What is the legal status of spectrum analyzers in United Kingdom? Is it considered 'interception', despite only analyzing the magnitude of a frequency?
Section 1(1) of RIPA makes it a criminal offence for a person intentionally, and without lawful authority, to intercept in the United Kingdom (UK) any communication in the course of its transmission if that communication is sent via a public postal service or a public telecommunication system.
The act apparently does not define what constitutes "interception".
A spectrum analyzer performs a kind of mathematical analysis on a physical signal which (very roughly put) says "here, the higher frequencies are loud and the mid-range frequencies are soft". You could subject a phone conversation to this analysis and get some information about an individual saying "Let's kill him" (information about "what that utterance sounds like"). In theory you might be able to figure out that a person said "Let's kill him" using that output, but that's close to the realm of science fiction than doable practice. But it's not clear whether that matters for the law. That is, the law isn't stated in terms of methods of extracting usable and possibly damning evidence from some transmitted signal, it simply prohibits "interception".
A frequency analyzer might be mostly hardware or mostly software, and points in between. The machinery itself is legal, and can be used legally or illegally (mostly legally), just as a tape recorder can be used legally or illegally. It would be illegal to record a phone conversation (recording is a form of interception, one that preserves most of the information in the signal). It would, technically, be illegal to spectrum-analyze a phone conversation (another form of interception, one that doesn't preserve enough of the information to be of use for prosecution). There isn't anything in the act that says "but interception is legal if the product is not practically usable in a prosecution".