by Edwin J. Somers, CAS
In this article, we will discuss frequencies available for wireless microphones in the United States. We are seeing a tremendous increase in the use of wireless and a decrease in frequency availability. This is causing an increase in congestion among wireless users as well as an increase in interference from other users, legal and otherwise.
The frequencies available for wireless use include the hydrological band 169-172 MHz, the VHF television band channels 7 through 13, 174-216 MHz, and the UHF television band channels 14 through 67, 470-794 MHz. Not all UHF channels are available in all areas.
We are allowed to transmit on these frequencies as secondary users. This means if we get interference from the primary users, such as the broadcasters, we must move to another frequency.
The FCC has allocated the broadcasters licenses to operate HDTV transmitters on the UHF television band. This effectively makes these channels unusable for radio mics because of the uniform distribution of power on each channel, as well as the tremendous power levels involved. We are going to see more and more HDTV stations on the air as we approach the deadline for startup.
I am often asked what the industry is doing about the loss of spectrum, and I do not see any efforts to obtain or reserve spectrum. I think the primary reason is the fact that, as operators, we are not required to have a transmitter license except under specific circumstances. In the eyes of the Federal Communications Commission, if we do not have a license, we do not exist.
The one thing that is happening is that manufacturers are offering very effective multi-frequency or frequency-agile systems which work exceedingly well in harsh interference environments.
The transmitting power for VHF transmitters is limited to 50 mw (milliwatts) in the United States, and 250 mw for UHF. Most manufacturers limit their UHF transmitters to about 100 mw due to the reduced battery life of a higher power transmitter. Some high power transmitters are offered with multiple batteries.
When operating multiple transmitters, frequency coordination is mandatory. With single channel transmitters, it was not much of a problem because the vendor usually coordinated the frequencies. With frequency-agile systems, some form of field coordination is required. All manufacturers have developed coordinating software for their respective systems and will readily assist users with coordination, as will the vendors. Some of the manufacturers even offer the software to the end users. Now how convenient is that?
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