Tuesday, October 27, 2009

Please do not interfere, we're busy here!

Interference is a frequent problem in all radio services, and the amateur service is no exception.  Interference is anything (natural phenomena, other signals, whatever) that hampers or prevents the successful receipt of a radio communication by its recipient other than the simple lack of a signal path between the transmitter and the receiver.  Hams distinguish between natural sources of interference (often called "atmospherics" or "QRN"), which are typically caused by things like lightning storms or coronal mass ejections hitting the magnetosphere, and artificial sources of interference (or "QRM"), which may be the result of other people's attempts at transmissions, deliberate jamming, or noise produced by noncommunicative uses of RF energy (like microwave ovens, computers, and hybrid cars). 

The exact definition of interference can be found in 47 CFR §2.1(c): "The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radiocommunication system, manifested by any performance degradation, misinterpretation, or loss of information which could be extracted in the absence of such unwanted energy."  As this regulation is taken directly from the ITU's Radio Regulations, this definition is effectively universal across the world.  The International Telecommunication Union, or ITU, is the international treaty organization that regulates radio worldwide; the United States is a member nation of the ITU, as are virtually all other countries in the world.  (An important side note here: while the regulations that apply to amateur radio specifically appear in Part 97, hams are also required to comply with the general regulations that appear in Parts 1 and 2, and reference to these parts sometimes clarifies matters that are left unclear by Part 97 standing alone.  Some familiarity with these parts, as well as with Part 97, is therefore a good idea for the conscientious ham.)

I'm writing this series of posts at least in part as an effort to discuss the questions on the various exams.  And in this one I've come to a question that I disagree with the NCVEC about.  The question in this case is T1A10, which I here quote in full:
What is a transmission called that disturbs other communications?
A. Interrupted CW
B. Harmful interference
C. Transponder signals
D. Unidentified transmissions
The NCVEC considers the correct answer to be "B", "harmful interference".  However, the FCC explicitly defines "harmful interference" (in §97.3(23)) as "Interference which endangers the functioning of a radionavigation service or of other safety services or seriously degrades, obstructs or repeatedly interrupts a radiocommunication service operating in accordance with the Radio Regulations."  (This same definition is found in §2.1(c) and also derives from the ITU Radio Regulations.) The FCC's definition requires more than just a simple "disturbance" of communications for a transmission to be "harmful".  In this case, "B" is the "most correct" answer, but it is not a correct answer, simply because not all transmissions that disturb other communications will qualify as "harmful interference", merely as "interference".

This post has been brought to you by pool questions T1A10 and T1B01.