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.

Wireless Microphone Receive Antennas

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

 

As we know wireless microphone systems use radio frequency, RF, waves to link a wireless mic and the receiver together. We can think of RF as the same thing as audio waves just in much higher of a frequency. To give you an example of how high of frequency, the human hearing spectrum is from 20Hz to 20,000Hz. The Shure wireless frequency band of “H5” starts at 518MHz which is 518,000,000Hz. Obviously this is way to high for us to hear, but we can treat it the same way we do as audio waves.

Antennas for RF can act much like microphones, we need to select the right microphone for the source. Well we have our omni-directional antennas such as a 1/4 or 1/2 wave dipole (which most wireless systems use), this acts almost the same way a omni-directional microphone does receiving signal from all angles.

Then we have our shotgun or hyper-cardioid antennas, which we call a directional antenna. These antennas include the two most popular Yagi-Uda (which we simply call a Yagi antenna) and a Log-Periodic. These have multiple elements (or wires) and are directional meaning they are more sensitive in one direction than the other.

One of the more popular directional antennas in the wireless microphone industry is the Log Periodic antenna. It is a multi element directional antenna with a wide bandwidth which means it has good sensitivity to a wide frequency range. Shure’s version is pictured below.

This is the Shure PA805SWB Directional Antenna.

Before get too involved in the technical lingo of antennas, lets bring this back to microphone talk.  What is the benefit of a directional microphone such as a cardioid or hyper-cardioid microphone? It is to record the source only and reject all of the background noise.  A directional antenna does the same thing, just with RF.

Time to get into the dirt of all of this stuff.  I am going to be talking about the frequency range of 518MHz-542MHz which happens to be Shure’s H5 band for their wireless microphones.  Anything I talk about here applies to any frequency for RF be it higher in frequency like 900MHz or lower in the 100MHz area.

The Federal Communications Commission, FCC, has quite a fun job of keeping all of the RF frequencies coordinated.  If they didn’t we wouldn’t have our Wi-Fi or Cellular devices working like they do.  Taking a look at the 518MHz-542MHz range we find ourselves in the middle of the TV spectrum.  The FCC decided to put our wireless microphones in the same spectrum as our TV stations.  The Shure H5 band fits into TV channels 21-27.

This is the FCC allotment for the frequencies for the television channels 21-27 which lay in the same frequency band as Shure’s “H5 Band.”

These are the different channels that Shure has in the H5 Band which ranges from 518MHz – 542MHz.

While this doesn’t seem like much of an issue at first, I will now explain why this can be such a large problem.

Television just made a large transition from analog to digital.  If you are like me, I enjoy the high definition DTV stations.  Each TV station gets 6MHz of space to themselves for use.  So channel 22 has 518MHz to 524MHz.  Before the switch to digital us audio engineers had an easier time finding spots for our wireless microphones because there were empty spaces in each TV channels spectrum.

A spectrum analysis of an analog and digital television transmission. Source: http://www.comsearch.com/newsletter/images/analog_tv_ch_display.gif.

In Figure 1 of the above illustration, we see 3 spikes on the display.  Going from left to right these are the Luminance Carrier, Chrominance Carrier and the FM Carrier.  Then we can see our digital TV spectrum in Figure 2.  We can see that the digital transmission takes up all 6MHz of the channel.

Before a sound engineer could place a wireless microphone in between the Luminance and the Chrominance carriers in frequency and have no issues with the TV station causing interference with the microphone.  As we can see with digital there is no spaces in the channel.

Lets take a look at channel 24, which in the Phoenix, Arizona area is KTVK-DT aka 3TV.  Channel 24 ranges from 530MHz to 536MHz.  Here is a photograph of the transmission spectrum from a different station, KTVP-LP (channel 22), from my RF Spectrum analyzer:

This is a photo of the screen of my RF spectrum analyzer showing channel 22, KTVP-LP.  KTVP-LP is a 42 kW station and even at this “lower power” can make a large impact on a wireless microphone. The digital transmission of television now takes up the entire bandwidth of this channel ranging from 518-524MHz.

As you can see the transmission takes up all of the frequency range there. Now you may be thinking, “Well that’s fine, I will just put my wireless microphone on top of that frequency, after all my wireless mic is closer to my receiver than that TV station.”  While this will sometimes work, the power output from the KTVK-DT station is 1000 kW, which is also known as 1,000,000 watts.  Now we can look at our Shure SLX wireless system.  It has a 30mW output, or 0.03 watts of power.  This is a VERY large difference in a audio reference, this is like listening to a flute playing a ballad next to a fighter jet taking off at an airport.

So how can we help our wireless transmitter win in the fight of David and Goliath?

1.) Use a higher output wireless transmitter (however the FCC limits this at 50mW and then you have to worry about battery life)

2.) Move the receiving antenna closer to the wireless microphone

3.) Use a directional antenna that points toward our wireless microphone and away from the TV station

4.) Move our wireless microphone frequency to a different “channel” where there is no interference with a TV station

As you can see, we have a few different options and I will be going more in depth about these in future posts.  Please if you have any questions feel free to post below.