VSWR expressed as 1:n

At Expression of VSWR as a simple decimal real number I put the case for expressing VSWR simply as a real (ie a decimal) number rather than in the ratio form.

Lets remind ourselves of the meaning of VSWR (SWR).

(Terman 1955) gives a meaning for the term SWR (or VSWR).

The character of the voltage (or current) distribution on a transmission line can be conveniently described in terms of the ratio of the maximum amplitude to minimum amplitude possessed by the distribution. This quantity is termed the standing wave ratio (often abbreviated SWR)…

Standing-wave ratio=S=Emax/Emin

Terman has not dealt with the complication of short lines and lossy lines.

Note that the use of capital E implies the magnitude of voltage, so Emax/Emin must always be a positive number greater than or equal to 1.0 under that definition. Under that definition (and it has shortcomings), VSWR expressed as a ratio of m:n (and n is usually 1), m MUST be equal to or greater than n.

Screenshot - 20_01_16 , 11_35_41

Above is an extract from the brochure for Icom’s newest offering, the IC-7300.

Note the specification of VSWR 1:1.5 or less… is Icom dumbing down to make its buyers more comfortable?

It is not an isolated instance.

Screenshot - 20_01_16 , 11_39_50

It is not new, above is an extract from the user manual for the IC-7410. Did you spot the nonsense? Now in this manual, they sometimes mention a ratio greater than unity, they are inconsistent… and that in itself indicates a lack of quality in their product.

It is common in ham radio that hams bandy terms about that they don’t really understand, and VSWR is probably at the top of the list for new and old hams.

In fairness, it is really easy to get all this confused… really easy if you don’t have a clue what the term really means.

My ARRL Handbook (Silver 2011) does contain a meaning for VSWR buried in text on p20.4, so insignificant to not justify an entry in the index:

For a lossless transmission line at least 1⁄4-λ long, the ratio of the maximum peak voltage anywhere on the line to the minimum value anywhere along the line is defined as the voltage standing-wave ratio, or VSWR.

The explanation is restricted to lossless lines, avoiding the problems of the real world where VSWR in those terms cannot be the property of a single point on a practical lossy transmission line… but the concept given is correct as given… in a theoretical lossless world.

The text goes on to say:

Since SWR is a ratio of maximum to minimum, it can never be less than one-to one. In other words, a perfectly flat line has an SWR of 1:1.

It is probably not as clear as my statement above Under that definition (and it has shortcomings), VSWR expressed as a ratio of m:n (and n is usually 1), m MUST be equal to or greater than n.

References / links

  • Silver, H Ward  ed. 2011. The ARRL handbook for radio communications. 2011 ed. Newington: ARRL.
  • Terman 1955. Electronic and Radio Engineering: McGraw-Hill New York.