So What’s Happening Inside My Coax?

This article is rather long but it may be worth your time.  If you’re in a hurry, jump to Conclusions.

A coax cable is like a steel beam in a bridge: You don’t see anything moving but there is a lot going on inside.  (There is outside, too, but that’s another article.)

Coaxial cable or “coax” is a special kind of shielded cable that is optimized for use as an RF transmission line.  It has an inside wire (the center conductor) and metallic shield “tubing” (copper braid, foil, or even pipe.).  The insulation that separates the center conductor from the shield is called the dielectric. Dielectrics can range from solid or foamed polyethylene, to Teflon, to nothing at all but air except for occasional spacers.  The electrical characteristics of the cable are primarily established by the diameter of the center conductor, the thickness and type of the dielectric, and the inside diameter of the shield “tube.”  We’ll save the finer points of that for another chat, too.

Coax comes in several major flavors: flexible, semi-rigid, and rigid.  Most of us are using flexible coax which is handy because, well, it’s flexible, can be routed almost anywhere, it’s reasonably priced, and can be effective when used the right way.  It can also be a long skinny dummy load when used the wrong way and, yes, you can actually melt it.

All transmission lines, be they coax, twisted pairs, or open wire feeders, have a characteristic impedance in “ohms”.  It’s not their resistance.  It’s the ratio of voltage to current (volts per amp) that the cable tries to establish and at which it passes RF power with the least loss.  If you feed a signal into one end of the cable and connect a resistor equal in value to the characteristic impedance across the other end as a load (like a 50 ohm resistor for a 50 ohm line), the wave from the source will pass cleanly through the line to the load and will disappear as heat in the load.  If you replace the load resistor (the dummy load) with some other load with the same voltage to current ratio, like a resonant antenna or a well-adjusted tuner, you get the same effect.  The wave passes cleanly from source to load with minimum loss.  This is a “matched” load.

All coax lines have loss.  It turns your RF into heat.  Coax lines have resistive heat loss in their copper conductors just like any other wire. The more current, the more heat.  But unlike 60 Hz house wiring, coax at radio frequencies has dielectric loss, too.  The dielectric plastic heats up.  The more voltage, the more heat.  Keep this in mind for the next concept.

If you connect a load that is other than a matched load, not all of the signal will be absorbed by the load on the first pass.  Some of the wave will be reflected back toward the source.  The outbound wave (forward power) “collides” with the reflected wave coming back.  There will be a place, or places, in the line where the forward voltage and the reflected voltage add to each other.  In that zone will be a voltage that’s higher than the original signal.  Elsewhere in the line the current waves will add to each other and will create a zone, or zones, of higher current than the original signal.  The zones of higher voltage will have higher dielectric heating than you would get with a matched load.  The zones of higher current will have more resistive heating than you would get with a matched load.  The greater the mismatch (higher SWR), the greater the heating and the less transmitted and received signal you’ll have.  With a high SWR when running high power, the coax could actually melt or arc.  Or when passing tiny received signals, loss means they never make it to your receiver.

And it gets worse.  Because of a magnetic phenomenon called “skin effect”, the higher the frequency, the more the current crowds to the surface of the conductors.  It’s like cramming four lanes of traffic into one.  This increases effective resistance, hence more heat.  The dielectric also heats more at higher frequencies.  The RF voltage causes the molecules in the plastic to twist back and forth.  The higher the frequency, the more twisting and the more heat.  A cable that works fine on the low bands might be a long black dummy load at 2 meters depending how well matched it is.  In coax, and especially at high frequencies, bigger is better even when running a matched load.  Because of dielectric loss, UHF TV stations running real power use coax made of concentric pipes with nothing by dry nitrogen and a few ceramic spacers to keep the center conductor centered.  No plastic, no loss.  Big pipes, lots of surface, less loss.  When you’re running 300kw, it matters.

Conclusions:

  1. When using coax you want low SWR in the line. Having a tuner that makes your transmitter happy with a crummy load doesn’t change the SWR in the line.  The RF that turns into heat is gone.
  1. The higher you go in frequency, the bigger the coax needs to be. A hundred feet of RG58 might be fine on 75 meters but will waste a lot of your 2 meter signal, even more if the SWR is greater than 1.5:1 or so.
  1. If you plan to use a non-resonant antenna, or the same antenna on several HF bands (like I do), you’ll want to seriously consider using open wire line which has a small fraction of the loss of coax. If a feedline has zero loss, high SWR means nothing.  Prior to the flood of WWII surplus coax hitting the market, hams used open wire line.  They rarely if ever measured SWR even if they knew how or cared.  It just didn’t matter.  The loss of open wire line isn’t zero.  But, to put it in perspective, while coax is rated in db loss per hundred feet, with open wire it’s more like db loss per mile.  At one time I multi-banded an 80 meter dipole, fed with coax.  Pretty dull results.  I converted to open wire.  My receiver LIT UP.

Thanks for staying awake.  See you on the air!

73,

Chuck, NA3CW

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