Fast Video Covers Coax Velocity Factor

We once saw an interview test for C programmers that showed a structure with a few integer, floating point, and pointer fields. The question: How big is this structure? The correct answer was either “It depends,” or “sizeof(struct x).” The same could be said of the question “What is the speed of light?” The flip answer is 186,282 miles per second, or 299,792,458 metres per second. However, a better answer is “It depends on what it is traveling in.” [KB9VBR] discusses how different transmission lines have different velocity factors and what that means when making RF measurements. A cable with a 0.6 velocity factor sees radio signals move at 60% of that 186,282 number.

This might seem like pedantry, but the velocity factor makes a difference because it changes the actual measurements of such things as dipole legs and coax stubs. The guys make a makeshift time domain reflectometer using a signal generator and an oscilloscope.

Working with more manageable imperial units than miles per second, they compute the speed of light is also 11.78 inches per nanosecond. Using that number and their scope, they can use the time a pulse takes to travel its length to compare the cable’s length to the measurement. A 25 foot piece of coax read out as 24.96 feet. A commercial instrument read out 24.87 feet. Pretty good. A time domain reflectometer can also find problems in coax and also tell you about where the problem is.

Keep in mind that these guys are ham radio operators and probably not physicists. So for example, when he casually says that electrons and subatomic particles travel at the speed of light, you might want to fact check that. However, from the radio perspective they know what they are talking about. For the record, we aren’t physicists either, but we are pretty sure anything with mass  such as an electron can’t quite get up to light speed.

The key to building a good time-domain reflectometer by the way lies in fast rise times on the signal injection. We’ve seen quite a few designs over time. If you think the speed of light measurement is part of the “round Earth conspiracy” feel free to make your own measurements.

14 thoughts on “Fast Video Covers Coax Velocity Factor

  1. From what I understand, electrical current in a wire travels fast (something like 0.6 of the speed of light in a vacuum) but it does so by transferring energy from one electron to the next rather than by a stream of elections zooming through the wire at such speeds (more like Newton’s cradle than a marble race, of for that matter more like a sound wave than a stream of air).

    What I am curious about is when you get up to RF frequencies and start talking about transmission lines and waveguides, is the signal really electric at that point or is it photons/EM waves? Or is it both? If it’s both is there one of those elegant brain-warping symmetries such that the propagation speed of the energy through one electron knocking into or repelling the next one down the line is always the same as the propagation speed of a photon down the waveguide?

    Can anyone point me at a good readable text (not a video, actual text) that explains this to someone for whom giant hairy equations do not just click into place?

    1. The nature of the signal propagation doesn’t change as the edge rate of your signal increases. When the rise/fall time gets to a few times the propagation delay, then your reflected signals are getting large enough to be significant. i.e. transmission line.

      I usually think of the wave front carrying information much like a radar, If you have a long piece of cable, the source has no idea what’s on the far end. Similarly, the far end don’t know about the voltage at the source. Only when the wave get reflected, it would know if it is shorted, opened or have an impedance. The reflected wave encodes the info in the voltage level (radar signal strength). If the far end impedance is same as the medium, you don’t get a reflection.

    2. ” is the signal really electric at that point or is it photons/EM waves?”

      There is no real difference between the two. At any frequency above zero, the signal can be described in terms of EM quanta/waves. Calling it “electricity” is just giving it another name.

      The fields of the electrons interact with each other by the EM field, which means it’s photons that are passing the signal energy along.

      1. Beg pardon Luke,
        Are you claiming there is no net transfer of electrons (ie including their mass, just massless photons) eg from the negative rich electrode to the positive in such things as electrochemical cells which plate out metals etc ?

          1. Luke probably meant that the interaction between two electrons is mediated via a photon. Which is equally right and pointless in this context. The dielectric properties (imagine electric equivalent to viscosity) do not affect the invisible & almost non existing intermediate photons, but those hefty and large electrons.

    3. > electrical current in a wire travels fast

      This is not quite correct – should be “CHANGE of electrical current in a wire travels fast”. What you’re talking about is signals and yes, even at RF frequencies the signal is carried by electron flow, but the signal is a change in the electron flow, rather than the electron flow itself, and thus propagates at a speed much higher than the electron flow.

  2. Thanks :-)
    Interesting post, timely for me as discussions on few forums recently in relation to Electro Motive Force (EMF) and thus the strength of the electromagnetic field from a moving charge (whether firmly relativistic velocity or not), some quite vocal even from an emeritus maths professor and a physics ‘theorist’ without formal training making dubious linguistically challenged statements muddying the waters or just plain wrong !

    This link describes the Velocity Factor (Vf) in more detail for different wire types given a Vf even close to c can be achieved :-)

    That growing number of people though still small, who claim electromagnetism is fully dependent on relativistic effects namely special relativity ie. Length contraction of the electron’s wave function just can’t qualify it with comparative math. The logic suggests if Special Relativity (SR) were the key factor in electromagnetism or even some change in Vf then logically different Vf would have to change the force and of course this should be measurable and with modern Metrology to high precision & high repeatability too…

    What has been stated/claimed is the electron’s wave function contracts (even at 60% of c) for a given fixed length of wire as Vf approaches c then charge imbalance due to that contraction (increasing no. of electrons) in that fixed length of wire goes up ie. More electrons in a given length (for progressively higher Vf) than the number of positive charges which obviously can’t move are not charge balanced. Therefore one would expect with higher Vf (for different wire topologies) the electromagnetic force would be proportionally higher (specifically by the Lorentz length contraction math or some ratio of it). Seems reasonable but, Lorentz’s correction isn’t around in the physics for that math and no history of noticing/measuring it – a Nobel perhaps ;-)

    Ie. The discontinuity in understanding this is Ampere’s law doesn’t include any term for Lorentz or Vf yet has been shown to be accurate such as in solenoid and motor designs. Therefore the Lorentz contraction either doesn’t apply Or if it does to wave function it’s cancelled out by the other (more abstract) SR effect of proton/electron interaction in the wire Or something else entirely. Yet the number of e- charges delivered per unit time should be a factor for force. I have a lot on my plate and possibly missed something but, I’m pretty sure SR is Definitely Not key to electromagnetic force at its base, maybe a (small) correction factor though and it should be small since it hasn’t ever cropped up Or has it and if so where ?

    For example take two cases:-
    a. 1. Wire with lowest Vf vs 2. Wire with highest Vf
    b. Construct solenoids with exact same number of turns in 1 & 2
    c. Place in experimental setup with same current and calibrated field strength meter.
    d. Would there be any difference And is it proportional to Vf difference if at all, if so then Ampere’s law incomplete.

    FWIW and easier both solenoids could instead be identical and connected in Wheatstone arrangement with corresponding sensors for tightest differential measurement thus allowing (physically too) for interaction eg wires leading to both identical solenoids fed by very different Vf lengths in the bridge series elements of suitable wire then the overall Vf must be controlled by the series section alone and swapped around as needed for best experimental method :-)

    Of course it’s difficult to wind both solenoids the exact same way (for wide Vf) as some wire topologies bulkier. So another way to extend the experimental method is to dope the wire alloy to get some different Vf for same diameter and same resistance eg dope both copper alloys with two elements converging on same DC resistance eg such as silver also but, with one dopant affecting Vf far more proportionally than it does resistance. Though I prefer the series approach then easier to go from Eg 70% c to even 95% c then the difference of any resulting force far easier to capture ostensibly even by the amateur with rudimentary commercial grade instruments…

    Anyways, here’s a couple of links and papers relevant:-

    If someone can’t get a relevant paper due to paywall, send me msg through quora as linked from my name on here and I can send it or link to my temporary blind link copy if copyright allows. On reflection after a review of two above links, maybe the clue is in a conjunction of links above, if so then (*grin*) it squarely refutes the claim of some dweeb elsewhere that electromagnetism is due to special relativity and length contraction, wonder if it’s obvious ;-)


  3. Painful to watch that video at 3:00, switches from miles to something more palatable aaaaand …
    remains in imperial, almost as if intentionally inflicting the slowness of it on himself.

    You know, done in your head using 300.000 km/sec for speed of light ->
    300.000 m/ms = 300.000 um/ns = 300mm/ns (= whatever 0,3m/ns, 30cm/ns, etc)

    1. Yeah, but it’s not 300,000 km/sec. It’s 299,792,458 m / s. You’re 207.5 km/sec off.

      A foot per nanosecond is close enough, and easier to remember and say than “Thirty centimeters per nanosecond”. The metric system is nice on paper, but it’s really built for people who speak Portugese and the like, so they can pronounce twenty syllables per second.

  4. You got it roughly right.

    It’s the electrical field what moves at 0.6 of the speed of light (akin to the forces the balls in Newton’s cradle exert on each other).

    The higher the frequency, the more the magnetic component counts (think Maxwell’s equations: you have derivatives in both directions, so the rate of change of the electric field induces a magnetic field and vice versa) — it becomes more electro-magnete-y.

    Likewise, the higher the frequency the more photone-y

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