Amateur Radio Stack Exchange is a question and answer site for amateur radio enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I would be very interested in any real life data collected by fellow radio enthusiasts in regard to the velocity factors stated for milspec coaxial cable. e.g., RG213, RG58.
How accurate and consistent are these stated velocity factors in the real world?
"military-grade" doesn't actually mean something.
There's various military standards, but not everything used by a military adheres to them.
In the case of MIL std. cabling / connectors, the respective standard will specify maximum tolerances. There's literally hundreds of different standards. If your cable's manufacturer knows that their cable is better than that, the cable datasheet will tell you.
So, not really a question we can answer at all: You need to read your data sheets or standards, whatever applies.
According to Wikipedia, the velocity factor of coaxial cable is given by:
$$ VF = \frac{1}{\sqrt{\varepsilon_r\varepsilon_0}} $$
where $\varepsilon_r$ is the relative permittivity, a material property of the insulator, and $\varepsilon_0$ is the permittivity of vacuum, a constant.
So the velocity factor is determined by a single material property of the insulator and not dimensions. (An earlier edit of this post incorrectly stated that dimensions were involved, sorry.) I would think that for coaxial cable with a solid (non-foamed) insulator, the velocity factor is fairly consistent, because the uniformity of polyethylene from commercial suppliers is quite good. The velocity factor of coaxial cable with foamed polyethylene insulator may not be as consistent, because its velocity factor depends upon how much air gets into the foam. Foamed polyethylene, as used in popular coaxial cables such as LMR-400, makes the cable have lower loss than solid-dialectric cable.
I haven't been able to find specification documents defining RG-213, RG-8, or RG-58, but all the examples for sale on the internet that I saw have solid polyethylene dialectric, so the military specifications probably require solid PE to be the insulator. So I would think that the velocity factor of milspec coaxial cable is fairly consistent.
It's hard to say how accurate the quoted velocity factor of a given coaxial cable is. You might get some anecdotal evidence from hams, but the accuracy of their measurement techniques is unknown. If you're counting on a piece of coaxial cable to be a certain electrical length as part of an antenna system design, and you require a certain degree of accuracy, then you're probably better off cutting the piece long, measuring its electrical length with the help of an antenna analyzer or VNA, and then trimming it to be exact. Once you've done that, calculating its true velocity factor would be easy, and then you'd know how accurate the specification is for your piece of cable.
During manufacturing the tolerances defined as part of any specification will be asserted by choosing lots at random and testing to see if the equipment meets or exceeds those tolerances. No matter what specification, military or otherwise, this doesn't change.
As to whether or not a claim made on a random hunk of cable you get from an online supplier actually meets those specifications is a different question.
The received wisdom is to buy well-known brands from reputable vendors. That is, we are using standards how they are intended: by allowing us to trust that some equipment is within some tolerances because there is a chain of trust that asserts that it is. Sometimes equipment goes through third-party certification to strengthen this chain of trust. That is, the specifications result in equipment that is accurate and consistent in the real world, unless you have a way of proving otherwise for some specimen.
If, for some reason, we are unable to trust this process, or we have doubts about the claims on some equipment, then we have to test empirically. For coax and other hookup cable this was often a complicated and expensive process for the Amateur. And it wasn't always clear if the tools and techniques available to us would allow us to determine out-of-spec cable in any greater detail than the printing on the cable.
This has changed quite a bit with tools like NanoVNA and others, which allow you to determine the electrical length of a piece of cable quite accurately. (Well, accurate to some tolerances for this test equipment; like turtles, tolerances go all the way down.) With that information and some clever software (or doing the complex math by hand) deriving the actual Velocity Factor to a pretty reasonable degree is well within the grasp of most Amateurs with an interest in the subject. At least for this hunk of cable in your possession.
So, even if I did all that for some RGXX/U I had in my possession it wouldn't help you much at all. Even if it was way out of tolerance this does not speak to a different lot at a different time from the same factory. If it is important to you, for a specific project, you will have to measure it yourself.
