VSWR meter trap for the unwary

From time to time one sees discussion online about consistency of ‘measured’ VSWR at different power levels (on the same instrument).

A question often asked is:

I tune up at 10W and achieve VSWR=1.5, and when I increase power to 100W, the VSWR increases. Which should I believe?

The first thing to note is that good antenna systems SHOULD be linear, VSWR should be independent of power, it is if the system IS linear.

For the most part they are linear, even though many antenna systems contain elements such as ferrite cored inductors that may exhibit some small level of non-linearity in ‘normal’ operation.

Non-linearity caused by for instance saturation of magnetic materials, loss of permeability where the temperature of ferrite cores reaches Curie point, arcing of capacitors or other insulating materials is NOT normal linear operation of a GOOD antenna system. If high indicated VSWR at high power is caused by any of these effects, it is flagging a problem that requires attention.

That said, a significant non-linear element may be the VSWR meter itself.

A common, if not most common way to make these meters is to use a half wave detector to convert the direction coupler RF outputs into DC to drive an ordinary moving coil meter. These meters commonly assume that the detector DC output voltage is exactly proportional to the RF input voltage.

Lets look at the accuracy of that process.

Above is a plot of the detector output vs RF input voltage for a commercial 200W VSWR meter. The measurements cover input power from 10 to 100W.

The points appear to be in a fairly straight line, but as the curve fit shows, that line does not pass through the origin, there is a non-zero intercept of -0.1984V and so the DC output is NOT exactly proportional to RF input voltage.

If this detector was used in the type of VSWR meter where is ‘calibrated’ by adjusting a pot for full scale deflection from the forward coupler, then the reflected coupler is displayed to directly read VSWR from the meter scale.

Here is the problem: the DC output voltage is not simply proportional to RF input voltage, and the indicated VSWR will be sensitive to power level at which the measurement is made.

Above is a plot of the expected indicated VSWR  for a VSWR=1.5 load given the detector characteristic. You will note that at 100W there is a small error which becomes worse at lower power levels.

The use of a half wave detector on each directional coupler is very common, though not the only technique used.

It is possible to compensate for the offset in linear or digital electronics, but although fairly easy, it is not very often done.

Some examples:

In answer to the intial question, when indicated VSWR decreases at low power, the lower reading is probably invalid. Use the reading at higher power where the reading becomes independent of the power level.