Sunday, November 08, 2009

How long is your band?

When talking about frequency, and especially about "bands" (divisions of spectrum made for the purpose of allocation), it's very common to see frequencies specified in units of length (meters and centimeters being most common) instead of in units of frequency (kilohertz, megahertz, or gigahertz, or, more archaically, kilocycles or megacycles).  When a length unit is used, it specifies the wavelength involved.  The wavelength of a light wave is, of course, directly related to its frequency: the wavelength is the distance the wave travels (at the fixed speed of light) over the period of one oscillation, and so wavelength is just speed divided by frequency (and frequency is speed divided by wavelength).

Fortunately, the math for this is pretty simple, especially in the metric system (which even us backward Americans use when doing ham radio, thankfully).  By pure serendipity, the speed of light (in a vacuum) is approximately 299,792,458 meters per second; this is close enough to 300,000,000 meters per second that for most purposes the latter value can be used.  So to convert a wavelength in meters to a frequency in megahertz (or vice versa), one must merely divide 300 by the wavelength (or frequency).  Hams routinely refer to all the amateur bands below 200 MHz by their wavelength instead of their frequency, and it's quite common to hear the higher frequency bands by wavelength as well, so it's important to be able to do this translation on the fly (at least until you memorize the more common ones).  Fortunately, dividing into 300 isn't that hard.

So when a ham talks about the "160 meter band", they're not talking about a really long parade; instead, they're talking about a band whose wavelength is approximately 160 meters, which corresponds to a frequency of about 1875 kHz.  The 160 meter band is actually 1800 to 2000 kHz, so this is pretty close.  Other examples (more pertinent to the student preparing for the Technician exam) include the 6 meter band (50 to 54 MHz), the 2 meter band (144 to 148 MHz), the 1.25 meter band (222 to 225 MHz), the 70 centimeter band (420 to 450 MHz), and the 23 centimeter band (1240 to 1300 MHz).  Please do note also that these band definitions apply only in ITU Region 2 (and obviously only in the United States or other places where the FCC regulates amateur radio), and that other restrictions based on location may also apply (especially with respect to the 70 centimeter band, which has a lot of interesting restrictions on it).

Of the questions on the Technician exam related to this, two (T1C04 and T1C06) present an additional challenge that cannot be resolved simply by dividing into 300, as there are incorrect answers that are "close enough" that you can't eliminate them based solely on approximate wavelength.  Therefore, it's very important that you commit to memory the limits of the 6 meter, 2 meter, and 70 centimeter bands given above, not only for the purpose of passing the exam but also for the purpose of responsible operation.

This post has been brought to you by pool questions T1C04, T1C05, T1C06, T1C07, and T1C08.