Talk:Waveguide

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Reverted changes by ANom OF DUBIOUS AND AMBIGUOUS NATURE. FrankB 20:34, 14 July 2005 (UTC)

Contents 1 Transmission lines are not waveguides 2 European v US views on WGs/TLS 3 Fed Stan (1037C)Definition of waveguide 4 Summary of nomenclature argument above

[edit] Transmission lines are not waveguides

TLs and WG work on different principles. For example: there's signal loss in TLs but no loss in waveguides. So how can they be the same thing?--Light current 00:04, 22 January 2006 (UTC)

No loss in waveguides?!! What?!! This is totally wrong. Think about the IR absorbtion bands in glass fiber which make periodic amplification necessary. Think about rectangular microwave waveguides at high power... with huge currents in the metal, problems with heat, and where adding internal silver plating greatly reduces the LOSSES. --Wjbeaty 07:07, 25 January 2006 (UTC)

Could you clarify, what is different about the operating principle of a transmission line from a waveguide? If there's a clear distinction to be made between WGs and TLs, I'd support simply removing most of the TL material from this article.

TLs operate on TEM mode. WG are TE or TM modes. TEM plane waves cannot propagagate in WG.--Light current 05:55, 23 January 2006 (UTC)

How do you classify microstrip lines? They also don't support TEM modes, but are often called "transmission lines". --The Photon 03:20, 24 January 2006 (UTC)

I classify them as TLs. Predominantly TEM propagation. See here [1]--Light current 03:35, 24 January 2006 (UTC)

The definition in Ramo, Whinnery, & Van Duzer (just got this from Amazon's "search inside") is "A waveguide is a structure, or part of a structure, that causes a wave to propogate in a chosen direction with some measure of confinement in the planes transverse to the direction of propogation." (p. 398) Their first example of a waveguide is "a transmission line." I'll stand on their authority to say that the established literature considers transmission lines to be a sub-category of waveguides, not a seperate concept. -- The Photon 04:12, 24 January 2006 (UTC)

I dont agree. All the books I have differentiate between the two structures.--Light current 05:02, 24 January 2006 (UTC)

I'm suspicious. Which books specifically? Kraus "Electromagnetics" treats TLs and WGs the same. Here's an important point: he puts them in two classes: "hollow" waveguides, and the non-hollow ones such as coax, helical lines, etc. Perhaps you're definition of the word "Waveguide" resembles Kraus' definition of "hollow waveguide?" Certainly the coax (etc.) can behave as hollow waveguides when wavelength is short enough to approach the line's width, so that other modes become possible. But twinlead and coax guide EM waves, so they're waveguides regardless of modes or bandwidth limitation. Even in a flashlight, the energy flows as propagating EM fields in the space outside the metal. Think in terms of Poynting vector fields, and you'll see the identity between a flashlight's wave-guiding circuitry and a rectangular microwave waveguide. --Wjbeaty 07:07, 25 January 2006 (UTC)

FE Terman 4 th ed treats WG and TLs in separate chapters. Also, Electromagnetism by Grant& Philips (Physics Dept Manchester Uni) pub John Wiley, treats them differently.--Light current 18:45, 25 January 2006 (UTC)

A transmission line can act as a waveguide if excited above its WG cutoff frequency when different propagtion modes are used. But TLs are overwhelmingly used in the TEM mode. I think you need to look at that book you quoted in detail (rather than just the abstract or snippet) to see what the author is really saying.--Light current 05:08, 24 January 2006 (UTC)

I am actually very familiar with the book [R, W, & vD]-- I used it as an undergrad. I mentioned its available on Amazon in case you want to check into it. Following up, I found that Federal Standard 1037C clearly uses "rectangular closed waveguides" as an example of a type of transmission lines. Finally, another highly respected textbook, Balanis' "Advanced Engineering Electromagnetics" regularly uses "transmission line" and "waveguide" as interchangeable terms.

Based on those sources, I can accept that TL are a subgroup of WGs, or that WG are a subgroup of TLs, or that the terms are interchangeable. I didn't find any source that defines a clear distinction between TLs and WGs. If you could cite a specific reference for your claim that WG and TL work on different principles, it might help me to understand where you're coming from.

The Photon 03:38, 25 January 2006 (UTC)

Well looking at it from that point of view, I suppose you could say that any conveyor of information is a 'transmision line'. If you want to go down that path, then I would say that a waveguide is part of the larger group called 'transmission lines'.

However, you have to be careful here, because one of the primary differences (apart from the modal one) is that WGs are narrowband structures (they have cutoff and go into different, unsatisfactory, propagation modes at higher frequencies) whereas TLs (coax etc) are generally thought of as wideband 'pipes' and in fact go right down to dc from say 20 GHz. WGs dont do dc or anywhere near. So, on an octave BW basis TLs are wider band than WG and I think this is one of the main differences. The other main difference is that WGs can do much higher frequencies with low attenuation than TLs can. Does this help at all?--Light current 05:15, 25 January 2006 (UTC)

Could you provide a citation for the claim that there's no loss in waveguides? If its intended to apply to real waveguides, I simply don't believe it. Even the longest-distance waveguides I know of, undersea fiber optics, have ~0.2 dB / km losses.

No citation because WG do have losses (my exaggeration- sorry). But much lower losses than TLs because the energy is bouncing off the walls. Highly conducting polished walls will give lower losses.--Light current 06:03, 23 January 2006 (UTC)

If the claim is intended to apply to the textbook "ideal lossless waveguide", then it doesn't create a distinction from the textbook "ideal lossless transmission line." -- The Photon 04:53, 23 January 2006 (UTC)

Light current is confused. Yes, one typically requires that a waveguide confine light losslessly in the limit of perfect structures made of lossless materials, but in this limit both TLs and e.g. dielectric waveguides (i.e. index-guided) are lossless. (However, even this restriction is commonly relaxed, since one sometimes considers "leaky" waveguides that have some small intrinsic loss even for lossless materials.) There is no particular restriction to TE, TM, or non-TEM modes (and in any case any waveguide with inhomogenous dielectrics does not generally have a simple classification by polarization). Saying that waveguides are only "narrow-band" is nonsense...for lossless materials, even an optical fiber would have guided modes at all frequencies, extending arbitrarily close to DC; you get a lower-frequency cutoff for non-coaxial hollow metallic waveguides or for e.g. index-guided dielectric waveguides sitting on a substrate. —Steven G. Johnson 17:45, 25 January 2006 (UTC)

(My feeling is that this whole article needs to be substantially revised, in any case...it's a mishmash of information that doesn't clearly distinguish statements that are generally true from properties that are specific to particular waveguide geometries and materials. It should also be moved to Waveguide (electromagnetism) or similar, since it's perfectly possible to make waveguides for acoustic waves etc.) —Steven G. Johnson 17:45, 25 January 2006 (UTC)

Thanks, Steven and Wjbeaty for jumping in. I thought I might be kicking up dust where no-one really cared. I don't know if Light Current is confused or just biased (like all of us) toward the terminology of either his locality or his particular sub-field of expertise.

I agree the article generally needs a lot of work, but I don't think a title change is required. There are lots of examples where the most common usage of a word gets the main article of a particular title, and other less common usages have a disambiguation page to seperate them. For example, Laser and Optical. Only if someone would write an article that covers waveguides for all types of wave phenomena, then it ought to get the main article, with this article relegated to Waveguide (electromagnetism), alongside of Waveguide (optics), Waveguide (acoustics), Waveguide (fluid dynamics), etc.

The Photon 04:42, 26 January 2006 (UTC)

I will admit to being biased/ignorant about the terminology because I, like some others, have not covered the whole gamut of WG and TLs. In fact I have never bolted pieces of hollow WG simply becuase I have never been working at those frequencies. Neither have I jointed two pieces of optical cable. However, I do agree that the subject appears to be large enough that a split based on Stephens/Photons suggestions seems sensible. I will then restrict myself to editing the electromagnetic WG and TL pages.--Light current 17:49, 26 January 2006 (UTC)

My personal opinion concurs with the following extract that Stephen Johnson has found:

The term waveguide can be applied to all types of transmission lines in the sense that they are all used to guide energy from one point to another. However, usage has generally limited the term to mean a hollow metal tube or a dielectric transmission line. In this chapter, we use the term waveguide only to mean "hollow metal tube."

--Light current 23:36, 26 January 2006 (UTC)

[edit] European v US views on WGs/TLS

Before everyone gets too agitated about differences and similarities between WG and TLs, Im going to suggest that it seems to me there is is difference in view here between people on each side of the Atlantic due to the books they have been reading/studying.

It would seem that the USA view is that a TL is anything that can convey info: from a piece of wet string to an optical fibre and beyond. Over here (the old world), rightly or wrongly we have a more refined view of what constitutes a waveguide or a TL. So Im going to stick a globalise tag on the article just to remind people to be careful about representing a world view on this subject. --Light current 18:52, 25 January 2006 (UTC)

I regularly go to international conferences dealing with electromagnetic wave propagation, and I have never heard of the "refined" view of what constitutes a waveguide that you are referring to. Can you give a reference that clearly states that a TL is not a waveguide? (Historically, there were lots of analysis of TLs prior to the development of a more-general electromagnetic waveguide theory, but the existence of TL-specific terminology and analyses does not show that they are not considered "waveguides" by modern scientists.) —Steven G. Johnson 19:13, 25 January 2006 (UTC)

Are you saying that all WG are TLs, or all TLs are WG or what?--Light current 19:54, 25 January 2006 (UTC)

I mean that all TLs are waveguides. I wouldn't say the converse; in my experience the term "transmission line" seems to be mainly used for waveguides based on metallic components and operating at RF or microwave frequencies.

However, maybe I spoke too soon. It seems that there may be some differences in terminology, not so much between Europe and US, but between e.g. the optics and microwave communities. For example, I found this site (which seems to be from US Navy training material):

The term waveguide can be applied to all types of transmission lines in the sense that they are all used to guide energy from one point to another. However, usage has generally limited the term to mean a hollow metal tube or a dielectric transmission line. In this chapter, we use the term waveguide only to mean "hollow metal tube." It is interesting to note that the transmission of electromagnetic energy along a waveguide travels at a velocity somewhat slower than electromagnetic energy traveling through free space. [2]

—Steven G. Johnson 02:55, 26 January 2006 (UTC)

As a matter of encyclopedic style, it seems there's a preference for each article to take on as broad a meaning as possible. This would seem to favor waveguide being used in its broadest meaning: anything that guides waves. On the other hand, this can be taken too far. For example, see the mess that is created in Optical communications by the initial assertion that the term encompasses any communication using light as its "transmission medium". I'd like to clean that article up, but starting with the assumption that it should cover everything from aposematism to wavelength division multiplexing makes it a nightmare to contemplate.

In this case, where one local definition of the term (in the UK) has a more restrictive definition, and another (the US) has a broader definition, I'd suggest the article ought to be allowed to cover the broader field, with a note on the restrictive usage in particular regions or fields. The main thing really ought to be to actually fill out the article with some real content. Hopefully my next contribution will be on the article page, not the Talk page. The Photon 04:42, 26 January 2006 (UTC)

[edit] Fed Stan (1037C)Definition of waveguide

waveguide: A material medium that confines and guides a propagating electromagnetic wave. (188) Note 1: In the microwave regime, a waveguide normally consists of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross section. This type of waveguide may, under certain conditions, contain a solid or gaseous dielectric material. Note 2: In the optical regime, a waveguide used as a long transmission line consists of a solid dielectric filament (optical fiber), usually circular in cross section. In integrated optical circuits an optical waveguide may consist of a thin dielectric film. Note 3: In the rf regime, ionized layers of the stratosphere and refractive surfaces of the troposphere may also act as a waveguide.

--Light current 23:28, 12 March 2006 (UTC)

[edit] Summary of nomenclature argument above

I wrote this out as part of another discussion, then realized it wasn't very useful there, but I didn't want to throw it away since I think it is helps to clarify the nomenclature discussion.

There are apparently three ways that the the categories of "waveguides" and "transmission lines" are defined:

• Waveguides are a type of transmission line. This view would come from looking at the function of the device, and this is the one used in 1037C. I don't know of anyone who uses this view, but I could imagine it being used in government contract bids or patent filings, for example.

• Transmission lines are a type of waveguide. This view comes from looking at the principle of operation: Solution of the electromagnetic wave equation with appropriate boundary conditions. This is the view used by American academics, for example in the Ramo, Whinnery, and Van Duzer book I've cited before.

• Transmission lines and waveguides are seperate non-overlapping categories. This view comes (I suspect) from practicality: it's more useful to have words that actually distinguish between the two concepts. This view seems to be used by UK academics -- for example Grant and Philips cited by LC, above.

I find the third view is used in practice by my colleagues in the fiber optics and high-speed electronics business, but when things get nasty we are likely to say "okay, so in this situation we have to consider all of the waveguide effects in this transmission line". On the other hand, the important thing here is, what gives the clearest explanation to the reader of an encyclopedia?

--The Photon 02:34, 13 March 2006 (UTC)

V useful summnary. Perhaps we could include it in the article under terminology or something!--Light current 03:30, 13 March 2006 (UTC)

Well, its hardly written in encyclopedic style... Also, this would dominate the article if inserted now, and I will stick to my proposition that an encyclopedia is about ideas, not words, so I'd find it distracting to have it stuck in there now. But it is GFDL, so I can't forbid it. -- The Photon 03:40, 13 March 2006 (UTC)

Well it would obviously need rewriting in an acceptable format. Perhaps it should go in the lede?--Light current 03:46, 13 March 2006 (UTC)

If you or someone else would like to re-write, it's GFDL. In my opinion, including the explanations it's too long for the lead. -- The Photon 04:11, 13 March 2006 (UTC)

The name "Waveguide" would suggest any structure capable of directing the propagation of electromagnetic waves (ie. anything that guides waves). A tranmission line directs the propagation of electromagnetic waves, but is specifically meant to propagate TEM waves. Therefore, isn't it logical to classify transmission lines as a subset of waveguides...? -- A Passing Student

Pleas refer to the above posts regarding this discussion.--Light current 02:14, 21 March 2006 (UTC)