Wireless Microphone Antenna Distribution on a Budget – Continued

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

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.

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Finding Gold in Random Places

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When coming into a new position at any location where you are the lead tech I strongly recommend looking at EVERY system in every venue of your location. This last week, I was introduced to a room that I have never been in that had an audio system installed in it. I for one didn’t know the room existed nor even had an audio system installed.

Installed in the audio rack that held the mixer was a digital CD/DVD audio recorder, the Tascam DV-RA1000 which is a high resolution audio mixdown recorder that can record 192kHz/24-Bit audio to a DVD or normal resolution to a CD.

The unit was not connected to anything and wasn’t even plugged in! This CD recorder was much better quality than the Tascam CD-RW750 which we have been using to record our sermons for years. The DV-RA1000 includes internal DSP for limiting/compression and EQ not to mention a lot of different inputs both digital and analog.

The largest benefit was the time it takes to finalize the CD. With the CD-RW750 it took about 1.5 minutes to complete a CD after the recording has been stopped. The DV-RA1000 takes less than 30 seconds. Also the time it takes the unit to stop recording and be ready to record a 2nd track is much less time.

All in all it was a great decision to switch to this unit, but even better is that I didn’t have to spend any money as we already had this. I just had to find it! So the moral of the story is that you need to look in all of your rooms/systems/venues and document every piece of gear you have.

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Stereo to Mono Conversions

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At North Ridge Community Church in our Worship Center we have a mono system.  Mono meaning that there is no left or right, there is just the one speaker.  Our room doesn’t have a good shape for everyone to get a good stereo feed, so mono it is.

Almost every CD player, computer, MP3 player or media player is in stereo.  On a sound board, to get these media devices connected, you often have to use a stereo channel or two mono channels (one for left and one for right).  With a mono audio system you combine the left and right channels inside the sound board to send to your speakers.

Most churches require a lot of media devices to play from.  At NRCC we have a computer, iPhone/MP3 player, CD Player and DVD player.  If you add that up that is 8 different connections into the board for 4 devices!  If your church has a smaller format audio console, those add up quick and take up your channels!

One way to fix this is by using a stereo to mono summing circuit.  This isolates the left and right signals and then joins them together to form one output that you can send to the sound board.

As we can see in Figure 1, we have our left and right channels.  The easiest way to think of this is, think of an RCA cable.  So you have your red and your white cables.  The pin that sticks out of the RCA is the positive, the part on the outside is the ground or negative.  This circuit takes the positives of both the left and right, puts them through a 1,000 ohm resistor, joins them together and then puts it into one single RCA jack.  This circuit works well, you may loose a bit of volume using this, but it’s nothing a gain adjustment can’t fix!

One way to improve on this is to add an audio isolation transformer to isolate the audio ground.  By isolating the audio ground you remove any chance of ground loops (humming/buzz).

As we see in Figure 2, there is a transformer added into the mix.  This transformer, in a simple way of thinking about it, copies what is on the left side and pastes it on the right side while keeping both sides electrically isolated from each other.

In the photograph above is a old broken “Live Wire Solutions Direct Box SPDI” in which I removed the transformer.  I separated the two 1/4 inch jacks and connected them together via 1k resistors, this goes to the -20dB pad switch, then goes into a 1:1 audio isolation transformer and then gets connected into the XLR connector.  Pin 3 and Pin 1 are connected together on the XLR connector.  Positive is connected to Pin 2.  This box is now converting my stereo feed into a mono feed.

By making cables or small converter boxes you can “sum” the stereo to mono before your mixer and save yourself some channels.  If you have any questions feel free to post below!

Live Mic’ing of an Acoustic Guitar

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North Ridge Community Church has added a smaller service later in the afternoon at 5PM in addition to our two earlier services at 9:00am and 10:30am. The new service is called Live @ 5 and is in our smaller venue called “Ridge Cafe.” The Ridge Cafe has a pretty good sounding system which is in stereo with QSC 3 way speakers and a single 15″ sub on each side. Our main worship center has a mono system so it is always fun to mix in a stereo rhelm.

Our guitarist was telling me that his pickup in his guitar is not picking up his high E string. I told him that was no problem that we would just pick up his acoustic with a few microphones. Which brings me to my favorite mic’ing technique for an acoustic guitar.

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I picked two small diaphram condenser microphones, the AKG CK91 which has a cardioid pattern. What I do is make an XY stereo mic configuration but orient it north south instead of left right. I then place this right in between the sound hole and the 12th fret a few inches from the strings. Then on the sound board I pan the two mics right and left. It gives a very surrounding stereo sense when listening to the acoustic.

The idea is that one mic picks up the lower strings, the other the higher strings. When you pan them it puts them in a very nice stereo image. This works really well when doing an acoustic type mix in stereo. I don’t think the stereo imaging comes across as well in a full band setup.

Lastly when you do this you need to make sure to be real careful of feedback. Each open microphone you add on stage the gain before feedback of the system drops. Basically be careful adding too much acoustic in his monitor. The XY stereo mic still behaves as a cardioid pickup so place the monitor in the same location you would normally with a normal cardioid microphone.

I WISH I had a recording of this live, this room doesn’t have a CD recorder currently. But the next time I mic a guitar like this I will make sure I get a recording of it.

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