Frequency Letter Bands

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There is confusion and controversy about the microwave frequency band definitions used in North America. Legend has it that the designators were originally contrived during World War II to confuse the enemy. Engineers in Fort Monmouth New Jersey came up with the letter codes, which were classified as Secret at the time. Naturally, a logical progression of A, B, C wouldn't do for that purpose, so they chose L, C, X and K, and a whole bunch of lower-case letter sub-band designators that have been all but forgotten, with the exception of the Ku and Ka bands. After the war, Uncle Sam didn't declassify the system for everyone's use, different companies such as Sperry, Motorola, Narda, Hewlett Packard and Raytheon made educated guesses on the secret frequency bands, with inconsistent results and little attempt to organize an industry-wide standard.

In 1959 the world came to an agreement on the designator letters, at the International Telecommunications Union meeting in Geneva. The 1959 approved designators (Article 2, Section 11 of the Radio Regulations) are available in the ITT Reference Data for Radio Engineers. Now this data is obsolete, perhaps because they never considered that anyone would be interested in frequencies above 40 GHz. Kind of like when the phone company standardized on seven digit phone numbers, thinking that one area code for each U. S. state would provide enough phone numbers to last forever. Thanks to this short sightedness, the misery of changing area codes has become routine. But I digress...

In 1984, the IEEE microwave nerds agreed on the standard letter-band designations shown in the table below, the first two columns of which you should commit to memory if you want to be taken seriously. The first IEEE standard was published in 1976, then updated in 1984 and now exists as IEEE Standard 521-2002. The next update is estimated for 2019. 

It's time for a Microwaves101 Rule of Thumb, which we loosely apply to memory aids as well as other useful microwave sound-bite info!! Want to remember the correct order of Ku, K and Ka radar bands? K is the middle band (18-27 GHz), while Ku-band is lower in frequency (think K-"under") and Ka-band is higher in frequency (think K-"above").

The frequency band designator story does not end with the IEEE, since United States waveguide manufacturers have adopted their own set of standards over time, corresponding to frequency passbands of the many different rectangular waveguide cross-sections that have become standard in the industry (waveguides behave like bandpass filters). Unlike the IEEE frequency standards, there is considerable overlap among the waveguide bands, so that you can choose the waveguide type where your application is nearest the center frequency.

We keep our waveguide letter band and dimensional info on another page (click here). Although there are disagreements between waveguide vendors, our table is representative of the US system (other countries such as the U.K. have their own weird nomenclature that is just as confusing).

New material!

The material below just came to our attention, it also speaks to the origin of the microwave frequency letter bands, and it seems to make a good deal of sense.

EW letter bands

This came in from Andy, the EW letter bands cannot be ignored! Caution, the information below came from a Canadian web site!

Note that the channel width is not the full band. For example, an 18 GHz Band J radio would have channel steps 1000 MHz apart (17,000-18,000-19,000...) Thanks to MN!

If anyone has any other information on the frequency letter bands (such as a reference for these definitions), send it in!

 For those that want to go beyond the letter band frequency ranges, take a look at the official FCC interactive spectrum allocation chart.