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.

Mayday, mayday, mayday: Emergency operations

Quite a lot of people get into amateur radio for the purpose of emergency communications.  There are two meanings to that phrase, too, and it's important to keep them straight.  The first, "emergency communications", is the provision of communication assistance during an "emergency"; that is, responding to a station in distress and, based on the ensuing communication, providing assistance directly or dispatching others to provide assistance.  The second is more strictly a "communications emergency", which is when an emergency situation cause (or is likely to cause) a failure or breakdown of ordinary communications systems.  Amateur radio operators have specific powers and duties in both of these situations.  Providing emergency communication is one of the declared purposes of the amateur radio service, so hams should have some understanding of these powers and responsibilities.

Amateurs are, generally speaking, not permitted to communicate with stations in other services using their amateur radio station.  This includes the licensed-by-rule Family Radio, General Mobile Radio, Citizen's Band, or Multi-Use Radio services.  Even though all of these services are available to anyone (other than a representative of a foreign country) who possesses the appropriate equipment, hams are not exempted from the requirements in these services to use type-accepted equipment and hams who wish to operate in these services must ordinarily meet these same equipment limitations and requirements as any non-ham would.  Nor may a ham use his or her equipment to communicate with public safety entities.  There are, of course, exceptions to this general rule, almost entirely for emergency situations. 

There are three major exceptions for "emergency" situations:  First, an amateur may use any means of radiocommunication at his or her disposal in order to provide essential communications needs during an emergency which immediately threatens human life or property, when other forms of communication are unavailable or disrupted.  Second, an amateur station which is actually in distress ("distress" also being a term of art which in this case means "in immediate threat of loss of life or serious permanent injury") may use any means at its disposal to attract attention and obtain assistance.  Third, an amateur station who is aware of a station in distress may respond to that station in distress or make any other communications necessary to obtain assistance for that station in distress, even if that would involve transmitting to stations outside the amateur service or using frequencies not authorized to the amateur.

These exceptions are to be construed very narrowly, in practice.  The communications must be "essential", the threat to life or property must be "immediate", and other forms of communication must be "unavailable".  Don't use this one lightly; if you have a choice between running next door to use their phone to call 911 or transmitting on the local fire department's general frequency, please go next door and call 911.  This is intended as an absolute last resort "when all else fails", not an option to used when other options are merely inconvenient.  Similarly, if you are monitoring, say, the marine HF bands and you hear a Mayday call, do not immediately respond; you might interfere with other ships or the Coast Guard responding.  Only if you do not hear a response, or if the calling station repeats the call (instead of responding to a response to their Mayday call) should you "unlock" your gear and respond to the calling station, or relay the call.

In addition, the FCC may declare a "communications emergency" when a disaster causes the loss of normal communications in an area.  During such a declaration, amateurs must refrain from using any frequencies designated in the disaster declaration unless they are actively involved in providing assistance.  Such declarations are common during major disasters, such as hurricanes and earthquakes, which tend to disrupt normal communications.   The FCC may also authorize amateurs involved in providing assistance to use frequencies other than those which are normally available to amateurs, or to communicate with stations not in the amateur service, during a communications emergency; amateurs are required to comply with the directions of the FCC and its Field Engineers in such situations.

Outside of an FCC declaration of an emergency, no frequencies are reserved for emergency communication; there is no amateur equivalent of CB channel 9 or marine channel 16.  In general, the best frequencies to use for declaring an emergency in the amateur bands would be the various national calling frequencies, because there's a better chance that someone will be listening on one of these.  However, emergency traffic always takes priority over non-emergency traffic, on all frequencies and at all times.

It should go without saying that if you hear a station calling with an emergency (for which the most proper method to call "Mayday" three times, followed by identification) any non-emergency communications on the same frequency must immediately suspend until the emergency is resolved.  At the very least, do not interfere with emergency communications, and to the extent that you are able, you should assist the station in distress.  (But avoid creating confusion; listen closely and think before you transmit.)  You should always assume that a declaration of emergency is real; if it's not, the station calling the emergency is the one on the hook for making a false distress call.  In the United States, making a false distress call is a federal felony; offenders will almost certainly lose their FCC licenses, and will face stiff fines (in one recent instance, $45,000) and even the possibility of prison time, so don't do that.

This post has been brought to you by pool questions T1C03, T8A01, T8A02, T8A03, T8A04, T8A06, T8A08, T8A09, T8A10, T8A11, T8A12, T8B08, T8B11, and T8C01.

Thursday, November 05, 2009

What's in a call sign, anyway?

Most people have some familiarity with call signs, if nothing else from broadcast radio stations, which in the US tend to be four letters starting with either K or W.  The more knowledgeable will know that in the broadcast services, K stations are west of the Mississippi and W stations east of it (with some grandfathered exceptions).  Those near the borders may know that Canadian stations start with a C, and Mexican stations with an X.  But, unless you're a shortwave listener or a ham, you probably don't know why the letters used are K and W, or what other letters might be used for, if anything.

The ITU, which I talked about in my last post, is the entity that makes the rules that everyone has to follow for call signs.  Each member country is assigned one or more one or two character prefixes (or, in two cases, three character), and all call issued by that country have to start with one of those prefixes.  There are nine one-character prefixes: B, F, G, I, K, M, N, R, and W; the United States, being a charter member of the ITU and the largest at the time that it was decided to regularize call signs, got three of them; (N, K, and W); the UK gets two (G and M), and the other four go to China (B), France (F),  Italy (I), and Russia (R).  Everyone else has to use at least two characters for national identification; Canada, for example, is allocated CF through CK, and Mexico is allocated XA through XI.  (Both countries have other allocations.)  Some countries even have prefixes that start with digits instead of letters.

So why N, K, and W for the US?  France, Italy, and Russia got their initials, after all.  Well, during the early years the US military used A call signs for Army stations and N call signs for Navy stations.  W (. - -) and K (- . -) are what you get when you add a dah to the Morse code for A (. -) and N (- .), and so commercial stations adopted those letters.  The United States has since given up part of A for the use of other nations, and now only has AA through AL (in addition to all of W, K, and N).

In addition to allocating prefixes to the many member nations, the ITU also sets standards for the parts of the callsign after the prefix, at least in certain services.  For amateur radio stations, the ITU mandates a one or two character national prefix, then a digit, and then up to four additional characters, the last of which must be a letter.  For the United States, which has three one-character prefixes, that means a callsign can be as short as three characters or (in theory) as long as 7.  The FCC, however, does not issue four-character suffixes yet, and reserves the so-called "one-over-one" callsigns (one letter, one digit, and one more letter) as "special event stations" (which are issued for short-term use by any ham who requests and obtains one from the NCVEC, who administers that program, and never permanently), so all FCC-issued amateur call signs are four to six characters long.  Not all combinations within this space are used, either; the FCC excludes for the moment all "two-over-three" (two letters, one digit, and three letters) call signs beginning with A and N from the amateur service, and excludes certain others combinations for a variety of other reasons.  Also, while the ITU regs would allow for the issuance of a call sign containing more than one digit, the FCC does not do this; US amateur call signs always contain exactly one digit.  And that digit is not entirely without meaning.

For the purpose of the amateur radio service, the FCC divides the country into 13 regions.  The first 10 regions, numbered 1 through 9, and 0, are groupings within the "lower 48" (see map to right).  Regions 11 through 13 are Alaska; Puerto Rico and other Caribbean islands; and Hawaii and other Pacific islands.  The FCC issues call signs based on what region the amateur's mailing address is in, and within each region based also on the amateur class of license, using what is called the sequential call sign system.  For those in regions 1 through 10, the digit in the call sign corresponds to the region number; for regions 11, 12, and 13 the digit used also depends on location (typically, which island you're on).  Every amateur gets a sequentially-issued call sign initially; after that a ham's call sign will only change (barring extraordinary events) if the amateur, applies for a vanity call sign or requests a new sequentially-issued call sign.  Also, once the FCC has issued a call sign, it will never issue it again via the sequential system; the only way for a call sign that has been abandoned to be used again is via the vanity call sign system. 

In addition to dividing the country into 13 regions, the call signs available in each region are further divided into four groups, A through D.  Calls in Group A are reserved only to Extras; those in Group B are available to Extras and Advanceds; those in Group C to everyone except Novices; and those in Group D are available to any amateurs.  Because of this, you can tell from a call sign what the minimum license class someone has: someone with a two-over-one or a one-over-two is necessarily an Extra.  But you cannot assume that someone with a two-over-three is a Novice (and in fact that's almost certainly not the case), for three reasons.  First, a ham who upgrades does not automatically get a new call (anymore; that used to be different), and so could have a higher license class than his or her call sign group indicates.  Second, a ham may request any call sign which is permitted to his or her license class through the vanity program.  The third, and perhaps most common, for call signs that don't fully reflect the licensee's operating class is that Group C is exhausted in almost all regions, which means new General and Technician licensees (in the lower 48, at least) now get two-over-three "Novice" calls (as do initial club licenses, which are applied for through Club Station Call Sign Administrators, instead of the usual VE process). 

This post has been brought to you by pool questions T1B03, T1B04, T1B05, T1B06, T1B08, T1B09, and T1B10.