So What’s Happening Inside My Coax? Part 2

Please read Part 1 if you haven’t read it yet.  This article picks up where Part 1 left off.

A couple of principles:

  1. Light travels slower through solids than it does through vacuum or air. Sometimes much slower It’s how optical lenses work.  And remember, light and RF are the same thing: electromagnetic waves of different frequencies.  Different colors, if you like.
  1. A wavelength is the physical distance (feet, meters, centimeters, etc.) between one wave peak and the next in an electromagnetic wave. Frequency is the number of waves that occur during a unit of time.  The speed of light (RF) is what relates wavelength and frequency.

In Part 1 we talked about electrical things happening in coax such as voltage, current, resistance, and dielectric loss.  Now we’re going to talk about the “light” in your coax.  The RF that’s flowing through your coax isn’t just in, or on, the copper surfaces.  It’s a beam of light that’s shining up through the dielectric layer.  (If you’re into extra reading, do a google search on “TEM mode.”)

Our Principle 1 says that light travels slower through solids.  That includes your RF and it includes the polyethylene dielectric layer in your coax:

  • If your coax has normal non-foamed polyethylene dielectric, the speed of RF through the poly layer is about 66% of the speed of RF (light) in a vacuum or dry air.
  • If your coax, like RG8X, has “foam” poly with lots of air bubbles, the RF speed is about 82% of the speed of light.
  • If you get the really fancy coax (Heliax) that’s mostly filled with air, RF runs about 95% of light speed.
  • Also note that there is a relationship between speed of RF and efficiency (loss). Generally speaking, the lower the speed, the lower the efficiency.  (There is more material for the RF to push through.)

In other words, it takes more time and effort for RF to get through an RG8 cable than it does in Heliax, or in free air.  The percentage of light speed in coax is called “Velocity Factor” or VF.  Every kind of coax or open wire line comes with a VF rating.  Mind you, that rating is not exactly accurate for every inch of coax that’s manufactured but it’s close enough for most uses.  So now the big question everyone wants to know:  What uses?

In your ham career you may want to connect multiple antennas together with a “phasing line” or need to use a quarter wave coax impedance matching section or want to avoid an unhappy length of feedline on a given band.  For all of these things you need to know the “electrical length” of the coax to achieve your goal.

As an example, let’s say you need a quarter wave of RG8 for some purpose on 40 meters centered on 7.2 MHz.  Remember the formula for a halfwave dipole?  (468/ f MHz). It’s 468 because a dipole has capacitance which makes it shorter than a free space half wave.  A free space half wave is 492/f MHz.  (Memorize that.)  So let’s calculate: 492/ 7.2 Mhz= 68.33 feet.  But remember, that’s a half wave and we want a quarter wave so: 68.23/2= 34.16 feet.  Now let’s apply our RG8 VF: 34.16 x .66= 22.5 feet.  That’s an electrical quarter wave of RG8 on 7.2 MHz.   If you used RG8X with its foam dielectric, it would be 34.16 x .82 = 28.01 feet.  See?  The higher the VF, the longer the electrical quarter wave length.  True open wire line with few spacers has a VF of about 99% so a quarter wave of open wire line would be nearly the same as free space.

An observation: If “hollow” coax is better than coax with solid dielectrics, why do we use the “inefficient” stuff?  One good reason: the solid stuff is very bendable.  You can bend it or flex it around your antenna rotator and the solid dielectric will maintain the position of center conductor in the center of the cable.  Foam coax, especially types with stiff, non-stranded center conductors, should only be bent with gentle radii because under stress the center conductor will slowly migrate through the foam dielectric and eventually short out to the shield.  The “hollow” coax like Heliax is really a corrugated copper pipe with either foam or helical/air dielectric.  It doesn’t take much imagination to figure what would happen if you flexed it back and forth.  It’s only used for fixed installations, like the W3GMS repeater antennas.

Conclusions:

  1. All other things being equal, the less solid stuff that is present in a feed line, the more efficient it is and the faster RF travels through it.
  2. The Velocity Factor (VF) is different for different dielectrics and configurations. It’s always a published specification and, for extra credit, there are ways of directly measuring it for a given piece of cable.
  3. When you need a specific electrical length of feedline, you need to use VF to calculate it.

Remember the beginning of Part 1 when I said there was a lot going on inside your coax?  Trust me, there still more.  But we’ll leave your coax alone for now.  😉

73,

Chuck NA3CW

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