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 the frequency is, 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 much to high for us to hear, but we can treat it the same way we do as audio waves.
Antennas are like microphones
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.
LPDA (Log Periodic Dipole Antenna)
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.
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.
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.
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:
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.
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