The effect of SWR on feedline loss ... a reminder

Now that we have a wider range of frequencies to use for phone (including, of course, AM) it is useful to review the effect of SWR on the losses of the feedlines we use with our antennas. All of this information is in the handbooks. You can also get a quick feel for it by observing the following:

If the SWR is 2:1, then, by definition (with some minor approximations), the net current (flowing through the conductors) at some points along the feedline is twice the net current at some other points along the feedline. The resistive loss per unit length of the feedline is four (4) times as high at those points along the feedline where the net current is twice as high: i.e., resistive loss ~ (I**2) x (R). Although it takes a little calculus to prove, this implies that the loss of the feedline per unit length in dB per foot is four times as high at those points along the feedline where the current is twice as high. The total end-to-end loss of the feedline in dB is a cumulative blend of the higher loss per unit length at the points where the net current is higher and the lower loss per unit length at points along the feedline where the net current is lower.

*Remember, the net current, at any point along the feedline, is the sum of the forward current and the reflected current. The forward current is the same at all points along the feedline, and the reflected current is the same at all points along the feedline. The net current, any any point along the feedline, depends upon the phase difference between the forward current and the reflected current. If they are in phase (but flowing in opposite directions) the reflected current substracts from the forward current. If they are out of phase (but flowing in opposite directions) the reflected current adds to the forward current. For example, if the forward current is 1 amp, and the reflected current is 0.5 amps... then the maximum net current is 1 + 0.5 = 1.5 amps. The minimum net current is 1-0.5 = 0.5 amps. The SWR is maximum net current/minimum net current = 1.5 / 0.5 = 3 (usually written as SWR= 3:1)

Going to the tables (e.g., 1997 ARRL Handbook, page 19.6 Figure 19.5), if the nominal (at 1:1 SWR) end-to-end loss of the feedline at the frequency of operation is 0.5 dB, then the actual end-to-end loss of the same feedline at an SWR of 3:1 will be 0.77 dB. [A 0.27 increase in end-to-end loss corresponds to a 6% loss of power reaching the antenna... which is, of course, not much in terms of the grand scheme of things]

Likewise, if the nominal (at 1:1 SWR) end-to-end loss of the feedline is 1.0 dB and the SWR is 5:1, then the actual loss of the feedline will be 2.0 dB. [A 1.0 dB increase in end-to-end loss corresponds to a 21% loss of power reaching the antenna... which is still not that much in terms of the grand scheme of things: i.e., 0.17 "S" units at the receive end of the link]

Best regards
Stu