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Context

I aim to start playing with RF and the first thing I wish to do is some EMI tests.

I've been watching YouTube videos on the matter as you do, and I've seen plenty of people using SMA connectors. This initially surprised me as I was for the most part only familiar with BNC connectors, so I went searching and found out why different connectors exist. They can have different impedances and most importantly different frequency ratings. Eventually I saw someone crimping an SMA connector onto some coaxial cable which lead me to go and see how expensive of an endeavor this was, and I found not expensive at all. I immediately found a 10-pack of connectors and a crimper for $30 on Amazon.

Upon trying to figure out what kind of coaxial cable they were using, I found people using RG58 and RG402. Later I found this chart with various coax cables and their characteristics. Looking on Amazon I found some badly reviewed cables with users complaining about the shielding being magnetic (meaning it wasn't copper).

Later I managed to find this article on optimal high frequency coaxial cabling, but still not anything that would mention a frequency rating of any kind. This was unlike the connectors, for which I immediately found a general frequency rating on Wikipedia.
Lastly I came upon this answer which explained resistive and dielectric losses are a function of length and frequency (for any given cable's characteristics).

Current Understanding

I'm now thinking that no coaxial cable designation has any inherent frequency limitation.
It's all down to attenuation and length. As I have (perhaps wrongfully) concluded from the aforementioned answer, attenuation will go up with frequency (and length). And so for any specified maximum attenuation, the useful length of coax that will simply reduce as the frequency increases, but it will still work.

So if we're trying to determine the 'quality' of a coaxial cable, seems like to me that we should go for the one with lowest attenuation. The only other criteria for 'quality' of the coax appears to be the effectiveness of its shielding, and that too seems irrespective of frequency. So for instance LMR-200 would be objectively better than RG-58C (same outside diameters).

Lastly, it seems perhaps lesser metals in the braid/shielding would result in higher attenuation due to a faster increase in resistive losses, though I'm wondering if the comparatively large volume and surface area (as compared to the center conductor) wouldn't render the difference negligible. Otherwise if attenuation per unit length is a part of the specification then it seems to me the material will hardly be relevant in electric terms.

Question

Is there a frequency rating for coaxial cable?
How do I know if I have good coax or poor coax (for the same designation)?

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There is an EM limit to coax, which is where waveguide modes manifest -- that is, signals no longer propagate in the TEM00 (current lengthwise on core, opposite on shield, radial electric field, tangential magnetic field -- the standard coax field diagram). Which is to say, the distance between core and shield, or the shield diameter or circumference, is comparable to 1/4 wavelength or more (dimensions and wavelength fraction depending on mode). This still isn't a hard cutoff, signals still pass -- but they don't pass in the orderly fashion that we expect from coax in normal (below-cutoff) use, that is, the impedance and velocity factor are all over the place; plus mode conversion occurs (due to asymmetries in the cable construction or orientation, energy couples into orthogonal modes, where it gets absorbed, causing transmission notches, impedance peaks/valleys, etc.). Which is further "a thing" because not all modes are coupled by the core-shield connection.

More often, there is a practical limit to coax, which is the attenuation rate per length, which increases dramatically with frequency, particularly for the poorer quality / simply constructed kinds like RG-58.

Generally, larger coax has lower losses, right up to (or near, anyway) mode breakup. Cell phone and television transmitters for example use inch-scale conduit, for signals from say a few hundred MHz, to low GHz; the dielectric might be mostly air, with a loose plastic support structure to hold the core in place, maximizing core diameter and thus conductive surface. When your electrical bills correlate directly with transmission line losses, investing in such specialized, low-loss materials is desirable.

Or compare RG-58 to RG-174, basically the same stuff, scaled differently; the latter has significantly more attenuation/length.


By the way, there are proper and ready sources for all of these data. Understand that Wikipedia is at best an aggregator of knowledge. YouTube videos and random web articles are user created so depend entirely on who wrote them, from what sources, and why (and, sources are almost never given, so you have no way of checking how well they've represented it). Popular videos are rarely well sourced/cited, and well sourced/cited videos are rarely popular (and guess which one YT is likely to return from searching?). Volunteer systems, like Wikipedia, Stackexchange, etc., have varying standards (e.g. Wikipedia requires references, if not necessarily good ones; only some Stacks do); and generally "good enough" passes, subject to social norms (e.g. it's not worth correcting something if it's going to "rock the boat"). Which when it comes to complex topics that are opinioned far more often than they are understood, can be quite frustrating to see; take basically any EMC topic on here, for example.

Even worse, there's AI large language models, which are designed to mimic written language, with (currently) little to no basis of knowledge, nor capacity for inference upon that knowledge (it's no accident that GPT, etc. sourced answers are currently prohibited here). Needless to say, don't attempt to source information from such services. Use them for what they're good at: language itself, not the things language represents.

Whatever the matter; in this case (cables and connectors), standards exist, which manufacturers follow and customers expect. Simply go shopping for cables and connectors, and look up datasheets! Datasheets are primary sources; units are given; and you can find definitions if you need to.

You may need supporting info to understand some things (like, what the heck are the various impedances within a multiconductor cable?), or a deeper understanding to intuit them (aha, the impedances are between wires in different configurations, e.g. parallel (common mode) or inverse (differential) pairings; and the impedance, inductance and capacitance are related by fundamental constants, material properties and geometry). These are good directions for further research. A number of these theoretical topics are explored in depth in university courses, which you may also find helpful. (Not to try and read too deeply into your level of knowledge, of course.)

Of course, manufacturers themselves span a range of quality. They might simply lie on the datasheet, or not provide one at all -- or the supplier you're buying them through doesn't provide such. Buying things on generic marketplaces (like Amazon, eBay, Ali, etc.) is very much caveat emptor. The temptation to sort by price is a strong one, but a risk laid barest through these sources. There are industry-recognized channels to go through; to source a particular brand-name manufacturer, find their list of authorized distributors. Parts quality, like knowledge itself, is a traceable path of connections, relationships; and like knowledge itself, quality is an ongoing process, a verb: knowledge is not something you learn once, but understand that memory itself decays over time, the assumptions that underlaid it change, and it must be forever checked and re-checked for truth.

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  • \$\begingroup\$ So as a rule of thumb, if I'm using 0.195" coax (~0.0975" from core to shield), I should expect to be able to do up to roughly 30GHz (frequency for 4*0.0975" wavelength) before the multimode problems start to kick in? (ignoring attenuation). Also it's my first time hearing about it, so would it be a good simplified (dumbed down really) explanation to say that smaller wavelengths will start being able to bounce around in the cable causing all the problems you described? \$\endgroup\$
    – TrisT
    Sep 17, 2023 at 10:23
  • \$\begingroup\$ As for the second part of your answer, I understand your points. I mostly use youtube to find out what I don't yet know about - channels can cover a variety of topics, and even for a single topic they often present many peripheral ones that are incredibly helpful to know/learn alongside the main one. Wikipedia/articles to get a general sense, and I use platforms like this when I'm still having trouble crossing over to the math-heavy or extremely complex literature which often seems like it's written for the author to show off how intelligent they are as opposed to actually teaching the reader. \$\endgroup\$
    – TrisT
    Sep 17, 2023 at 10:31
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    \$\begingroup\$ Indeed, "waves bouncing between the walls" isn't a bad description of waveguide; for rectangular waveguide, the propagation is not so much straight-down but diagonally-back-and-forth, which gives the odd result that phase velocity is greater than group velocity. (Ideally we want them equal, so the wave arrives at the destination in-phase, i.e. pulse goes in, pulse comes out; this is a desirable property of coax.) \$\endgroup\$ Sep 17, 2023 at 18:54
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Is there a frequency rating for coaxial cable?

No, there isn't a frequency rating but, if you look into the data sheets of cables from reputable suppliers they will indicate what the attenuation figure is versus several spot frequencies versus cable lengths. Typical cross comparison: -

enter image description here

Image from Coax Guide by ABR industries found by googling "coax cable attenuation vs frequency".

How do I know if I have good coax or poor coax (for the same designation)?

You buy cable from a reputable source (as with any electronic part) and the cable you buy should be supported by a decent data sheet. Here's another useful resource on attenuation vs frequency. Typical example of some of the data in the document: -

enter image description here

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  • \$\begingroup\$ Seems I failed to consider cables would have datasheets. Most if not all the cable I've bought in my life has been from hardware stores or the internet and they came with no information attached - and the ones for which I found information had little more than voltage rating and so I never thought coaxial would be such a world of difference in that regard. Thank you. \$\endgroup\$
    – TrisT
    Sep 17, 2023 at 10:36
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I'm now thinking that no coaxial cable designation has any inherent frequency limitation. It's all down to attenuation and length.

For a given inner and outer diameter, at some high frequency, the coaxial cable will become multi-mode. That is, it will begin to transmit modes other than the TEM mode.

When this happens, the behavior becomes somewhat unpredictable, since the coupling between the modes can depend on details of how the cable is bent and how the exact coupling geometry at the launch point.

But once some signal is launched into the higher order mode, it travels at a different velocity from in the TEM mode, and the signal can become highly distorted.

To avoid this, for very high frequency signals, the cable diameter must be kept small. But this tends to increase the attenuation because of the reduced copper area leading to higher resistive loss.

The result is, as you search for cables rated for higher and higher frequecies (10, 20, 40, 50 GHz) the attenuation ratings unavoidably increase as well.

To answer your specific questions,

Is there a frequency rating for coaxial cable?

Yes. Cable intended for high frequency use will include a maximum frequency rating in the datasheet or catalog.

How do I know if I have good coax or poor coax (for the same designation)?

The RG designations are in fact references to very old US military specifications. If a cable is sold as RG-178, or whatever, then it isn't promised to achieve any better attenuation or leakage than what is specified in the RG-178 standard.

But you can choose from numerous cable types, either with RG designations, or proprietary ones, to find a cable with lower attenuation at your operating frequency, if you want to pick the "best" cable for your system. (Or you can pick the lowest cost cable that has low enough attenuation to meet your system requirements)

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