Topband: Re: Coax capacitor losses

[John Kaufmann]

Let me update my earlier post about losses in coax lines, where I cautioned people about using them as reactance elements (capacitors or inductors).  If you are familiar with the Smith Chart, you can calculate the loss resistance in a section of line, based on its length and its loss specs (dB/ft or dB/m).  Then you can determine whether or not you will have a problem in your application.

There is a section in the ARRL Antenna Handbook entitled "Line-Loss Considerations with the Smith Chart".  In my copy of the Handbook (17th edition, 1994) it's on page 28-6.  It may be different (or perhaps not even present) in newer or older Handbooks.  It describes how to do Smith Chart calculations taking line loss into account.  If you follow this, you can see that lossy coax used as a capacitor (or inductor) will always produce a series resistance along with the desired reactance.  The amount of resistance may or may not be significant, depending upon the application.  Low-loss coax, like large-diameter hardline, is best while the RG-58 and RG-59 types of lines are worst.  Antennas with low feedpoint resistance are the most sensitive to resistive loss (just like ground loss in verticals).  

In general, the coax line loss is greater when the line is longer (up to 1/4 wave or more) and when the reactance being created ends up on the right half of the Smith Chart.  It's worst when the reactance is near the 3 o'clock position on the chart.

The measurements I've done on coax line loss resistance (using my General Radio R-X bridge) agree quite well with the predictions from the Smith Chart.  In general, though, you'll need a pretty accurate instrument to measure the resistance because it tends to fall in an impedance range where the commonly used antenna analyzers are less accurate.

73, John W1FV