RF Coordination

Most of us in the pro audio industry deal with RF (wireless mics, clearcom, assisted listening, walkie talkies, etc…) but few actually have a solid grasp of all the essential elements that make it work properly (knowledge of the hardware, understanding of the physics, antenna theory, frequency coordination, intermodulation distortion, etc…) There is nothing easy or simple about doing RF well and as the FCC sells off more and more bandwidth, our lives regarding RF are going to get more and more difficult. Might as well begin the process of preparing now.

Recently, after having some RF issues, I bought an RF scanner and the software necessary to monitor and save RF spectrum scans. Interestingly the universe is now handing me a cornucopia of RF issues to work with. This morning I received a text message from a staff member at a church that has (5) Sennheiser EW100 series A band (516mHz to 558mHz) RF mics. Via Teamviewer app, I was able to log into the local computer that records the services and see that (4) out of (5) RF receivers were being stomped on. I was also able to mute those channels so the church could get through their morning service without interruption from RF blasts. I headed to the church as soon as possible and we retuned everything to a new area of frequencies that is hopefully clear. Note that the existing frequencies were clear when I arrived but suffered so RFI (radio frequency interference) earlier in the day. Something is intermittently causing issues. No choice but to run from it.

In order to coordinate RF correctly, you need a few resources. An RF scanner is mandatory so you can scan the local RF spectrum and see if there are any openings. An RF scanner is not a panacea because it only scans when it’s active so if you scan for 5 minutes and then stop and then some RFI comes along, you won’t know it via the scanner. In addition to scanning locally, you also need to know what to expect regarding broadcast signals in the local area. In theory your RF scanner would show you the same data but sometimes the FCC data isn’t up to date in which case, if you used only their data to coordinate frequencies, you might avoid using some of the RF spectrum that is actually clear. For example, if a broadcaster becomes dormant.

Some of the information needed is readily available on the internet.
For the USA, here is a wiki article that explains the frequencies that DTV channels use.
WIKI – North American Television Frequencies
WIKI – North American Television Frequencies Broadcast Television

Once we know that X TV channel uses between Y&Z frequencies, we can understand what spectrum may be fair game for our purposes. Remember, there are no guarantees.

Here is a chart that shows which DTV channels are active in X location.

Programs like IAS, Clear Waves and RF Guru will include this information and use it when helping choose frequencies. Meaning that you can figure it all out for yourself by piecing all the data together or you can use an existing set of tools to do it for you.

Manufacturers like Sennheiser provide a website where you can check for known frequencies.

Sennheiser Frequency Finder webpage

You fill out the zip code or city and then set a few parameters and the webpage will provide you with a suggestion for where you tune your mics.

Here is the website for checking Shure gear:
Shure – wireless frequency finder website

The main problem with using the manufacturers information is that they logically only support their devices and the frequencies their devices operate on so if you have multiple brands of models of RF gear, you’re on your own to coordinate the RF system. This is where software like IAS is invaluable. IAS has gear lists so in addition to taking the data generated with an RF scanner, you can configure it so that it knows that you have X number of channels of RF mics, wireless com, etc… and it will know which device is tuneable and which ones aren’t and also the range they can tune over. Then IAS will provide you with a master list of frequencies you should try to avoid IMD and still make your rig work. IAS will also indicate if you aren’t going to be able to make your rig work as specified. $550 might seem like a lot of money until you ruin a show trying to coordinate your RF without it.