log periodic can be grabbed for $25 off of his eBay store.  The antenna comes with a spot for an SMA connector to be soldered onto the antenna.  To complete the system, I needed to get an SMA to BNC adaptor cable which was sourced for $10-$15 off of eBay.

Log Periodic Antenna

Log Periodic Antenna

My next project was testing the Log Periodic antenna with the spectrum analyzer.  I setup my test again with the sine wave into the transmitter and placed that onstage.  The measured signal level came in at -43 dBm.  This is a +25dB gain over the stock 1/4 wave vertical mounted in the rack.  This antenna was attached to a clamp using a spring clamp, better results would be yielded if I removed the metal clamp and used a non-metalic mounting system.

Now that we have an antenna that yeilds better results than the stock 1/4 wave vertical, I want to be able to split this signal out to the different receivers.  When you split a signal from one to two you will have a 3dB or more drop in signal.  When you start splitting the signal to more devices you will have more dB loss.  This is one reason I wanted to have a high gain antenna like the log periodic, to be able to have enough original signal to split to the 7 receivers.

There are two main types of Coaxial Splitters, active and passive.  A passive splitter will use transformers and/or resistors to split the signal.  An active will split the signal using powered circuitry and most of the time add a small signal amplifier to boost the signal.  My first series of tests were to see if a passive two way splitter would produce the same results as the Shure UA221 splitter.

SVI SV-2G Two-Way Coaxial Splitter

I sourced a SVI SV-2G two way passive splitter with a bandwidth of 5-1000MHz.  One issue is that the TV Coaxial splitters use F-Connectors, and most of the wireless products in the audio industry uses BNC.  I decided to use an adaptor from F-Connector to BNC for the tests.

I attached the 1/4 wave stock antenna from Shure to the antenna port of the splitter.  I then tested both outputs of the splitter and measured an expected 3.5 dB loss (the specs called for a 3.5 dB loss).  The interesting thing is that the Shure UA221 measured a 4.0 dB loss.  By these measurements the SVI coaxial splitter outperformed the UA221.  I also listened to different audio coming through the wireless system to make sure that no audio degradation was present.  No distortion or degradation was present on the signal.

SVI SV-3BG Three-Way Coaxial Splitter

Next, was to test a three way splitter.  The SVI SV-3BG, same bandwidth of 5-1000MHz and an expected loss of 6.0 dB.  After connecting and testing the three way splitter, I found the same expected loss as detailed on the unit.

PCT-MA2-4P Four-Way Active Coaxial Splitter

After testing both the two way and three way passive splitters, I wanted to test an active splitter.  I found a 4 way PCT-MA2-4P active coaxial splitter, this requires a DC power source at 12-15v at 300mA.  The spec’ed gain is +8dB.  I connected and found the expected gain of +8dB.  I also tested audio quality and found no change compared to the UA221.

I wanted to go into further testing of the amplified version compared to the UA221.  I was worried at first of the amplifier inside of the splitter adding in noise.  I attached the log periodic antenna to both the Shure UA221 and the PCT-MA2-4P and tested signal strength, noise floor level, and then calculated the signal to noise dB difference.  Here is what I found:

Log Periodic into UA221

  • Signal Strength: -46 dBm
  • Noise Floor: -94 dBm
  • Signal To Noise: 48 dB
  • Gain Compared to 1/4 Wave rack mounted Antenna: +22 dB

Log Periodic into PCT-MA2-4P

  • Signal Strength: -36 dBm
  • Noise Floor: -88 dBm
  • Signal To Noise: 54 dB
  • Gain Compared to 1/4 Wave rack mounted Antenna: +34 dB

Not only did the PCT-MA2-4P allow the connection to twice the amount of devices, but also outperformed in the signal strength and signal to noise measurement.  Comparing the signal strength of the amplified PCT-MA2-4P (-36 dBm) to the SLX transmitter at 10 feet (-30.5 dBm) has this still in a safe range for not overloading the receiver.

At this point, I knew this was promising!  The use of inexpensive television coaxial splitters was a formidable option for wireless system antenna splitting.  I am going to lead this post to an end.  In the next post, in the following weeks, I will show you the application of using an 8 way active coaxial splitter in a wireless microphone system.  Please if you have any questions, feel free to leave me a post below.

P.S. Some of you may argue that the TV Coaxial splitters are 75 ohm and that the conversion of a 50 ohm antenna to the 75 ohm splitter back to the receiver looking for a 50 ohm load will apply more loss than I measured here.  While yes you do have a bit of loss from the conversion of 75ohm to 50ohm (less than a dB of loss), the spectrum analyzer, just like the wireless receiver has a 50 ohm connector on it.  So any losses created by the 75-50 ohm conversion would be already calculated in my tests.

Wireless Microphone Antenna Distribution on a Budget – Introduction

WE HAVE MOVED! Please navigate to the current article links below to see the latest from dBB Audio!

  1. Wireless Microphone Dead Zones?!
  2. DIY Wireless Microphone Antenna Distribution – Antennas
  3. DIY Wireless Microphone Antenna Distribution – Introduction
  4. DIY Wireless Microphone Antenna Distribution – Part 2

The microphone is an essential tool to an audio engineeer, being able to reproduce a sound wave into an electrical current created the ability to use a audio reinforcement system.  In 1953, Shure Brothers designed and marketed a “Vagabond” system which dates to be the 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 a 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 Commision, 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.

Power Supply Woes

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This last week I got the call that our soundboard in our choir practice room took a fiery and smokey trip to heaven and is not working! Turns out that the ac power fuse blew which was then replaced. Once powered on again, sparks, smoke and flames incued.

The board is a Soundcraft Spirit M12 which is a 16 channel board. The power supply was pretty easy to get to after removing 8 screws and disconnecting two wiring harnesses.

I shot the following photos of the power supply which has some pretty good damage. R24 burned a hole through the PCB board. The R27, R35, R36, R45, R47 resistors blackened the board as well as cracked from the heat. L2 boiled the casing and windings. Lastly the filter capacitor 100uF 400V has a hole in the side of it.










I called the Soundcraft USA parts department and they had a power supply module on hand for $137. So that is being shipped to the church and will be replaced and back in service about 1-2 weeks after the incident. It will be interesting to see what revisions they have made on the power supply board in resent years.

Not too shabby of a repair. Pretty simple really. The benefit of this mishap is that I will probably mount this mixer in a portable rack. Along with adding a few amps, wireless mic and a few eq’s will make a portable system for the church to be able to use whenever they need it. Which is something that the facility currently does not have.

Monitors – Board Unity Gain vs Amplifier Output Gain

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Today’s post is regarding monitors.  At North Ridge Community Church, NRCC, we run our three separate monitor mixes from the front of house.  These monitor mixes are fed via the pre-fader aux sends, go through a graphic equalizer, amplified by a Crest Audio amplifier and then show up to our JBL floor wedges.

This is a very normal way to do monitor mixes and all is great, until… I discover how it is truly set up.

  • The amplifiers in the amp room are all turned up to full.
  • The output of the board aux sends are not full signal.  In fact they don’t have enough signal to light up the -21dB light on my equalizer.
  • 100-105dB volume normally output by monitors.
  • AC hum present when system is turned on.

Because the are turned up so loud, the aux send knobs are in the 9 o’clock to 11 o’clock range.  While this doesn’t sound that bad, on the Allen & Heath ML4000 the unity gain (+0dB) for aux sends is at 3 o’clock and the knobs are a logarithmic potentiometer.  They act just as a normal fader does on the sound board, where there is more resolution in the higher areas of the fader.  Basically if you move the aux send knob from 9 o’clock to 10 o’clock you would increase the volume by 10dB.  If you were to move the knob from the 2 o’clock to the 3 o’clock position you would increase the volume by 1-2dB.

Found at http://www.bigmuffpage.com is a perfect graphic for explaining the logarithmic taper of the potentiometers that are used in aux sends.

Another thing that is happening by having such a low output from the board and the amps turned up high is easily explained in the graphic below that I made:

By having the board turned down and the amplifer turned up to full, you have made your signal to noise ratio very small. But when changed to a full board output and to a lower gain on the output of the amplifier you will have a high signal to noise ratio giving you less noise in the monitor system and better clarity.

As shown in the above graphic, by having the board aux output turned down and the amplifer turned up to full, you have made your signal to noise ratio very small. By keeping it this way you amplify the noise along with the small signal sent from the board.  This makes for a noisy system. But when changed to a full board output and a lower gain on the output of the amplifier you will have a high signal to noise ratio giving you less noise in the monitor system and better clarity.

By keeping your board output in a unity gain region you have higher resolution on the aux knob giving you an easier time mixing the monitors.  Also this leads to when you do an AFL (after fader listen solo) you don’t need to turn your headphones up to hear it.

After turning the amplifiers down and turning the board up I was able to keep the same overall volume with the monitors but now there is no audible noise in the room from the system being on.  If you find yourself having a hard time getting a consistent monitor mix or a noisy system, you might want to double check your amplifiers output gains to make sure they are receiving a unity gain signal.

Dead Zones?!

WE HAVE MOVED! Please navigate to the current article links below to see the latest from dBB Audio!

  1. Wireless Microphone Dead Zones?!
  2. DIY Wireless Microphone Antenna Distribution – Antennas
  3. DIY Wireless Microphone Antenna Distribution – Introduction
  4. DIY Wireless Microphone Antenna Distribution – Part 2


All of us remember the advertising of Verizon Wireless where the technician is walking around to different areas saying “Can You Hear Me Now?”  Well, they never let us hear the other side of the conversation…  Over the next few weeks, I am going to be dedicating a few posts to wireless microphones and how they work.  Specifically, the Radio Frequency part of the wireless microphone system.

A few months ago, I was listening to the pastor speaking at my church and I kept hearing a small spot of sound where he wasn’t coming through the PA.  Our church uses a Shure wireless microphone system on the pastor.  Whenever he would walk around, it seemed that there was a dead spot in the antenna coverage in one small area.  During those brief moments, no sound from the pastor was coming through the speakers.

Being a ham radio operator, KD7QCU, I am intrigued in the idea of trying to devise a cost effective way of solving the reception issues.  In my next blog posts I will be talking about my ideas for solving these issues in a cost effective way.  So stay “tuned”, pun intended!

Polarity issues in CD recording from bad wiring

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Balanced audio signals are a great thing for the audio industry. If you don’t know much about balanced vs. unbalanced signals, I highly suggest doing some reading on Wikipedia.

Balanced signals have a differential amplifier on the output and the input side. They basically carry a mono signal down 3 conductors. Two wires carry the signal and then the last wire carries the ground. The beauty of the differential amplifier is that two wires that carry the signal. One wire carries the normal signal, then the other inverts the polarity. So, the two signals are out of polarity of each other. At the receiving end of the balanced signal, there is another differential amplifier which inverts one of the wires and sums them together to get the signal back to normal.

If there is any noise injected into the wire on its run, it is injected into both wires in polarity. When the signal reaches the receiving end of the wire, that differential amplifier inverts the polarity of one of the wires which cancels out the noise. This is because the noise is now out of polarity with each other and, in the summing process, cancels out itself.

Illustration from http://www.ians-net.co.uk/images/articles/balanced/balanced.gif showing how noise is rejected in a balanced signal.

At North Ridge Community Church, NRCC, we have a Tascam CD recorder which we use to record the spoken word of our services. I noticed one day that I had to turn up the CD really loud in my car to be able to hear it. This surprised me because earlier in the day I was almost clipping the meters with the CD recording levels. I decided to rip the CD into Cubase 5 to see what was going on. Here is what I found:

Out of Polarity signals which were recorded to the CD like this.

As you can see, the left and right channels are recorded out of polarity to each other. Basically, they are cancelling each other out from the inversion.

After seeing this on the screen, I went back to the church to find that the CD recording feed is coming from a balanced mono send off of a matrix on the Allen & Heath ML4000. The cable then goes into a 1/4 inch stereo to R and L RCA adapter. The way this adapter works is for a headphone cable which takes the left from the tip and sleeve and the right from the ring and sleeve. With a balanced signal, the tip and ring are inverted in polarity. So this is sending the + into the left while sending the – into the right.

I recorded a video of myself explaining what is going on when you have a signal that is out of polarity. In the video, I accidentally mention that this could be called 180 degs out of phase which is incorrect. The only term to call this is out of polarity.

After making the correct type of adapter with the balanced to RCA, the recordings will now have full volume on the CD recording with a full spectrum sound. This is another reason not to trust any installed wires without checking them.

Drew Brashler