The Coax Conundrum: Understanding Velocity Factor and Why It Changes Your Antenna Length
When you’re building an antenna, you quickly realize that the math on paper rarely matches the reality on the roof. One of the biggest culprits is the Velocity Factor (VF).
It’s the reason why a “half-wave” dipole isn’t actually a half-wavelength long in physical wire, and why your coaxial feedline behaves differently than a radio wave traveling through the clouds.
What is Velocity Factor?
In a perfect vacuum, electromagnetic waves travel at the speed of light (c), which is approximately 299,792,458 meters per second. However, once you introduce a medium—like the copper in a wire or the plastic insulation in a coax cable—the wave slows down.
Velocity Factor is the ratio of the speed at which an electromagnetic wave passes through a medium to the speed of light in a vacuum. It is expressed as a decimal or a percentage:
VF = v/c
- Vacuum: VF = 1.0 (100%)
- Solid Polyethylene (Coax dielectric): ~ 0.66 (66%)
- Foam Polyethylene (Coax dielectric): VF ~ 0.80 to 0.85 (80-85%)
- Bare Copper Wire: VF 0.95 to 0.98 (95-98%)
Why It Changes Your Antenna Length
When we calculate the length of an antenna, we use the formula for wavelength (λ):
λ = c/f
But since the signal travels slower in the wire than in a vacuum, the physical length required to “contain” that electrical wave is shorter. If you cut a wire to the theoretical vacuum length, it will be electrically “too long” and resonant at a lower frequency than intended.
The “End Effect”
Even for a simple wire dipole, we usually multiply the result by a factor of roughly 0.95. This accounts for the velocity factor of the wire itself and “end effect,” where the capacitance at the tips of the wire makes the antenna appear electrically longer than it is physically.
The Coax Connection: Dielectrics Matter
In coaxial cable, the velocity factor is determined almost entirely by the dielectric (the insulation between the center conductor and the shield).
| Dielectric Type | Typical VF | Why? |
| Solid Polyethylene | 0.66 | Dense material slows the wave significantly. |
| Foam Polyethylene | 0.80+ | Air bubbles in the foam mean the wave travels mostly through air/gas. |
| Air-spaced/Teflon | 0.90+ | Closest to vacuum performance; used in high-end or high-power lines. |
When does Coax VF matter?
For a simple feedline (connecting your radio to your antenna), the VF doesn’t actually change your SWR or the antenna’s resonance. However, it is critical in these scenarios:
- Phasing Lines: If you are feeding two vertical antennas to create a directional pattern, the cables must be exact electrical lengths (e.g., 90° apart – one fourth of 360° full wave cycle or 1/4λ).
- Antenna Stubs: If you are using a “matching stub” (a piece of coax used as a capacitor or inductor), the physical length is determined by the VF.
- Ugly Baluns: The length of the coax wound around the PVC pipe in an ugly balun determines the frequency where the common-mode impedance is highest.
Calculating Physical Length
To find the physical length (L) for a specific electrical wavelength, the formula is:
L = (c x VF)/f
If you want to calculate a 1/4 wave 75-ohm matching section for a frequency of 14.2 MHz using RG-11 (solid PE, VF=0.66):
- Vacuum 1/4 Wave: ~ 5.28 meters
- Adjusted for VF: 5.28 x 0.66 = 3.48 meters
Without accounting for that 0.66, your matching section would be nearly 2 meters too long!