Talk:directivity

RFC discussion: October 2015

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directivity

There are currently two senses:

1. (geology) The effect of earthquake motion propagation being greater in the direction of the rupture
2. (physics,mathematics) The direction of motion of a moving body; Both the address and the direction of a vector.

The second is my main concern, because of who added it, but I suspect that neither really captures the core meaning of this term.

My take on the IP who added the second term is of someone of above-average intelligence who's developed a seriously-misplaced confidence in their ability to master a subject by just reading about it. They really, really want to be an expert, so they gloss over any difficulties and fill in the blanks with bad guesses. This is extremely dangerous, because 1) Most of us don't know enough about the subject matter to spot the problems, and 2) Even if they're correct on 9 details in a row, there's no guarantee they won't be massively, hideously wrong on the 10th.

In checking on the second sense, it looks like the core meaning is something like "the quality of being oriented, or of having effects/motion distributed more strongly, in a specific direction", which would sort of make it a synonym of directionality. For instance, passages in Google Books on the subject of antenna design talk about calculating the degree of directivity, which is incompatible with either existing definition. Unlike the IP, though, I know my limitations when it comes to mathematics and the mathematical aspects of the sciences, so I'm not going to mess with the entry myself.

I would appreciate it if someone with the requisite background (@Choor monster? @Msh210?) would make sense out of this entry, and give some guidance as to whether anything needs to be rfved. Thanks! Chuck Entz (talk) 22:49, 24 October 2015 (UTC)

[1] and [2] make it sound like "the degree to which something's directed toward one direction". (That's terrible wording. I mean "the oomph with which it's directed", except that that's even worse wording.) Afaict from cites, it seems to be used for both sound and electromagnetic waves. But my physics is weak.​—msh210℠ (talk) 07:09, 25 October 2015 (UTC)

"Synonym of directionality"? Equinox ◑ 07:42, 25 October 2015 (UTC)

It seems to be a measure of the degree to which a transmission (of sound or electromagnetic signal) is directionally focused.

The Wikipedia article describes it as a performance measure ("figure of merit") for antennae. It states that directivity is the maximum value (over all solid angles) of the directive gain, which is the ratio of the radiation intensity (power / solid angle) to the average power per solid angle (total power / 4π). It also states that directivity is also used as a synonym for directive gain.

Of mostly usage relevance is that directivity is rarely expressed as a pure ratio, but rather on a decibel scale (10 * log₁₀ of the ratio). In any case, it is essentially dimensionless.

From [3] it seems that it may be called directivity factor when a pure ratio, and directivity index when converted to the decibel scale.— Pingku^dimmi 09:23, 25 October 2015 (UTC)

I'm pretty certain the given definition #2 is trash. But WP and Pingku are correct, the term is definitely real and well-attested. Searching on MathSciNet for the term "directivity", there were 131 hits. (MSN does not search the articles, but AMS reviews, so actual usage is much higher.) For example, Blanco et al, "Directivity enhancement and spurious radiation suppression in leaky-wave antennas using inductive grid metasurfaces" IEEE Trans. Antennas and Propagation 63 (2015), no. 3, 891–900. In short, the better your antenna aims its signal, the higher the directivity. Choor monster (talk) 16:50, 25 October 2015 (UTC)

Luckily, antenna directivity is something that I actually do know something about (earthquake directivity is effectively an analogue to antenna directivity - you treat the rupture point like an antenna, and the vibrations of the quake more or less like radio waves). I've tried to clean up the article. While I can find a few hits for "directivity" meaning the directionality of a vector, these are mostly either translated science papers or pseudoscience books. Smurrayinchester (talk) 09:31, 26 October 2015 (UTC)

Many thanks! Striking. If someone wants to RFV the marked-rare sense, it may be worthwhile.​—msh210℠ (talk) 14:44, 26 October 2015 (UTC)

I've added cites (not sure how worthwhile that was in the end, but hey ho) Smurrayinchester (talk) 15:44, 26 October 2015 (UTC)