Velocity Factor of Coax
“The velocity factor (VF), also called wave propagation speed or velocity of propagation, of a transmission medium, is the speed at which a wavefront (of an acoustic signal, for example, or an electromagnetic signal, a radio signal, a light pulse in a fibre channel, or a change of the electrical voltage on a copper wire) passes through the medium, relative to the speed of light. For optical signals, the velocity factor is the reciprocal of the refractive index.
The speed of radio signals in a vacuum, for example, is the speed of light, and so the velocity factor of a radio wave in a vacuum is unity, or 100%. In electrical cables, the velocity factor mainly depends on the insulating material.
The use of the terms velocity of propagation and wave propagation speed to mean a ratio of speeds is confined to the computer networking and cable industries. In a general science and engineering context, these terms would be understood to mean a true speed or velocity in units of distance per time, while velocity factor is used for the ratio…….“
The 3 paragraphs above were quoted directly from Wikipedia.org on the subject of “Velocity Factor”. For simplicity from this point forward, the term “Velocity Factor” will now be referred to simply as “VF”.
You are about to learn the facts and dispell the myths about the VF of coax. Many of you may disagree and argue with what you are going to read, but these are the facts. There are many articles by ham operators that use all kinds of big detailed (and well drawn) charts and fancy colorful graphs, and 20 different calculation formulae from other sources that they consider proof of their arguments about the importance of the VF of coax. But you will find no BS here, no charts or graphs or a bunch of formulae to play with in your calculator. Just the plain truth taken from the facts and plain simple common sense. Take it for what it’s worth. You can walk away laughing and still believing in a myth created by the coax manufacturers to sell you their product, or you could actually learn something and be a little smarter. The choice is yours.
I have received several emails from readers asking about the VF of coax that is used to construct coaxial antennas such as the Double Bazooka or coaxial Inverted “L” antennas. The calculating measurement formulae of those coaxial type antennas are designed and stated specifically for the frequency that the antennas will be cut and constructed for. But readers have asked about how the VF of different types of coax would change the measurement formulae.
The answer is very simple. The VF does not change the calculating measurements of the antennas. As stated above from Wikipedia, the VF is a measurement of the radio wave speed as it travels through a coaxial cable. It has nothing to do with the length of an antenna. Radio waves already travel at the speed of light. The VF of the coax determines how fast that RF signal will travel the length of the coax between the radio and the antenna.
ONE SMALL NOTE: Before we continue, I apologize deeply if I tend to be redundant at times, but there are so many people that just refuse to accept that this stuff is absolute fact. Psych doctors use a method that claims, “repeating the same thing often is said to have a slightly longer lasting effect on those who wear invisible aluminum foil hats”. Sigmond Freud once described this method as “an extra big crowbar to help pry open very tightly closed minds”. So please bare with me if I repeate myself often. It is not intended to annoy the smarter open minded readers who are just willing to learn something.
Thank you for you patience.
If a coax has a VF rating of 95, it means the coax has a dielectric insulator and shielding good enough to send the RF signal at 95% the speed of light to the antenna. So with the speed of light being 186,000 miles per second, that means that the RF will travel at 176,700 miles per second through this coax. Still pretty fast, wouldn’t you say?
Now let’s use a really cheap coax with a poor VF rating of 55. This now means that the RF signal will travel at 55% the speed of light which is 102,300 miles per second. I don’t know about you, but I am still thinking, “Pretty damn fast”.
The RF speed difference between those 2 types of coax seems like a lot. And I guess 86,000 miles per second really is a big difference. But to our slow brains and our radio electronics, it is still in much less than the “Blink-of-an-eye”. Do you honestly think that anyone could tell the difference in the delay? Is your brain so large and jam packed with super-dooper intelligence that you could hear the signal come in and say, “Yep! That was about 84,000 miles per second slower. He must have used a crappy coax.” But the most important question that you may ask is, how can this difference in speed have any effect on the length of the antenna?
Actually, IT DOESN’T. None. Zip. Zero. Nada. Keine. Aucun. 沒有. ничто….
These definitions are given in the context of radio and RF signal generation.
Velocity = Speed at which the signal travels compared to the speed of light.
Wavelength = The frequency spectrum length of each wave per second of a radio RF signal.
The only thing that determines the length of any antenna is, the frequency at which the antenna is tuned for. The VF of the coax will have no effect on the wavelength of the antenna. The VF can not change the length of a radio wave. Only changing the frequency can do that. One can not change the other. A Bazooka antenna at 28.500MHz made with a coax that has a 95 VF rating is 16 feet 2 inches. A Bazooka antenna at 28.500MHz made with a coax that has a 9 VF rating, is STILL 16 feet 2 inches. No matter what type of coax you use to build it, or what its VF is, it will always be 16 feet 2 inches. The reason (once again) is because the length of the Bazooka is determined only by the frequency it is cut for. P-E-R-I-O-D !
What About The VF of Digital Data Wire?
The VF of wire plays an important roll when sending a digital signal on a metal wire or fiber type cable. Digital data is not RF. It is bursts of digital packets at low power which requires the best conductor possible for the job it is expected to do. For example…. To send a good strong digital video signal between a BluRay disc player to the TV set, the best type of cable would be the HDMI cable. HDMI means High Definition Media Interface. What determines how good the HDMI is in carrying the strong digital signal would be the VF of the cable. An HDMI cable with a very high VF will carry a more steady and stronger digital signal between the 2 digital devices.
Another good example is the CAT cable that connects your computer to other computers, a network, or a digital interface such as an internet modem. The higher the VF rating of the CAT cable, the faster the digital data can travel between the computer and the modem. This provides a stronger signal for a consistant and strong digital signal.
The most important thing we must remember, and what so many antenna builders do not consider, is that when we are constructing a coaxial type of antenna like a Bazooka, the coax is not being used as coax. The coax is not carrying a shielded RF signal between the radio and the antenna, because the coax IS THE ANTENNA.
The coax used in making a coaxial antenna, is simply a radiator. The shield of the coax becomes the radiator and is no longer used as a shield to the center conductor. The shield of the coaxial antenna is now the actual antenna. It is just a wire radiating an RF signal. The VF of the coax used to build the coaxial antenna, has no meaning of any kind or in any way to the frequency in which the antenna is designed for. The total length of the antenna remains the same no matter what type of coax is used to build the antenna and no matter what the VF of the coax is.
Truth Of The Matter
When it comes to RF type ham radio coax, the term “Velocity Factor” was nothing more than a manufacturers way of advertising their coax quality compared to other types or brands of coax. It is just another specification they can add to the spec sheet to make it look good. But for typical ham radio RF applications, the VF really does not mean anything important. The speed at which your ham radio signal travels between your radio and your antenna will always be in “The-Blink-Of-An-Eye”, no matter what the VF of the coax is.
Most people use the VF rating as a way to judge how good the dielectric of the center wire insulator is, and how dense the braided shield is. A VF rating of 80 or higher usually means it is a pretty good quality of coax. A VF rating of 95 or better usually means the coax has a very low dielectric for low loss, and a good shield for a lower SWR on longer runs of coax. So it is reasonable to use the VF to determine the quality of the coax, but nothing more.
As I said before, the VF has important meaning for digital transmission of data between digital devices and media. But for the use of coax as an RF antenna for ham radio, the VF is actually pretty meaningless.
A Different Perspective To Ponder Over.
I have an interesting last minute way to look at this issue that just might put it into a more realistic perspective for the aluminum foil hat people. The rest of you will just get a little laugh from it. So after you read the next paragraph, stop and just think about it for a while.
Let us imagine for a few moments, that you could find the worse coax every made in the universe. This coax is soooooooooo pathetic, so horrible, that it actually has the horrible pathetic awful VF of just 1. YES I SAID JUST (ONE). This coax is so leaky and poorly shielded, you might as well just use 40 year old twisted up and kinked 22 gauge speaker wire as your coax. But no matter how horrible this coax is, a VF of just 1 would mean that your RF signal is still traveling at the lightning speed of 1,860 miles per second, which is equal to, (Are you ready for this) 4,776,480 miles per hour. Almost 5 million MPH. You could not blink your eyes that fast with a gun to your head and your life depending on it. Now seriously, (and excuse me but you really shouldn’t need to think very hard to answer this one,) but do you really think it actually makes any difference to the antenna that it took the RF signal, a vey slow Turtle’s pace of a whole whopping 5 MILLION MILES PER HOUR?
If you answered yes, then you need to get a dictionary and look up the term, “COMMON SENSE”. Because you don’t have any.