Archive for February, 2011

What Local TV Stations Should I Care About?

Thursday, February 24th, 2011

When setting up a wireless microphone system, it can be difficult to determine what TV stations have the potential to cause interference, and even harder to know which ones WILL cause interference. The only surefire way to do it is to bring in a real spectrum analyzer, which will cost ten’s of thousands of dollars. However, it is possible to get a pretty good idea of the RF environment with a bit of effort.

To get an idea of what TV stations are in your area, go to TV Fool and use their TV Signal Locater tool. Plug in your venue’s address (or coordinates, if there isn’t a physical address). If you are inside, leave the Antenna Height box blank. If you are outside, enter the height you expect to place your Wireless Mic antennas at. The click “Find Local Channels.”

TV Fool will search its database and tell you what channels you are likely to receive, and how strong they will be. This information is based on topographic information and the FCC’s TV database, and is a pretty good approximation. Scroll down to the table that TV Fool generated.

Each row of this table represents a TV station. The ones at the top are the strongest, and the ones at the bottom are the weakest. Generally speaking, if you are indoors you probably only need to worry about the ones with a light green, yellow, or red background. The green ones are the most likely to cause interference, and the yellow ones may cause interference. The red ones might be a problem, but it’s much less likely.

Make a note of the channel number under the “Real” column of the table for all green and yellow stations, and make a separate list of the red stations. Don’t bother with the gray ones; they won’t be an issue.

Now, you need to convert the TV channel numbers to frequencies. This website has a nice table which you can refer to. Avoid setting your mics to any frequencies within the TV channels you jotted down above.

This should help you get on your way with frequencies which are not occupied by TV stations in your area!

Do Multicouplers (Antenna Distros) Prevent Intermod?

Thursday, February 17th, 2011

I recently heard someone mention on a forum that if you do not use an antenna distribution unit (multicoupler) with your wireless microphones, then you would be facing intermodulation problems. This post will hopefully dispel that belief and explain why antenna distros are useful, and what they can and can’t do,

Antenna distro units are composed of two things, typically. First, the antenna distro unit amplifies the signal with a (typically wideband) RF amplifier by about 7 dB. Second, the unit splits it with a (close to) ideal splitter. The net result is that the signal at the output ports is pretty close in power to the signal coming into the input port.

Now, here comes the problem. When you run the signal through an amplifier, the amplifier will generate intermod products. Period. The good news is that if the amplifier is well designed these products will be very weak and hence will not cause much in the way of issues. If your frequencies are coordinated, none of the third-order (most powerful) products (IM3) will fall on any of your operating frequencies anyway. But, if any one signal is hot (a relative term), then any IM3 product involving that guy will also be increased in amplitude. The amplifier also adds noise to the signal, but if you did things right that’s not an issue.

So, the bottom line is that antenna distro systems do nothing to *prevent* intermod. This is not to say they’re useless by any stretch, though. What they do for you is let you share a (good) pair of antennas while keeping the signal at roughly unity gain, and help to reduce local oscillator leakage from one receiver to another*. They also often provide power distribution, which is valuable when you have >12 systems in a rack.

On a side note, when you have a high density of transmitters on stage, and they’re running high power (>10 mW or so), the transmitters themselves can create intermod! Remember that a transmitter is also a receiver, so while the signal from your power amplifier is going out of the antenna (most of it, at least…but some will go back in too because the antenna is imperfect!), signals from all around it are also going *in* to the PA. If the transmitter does not have a way to separate those signals and get rid of them, then they will leak into the PA and mix with the generated signal, creating intermod products. These products are potentially amplified, and transmitted over the air. No amount of receiver filtering will help you at this point.

The solution to this issue is to run the lowest power necessary for a solid link. Another tip is when your mics are lined up waiting for performers, put them all on a metal table and put a coffee can over each mic. This will reduce crosstalk and hence, intermod.

*In a receiver, there are signal generators (local oscillators) that are used to bring the signal down to a frequency that can be demodulated. These signals can leak out of the receiver port and, if an antenna is directly connected, they can actually be transmitted. If a bunch of antennas are located in close proximity, then it’s quite possible that another receiver will see that signal and it’ll get added to the mix and potentially cause direct interference or more intermodulation products.

What’s Really In The TV Bands?

Wednesday, February 2nd, 2011

The TV Bands in the United States get a lot of use, despite what you might think based on the White Spaces proceedings in the past few years. Some of it may be obvious, but other users might be downright surprising. This post is an attempt at a comprehensive list of users of the TV Bands in the United States.

Before we begin, here is a list of channels and what frequencies they correspond to:
2-4: 54-72 MHz
5-6: 76-88 MHz
7-13: 174-216 MHz
14-36: 470-608 MHz
38-51: 614-698 MHz