Velocity Factotr

Transmission Lines and Velocity Factor

Keith Greiner

May 27, 2026

This is about the little red dot and the little blue dot on the Smith Chart diagram on this site.

The red dot represents the value that you obtained from a NanoVNA or the MFJ 259D and entered at the top of the Smith Chart tool.  It is the starting point for your calculations to find the L-network components needed to match your source (transmitter) with your load (transmission line or antenna).  

The blue dot represents the values that are 0.65 (65%) of a wavelength away from the red dot. It is an adjustment for the speed of the signal along a coaxial cable. We all know that light travels at 299,792,458 meters per second.  In school you probably learned the rounded number of 300 million meters per second, and you learned that the radio signals from your transmitter to a far-away receiver travel at the speed of light.  That’s true, and it’s the reason that it takes a moon-bounce signal 2.56 seconds to leave your transmitter, go to the moon and back.  However, the speed of the signal from your transmitter, down the coaxial cable to the antenna is less than the speed of light, by a factor known as the Velocity Factor (VF) or Velocity of Propagation (VOP).

Here are some common velocity factors shown as a proportion and as a percent:

First, based on the important dielectric:

Solid polyethylene      0.66     66%

Teflon                         0.685   68.5%

Foam polyethylene     0.78 to 0.85     78% to 85%

Air                              0.95     95%

And now by the type of cable:

RG6U 75Ω coax                                                         0.645   64.5%

RG58U 50 coax with polyethylene dielectric          0.66    66% 

Fiber optic cable                                                         0.67     67%

Open wire ladder line with air dielectric                    0.99     99%     

Using the SeeSii NanoVNA-F V2

I looked at a number of methods for calculating velocity factor, and found a majority that did not lead to a clear value. Many had incorrect information that did not apply to the SeeSii NanoVNA-F V2.  Here is list of step for this VNA. 

1.     Measure some RG6U at 25’ 2” or 7.671 meters

2.     Make sure the coaxial cable is open at the opposite end

3.     Turn on and calibrate the NanoVNA-F V2

4.     Connect the cable under test

5.     Set up the screen: Trace0 only

6.     Connect the coaxial cable to the NanoVNA on Port 1, S11Reflectrometry

7.     Open the main menu and select TDR (Time Domain Reflectometry)

8.     Set check to TDR ON. Notice how the x axis and changes to fractions of a second

9.     Set Low Pass Impulse radio button

10.  Select Velocity Factor to 100%

11.  Set the frequency range to 10KHz to 200 MHz

12.  Open the main menu to Display and set FORMAT to LOG MAG.

13.  You should see a minimum like the one showing in Figure 1.

14.  Set main menu Marker to search for minimum

15.  Observe the TDR display 

16.  The time for the signal to be sent and then return is 55.68 nanoseconds

17.  Half of that is the time for the signal to travel one way and that is 27.84 nanoseconds 

18.  Read the distance value in meters, from the right, two cells and down 1. 

19.  My reading is TDR = 8.35 meters.   

Figure 2 shows the same analysis but with "PHASE" selected.  

Let’s look at the physics.  The NanoVNA measures the time that it takes for a signal to travel from the VNA device, down the length of the coaxial cable to the open end.  It does this my measuring the time and dividing by two.  With the velocity factor set to 100%, the device calculates the distance is with the assumption that the signal is traveling at the speed of light.  In my example, the time for the signal to be sent, and then return is is 55.68 nano seconds.  Half of that, is 27.84 nano seconds. and that means the distance, at the speed of light is 8.35 meters.  But we know that the actual length is 7.61 meters.   If we go back to the NanoVNA menu where the velocity factor is entered, and enter 91.2% instead of 100%, the reported distance now becomes 7.61 meters – the correct value.  That adjustment value, (91.2%) is the velocity factor. 

We can now write an equation to solve for the velocity Factor.   

Measured_Length = Reported_Value * Velocity_Factor

Therefore, 

Velocity_Factor = Measured_Length / Reported_Value

Velocity_Factor = 7.61 / 8.35  = 0.9113 = 91.13%

That is unusually high for many coaxial cables and suggests that the dielectric is a foam construction with a lot of air bubbles.  My inspection of this cable suggested that is true.   It has an appearance of looking like foam, and it melts easily when a soldering iron is in the viscinity. 

Using the MFJ 295D

The MFJ 259 D antenna analyzer may also be used to find velocity factor.  Although it is no longer being manufactured, if you can get one, it is a helpful tool for quick analysis and the screen is more visible in an outdoor sunny environment.

Here are the steps I used to obtain velocity factor values on an MFJ 259D.

These steps require that the far end of the coax be shorted.  It is equally possible to have the coax open.  The frequencies found, and the result will be slightly different. That’s ok as variation is normal.

I followed the steps with the test length of coax and with the far end open, and obtained spurious results, so I shorted the far end and obtained excellent results.   The main method is to use the 259D to check for nulls at two frequencies near the resonant frequency of the coaxial cable.  The difference in wavelength is then compared to the physical length to obtain the velocity factor.  But you have to follow some specific steps that are not necessarily all that clear in the 259D’s manual. The analysis is made more difficult because the 259D’s mechanical buttons don’t necessarily make contact every time you push them.   

1.     Short the far end of the cable

2.     Connect the 259D to the cable.

3.     Power up the 259D

4.     Once it is stable, use the VFO buttons and dial, starting at the lowest frequency and increase frequency until you find a deep null in the Ohms analog meter.  Make note of that frequency and continue until you find a second deep null.  Note that frequency and reset the VFO to just below the first deep null.

5.     Press Gate and Mode until you see a menu item “Advanced Mode”. I found it helpful to use two fingers and press Gate slightly before Mode.  It may be different for you.

6.     One problem with the 259D is that the mechanical buttons don’t necessary engage reliably and you may need to do this step several times until you see “Advanced Mode”

7.     Press the “Mode” button until you see “Dist to Fault”

8.     Set the VFO to a frequency lower than you expect a null.  Scan upward in frequency until you obtain the first deep null.  

9.     Press the Gate button. You should see a flashing “1st ” and when you press it, it will change to “2nd”.  The flashing text indicates that if you press Gate, you save the value, “1st” or “2nd”, whichever is indicated. While your actions may vary, you will want to have the sequence, indicated.  

10.  Tune to the second null.

11.  At this point, the “2nd” should be flashing.

12.  Press Gate again, and you should see a message “Dist to fault”,  and some value in feet and “ x VF”.

Here are results of five tests with the 259D on the test cable.  Frequencies are in MHz and distance is in feet.

First Frequency           Second Frequency      Distance to Fault

32.726                         48.760                         30.6

32.56                           49.155                         29.6

32.604                         48.700                         30.5

32.692                         48.740                         30.6

32.381                         49.021                         29.5

The average is 30.16 feet.

We know that the physical length is 25’2”, and that is 25.167’

Now, the velocity factor is 25.167 / 30.16  =  0.834 

The MFJ 259D-based velocity factor of 0.834 falls within the published range for polyethylene foam dielectric coaxial cable. 

The MFJ 259D process is intriguing and simple.   It could actually be done (and probably has been done) with little more than a grid dip meter or a noise bridge.  The process is this.  The 259D uses a grid dip process to identify two frequencies.  You search the entire range of 259D frequencies for two deep nulls that are close to each other.  Lets say you find that  F1 =  32.726 and F2 = 48.760.  Then we find the difference between the two.  Fd = 16.034

Now, we have a formula. 

VF = (2 L FD)  / c

Where: 

   VF = velocity factor

   L = physical length in meters

   Fd = the absolute difference between F2 and F1 in Hz  

   c = is the speed of light in meters per second 299,792,458 meters per second

In this example, 

VF = (2 * 7.61 * 16034000) / 299,792,458 = 0.81

How might you use the velocity factor?   Here are a few ideas..  

Impedance at a distance

We know that a half wavelength of coaxial cable returns an impedance value that is the same at both ends.  If you have a 75-ohm dipole up in the air, and you want to check the impedance, install a coaxial cable that has an electrical length of ½ wavelength.   That is, calculate the wavelength  𝜆  = c/f   = speed_of_light/frequency, divide it by 2, multiply by the velocity factor, and cut your coax to that length.  I’d cut a little long and trim using a NanoVNA. Double check everything, and you should be able to obtain a reasonable indication of the impedance up in the air.  

Quarter wavelength piece

Now that we know the velocity factor, we can find the length of a quarter wavelength matching stub.   Let’s say we want a quarter wavelength of this coaxial cable to operate on 14.040 MHz.

Since v = f * 𝜆    then   𝜆 = v / f    =  299,792,458 / 14,040,000  = 21.35 meters for one wavelength.

One quarter wavelength = 21.35 meters / 4   = 5.34 meters 

Adjusting for the velocity factor we have 5.34 meters * .9113 = 4.86 meters.  And that is, 15.94 feet.   

Other lengths 

Use the table at this link to find the lengths of coax needed to match the impedance found using the NanoVNA.

Phased arrays

If you put two vertical antennas 180 degrees apart and make sure that they are fed with in-phase signals, based of course on cable electrical length, then the phased array of two will hve strong signals at right angle to a line between the two antennas. 

Locating line breaks

This method may be used to locate line breaks.  Knowledge of the velocity factor is critical when  attempting to locate line breaks.