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The Last Empty Channel - TvTechnology

The Last Empty Channel

The band of frequencies from 608 to 614 MHz is commonly known as TV Chan-nel 37, but it has never been occupied by any over-the-air television station in North America.
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The story behind Channel 37

DALLAS

The band of frequencies from 608 to 614 MHz is commonly known as TV Chan-nel 37, but it has never been occupied by any over-the-air television station in North America.

It is a quiet channel reserved for radioastronomy, and since July 2000, has also been allocated for medical telemetry equipment on a co-primary basis. The pristine condition of Channel 37 has gone unnoticed by almost everyone--including many broadcast professionals.

But upon close examination, it is a conspicuous gap in an otherwise crowded, lucrative, and gradually shrinking broadcast band.

On a Web site about the old DuMont network, Channel 37 is listed as once having been used by WGOV-TV, Valdosta, Ga; however, FCC records do not indicate that WGOV has ever been assigned to a television station.

Thirty years ago, the emptiness of this channel wasn't remarkable because UHF stations were somewhat uncommon except in the largest markets; several channels were practically empty. Deregulation, especially in the 1980s, introduced radical changes in the broadcast industry, which led to a steady stream of new TV stations. There were 176 UHF stations on the air in 1970, but that number increased to 752 UHF stations by 2002.

In the same period, the total number of TV stations (VHF and UHF) doubled, from 862 stations in 1970 to 1,714 stations in 2002. With this one exception, the UHF channels are filling up.

CHANGING BOUNDARIES

When the boundaries are moved between one service and another, or new services displace old ones, the FCC's reallocation decisions are (supposed to be) based upon the public interest, convenience and necessity. This usually means that spectrum space is allocated in such a way as to generate the greatest benefit to the national economy's bottom line, the gross domestic product. Because the demand for RF bandwidth constantly increases, TV Channels 70 to 83 were reallocated for common carrier and private radio land mobile use in 1970. Other reallocation actions followed, affecting Channels 14 through 20, and eventually all TV channels above 51 will be reclaimed.

PART 15 DEVICES

Currently there are proposals to let users of Wi-Fi devices and other systems operate under Part 15 of the FCC rules on vacant TV channels.

Locally vacant TV channels have been used in the past for wireless microphones as well as medical telemetry. But it remains to be seen if appliance operators can make accurate assessments about whether or not a TV channel is really vacant.

Spectrum squatters can sometimes encounter or generate unfortunate surprises.

In 1998, WFAA-DT signed on using RF Channel 9 and caused unexpected problems for medical telemetry users in Dallas hospitals, which had been using Part 15 devices on this suddenly not-so-quiet channel.

WFAA-DT shut down for awhile so that the interference problems could be resolved, even though the station had no obligation to protect Part 15 users. Fortunately the interference problems were quickly resolved, and this incident served as a warning to others around the country in similar situations. Soon after the Dallas incident, and similar incidents around the country, Channel 37 was allocated for medical telemetry on a co-primary basis. Transmitters in this service operate indoors (inside hospitals) with an ERP of less than 1 Watt, so this channel is still very quiet.

RADIOASTRONOMY

The communications act states that one of the objectives for competitive bidding, that is, the auctioning spectrum space, is the "efficient and intensive use of the electromagnetic spectrum." Quietly listening for signals from outer space doesn't seem "intensive." So it is surprising that the FCC has protected this channel because radioastronomy generates no revenue and appears to have no tangible end product.

Astrophysicists engaged in radioastronomy attach considerable importance to the maintenance of this allocation, since without it there would be a large gap between the 410 and the 1,400 MHz allocations in one of the most interesting parts of the spectrum. The band at 608 to 614 MHz is of special importance for worldwide Very Long Baseline Interferometry (VLBI) observations.

There is a well-known hydrogen line--and a radioastronomy allocation--at 1,420 MHz, but there isn't a specific frequency in the 608-to-614 MHz range that is of interest.

Radioastronomers use as much bandwidth as they can for their observations, not just specific narrow frequencies, and they deal with extremely weak signals. The signal received in a typical spectral observation of a strong source is less than 10 to the -18th Watts (a billionth of a billionth of a watt). If a typical 1 milliwatt garage door opener were operated on the moon, it would be one of the strongest radio sources in the sky as seen from a terrestrial radio telescope.

Obviously then, high-power broadcast signals on Channels 36 and 38 are of great concern to the people who depend on Channel 37 being kept quiet.

OTHER APPLICATIONS

There may be practical uses for Channel 37 in works of fiction. Telephone numbers in the range of 555-0100 to 555-0199 are reserved for fictitious use in radio, TV and film. These numbers are generally recognized as invalid placeholders. Similar uses come to mind for Channel 37, in cases where a speaker would like to make a hypothetical example without naming anyone in particular, in the same way that you might use "example@whatever.com" in a discussion of e-mail protocol, without intending that anyone take it literally.

Channel 37 should not be considered wasted spectrum space, just because it is quiet. It is simply an anomaly of which most Americans--even most broadcast engineers--are unaware.