Channel 37: Why Is It Reserved?
For as long as there has been a UHF TV band, there has not been a single television station given the authority to operate on channel 37 in the entire country. Any applications for that channel are dismissed without explanation, and most literature treats channel 37 like it does not even exist. Many who are familiar with the subject know that channel 37 is reserved for “radio astronomy,” but what specifically caused that specific channel to be reserved over any other?
The first part of the investigation is to observe that the frequency at the center of the channel 37 spectrum, 611 MHz, is preserved through international treaty. Not only is it considered so important that no broadcasts exist on that frequency in the United States, but no broadcasts are permitted on that frequency in most of the rest of the world either. This should make it exceptionally clear that this piece of spectrum was not chosen arbitrarily; there is something important about 611 MHz.
Over the years, I have heard several rumors about just why channel 37 is reserved across the country. There are only two that have stuck with me to this point, however, and I cannot source them or tell you where I heard them, so it could have been random speculation by a person with limited knowledge. One is that channel 37 was being reserved in case of nation-wide disaster, and could be activated as some type of emergency frequency. This is certainly false, as the frequency is reserved specifically for radio astronomy. The second is that 611 MHz is the resonant frequency of hydrogen. This is not true either, but there is a relationship to hydrogen in the ultimate explanation, so it is quite possible that someone simply misinterpreted.
Two sources contained answers I needed. The first was a paper entitled, “A Survey for Transient Astronomical Radio Emission at 611 MHz.” This paper, written by several individuals at MIT and other institutions, contains the first major reason that 611 MHz is such an important frequency. On page 10, it reads, “many sources produce radio emission at 611 MHz by the synchrotron mechanism, in which high-energy electrons are accelerated through a spiral path by a magnetic field.” I admit to not being a physics major, but this certainly sounds like the type of thing that an astrophysicist would want to study without interference from high-powered broadcast television stations on the same channel.
The second source was a comment by the National Academy of Sciences’ Committee on Radio Frequencies in an FCC proceeding from 2004. In it, the many uses of the channel 37 spectrum are laid out:
Of particular concern in this proceeding is protection of RAS observations at 608-614 MHz (TV Channel 37). This band is one of the preferred RAS bands for continuum observations. For continuum observations, the approximate spacing between bands throughout the spectrum should be no greater than a factor of two in frequency (see ITUR Recommendation RA.314). The loss of Channel 37 for radio astronomy would result in an unacceptably wide gap.
In addition to continuum observations, this band is used for studying the interstellarmedium, pulsars, and the Sun. In regard to the interstellar medium, the 608-614 MHz band has great importance for observations of thermal and non-thermal diffuse radiation in our own Milky Way Galaxy. Such observations give information on the high-energy cosmic-ray particles in our galaxy and their distribution, and also on the hot ionized plasma in the disk of our galaxy.
Observations in this band are also important for the study of pulsars, which are highly condensed neutron stars that rotate with a period as short as a millisecond. Pulsars are commonly the remains of supernova outbursts, and the discovery and study of such objects in the last two decades have opened up a major new chapter in the physics of highly condensed matter, and have contributed immensely to our understanding of black holes and the final state in stellar evolution. Observations of binary pulsars by radio astronomers have verified the existence of gravitational radiation at the level predicted by the theory of relativity.
Observations are made of the neutral hydrogen line (rest frequency of 1420 MHz) reduced in frequency by the Doppler effect (redshifted) into the 608-614 MHz band. Such observations are used to investigate interstellar matter and the rotation and evolution of galaxies.
Lastly, important observations in the 608-614 MHz band are made of radio frequency outbursts from our Sun. These bursts of high-energy particles interact with Earth’s atmosphere and can cause severe interruptions in radio communications and power systems, and can also have dangerous effects on aircraft flights at altitudes above 15,000 meters. Study of these solar bursts aims to allow prediction of failures in radio communications. In addition, knowledge regarding high-energy solar bursts is essential for successful space exploration, both manned and unmanned.
It is clear that channel 37 has many uses, and that they seem to be pretty particular to the spectrum within channel 37. To read the raw documents, see the links below.Catz, C. A., Hewitt, J. N., Corey, B. E., & Moore, C. B. (2003). A Survey for Transient Astronomical Radio Emission at 611 MHz. Astronomical Society of the Paciﬁc, 115.
Retrieved from http://www.phys.vt.edu/~jhs/eta/references/KatzEtal2003.pdf
Comments of the National Academy of Sciences’ Committee on Radio Frequencies. (2004, September 1). Retrieved from http://sites.nationalacademies.org/xpedio/groups/bpasite/documents/webpage/bpa_048869.pdf