Transformers are used to obtain both the current and voltage samples in this design, whereas many use a capacitive divider for the latter. Both transformers have windings of 20 turns. The detector diodes are HP 5802-2800.
The parallel line arrangement, seen in some s.w.r. meters, has the disadvantage of the sensitivity falling with frequency so unless one is interested in only a small range of frequencies (one amateur band for instance) it can't be calibrated in terms of power.
The scheme used here has reasonably constant sensitivity over a 10:1 frequency range, about 3 to 30MHz.
Two examples are shown. One has a maximum power at f.s.d. of 100W and uses a meter movement from a commercial s.w.r. meter. The sensisitivity pot is calibrated in power for f.s.d. The other has two switched ranges, 30 and 300W f.s.d., the scale being calibrated in watts.
This is a simplified description of how these meters work.
All the meters you come across work on the same principle although the details of voltage and current sensing may be different.
In all these meters the forward power indication minus the reflected power indication gives the power dissipated in the termination (the load/antenna or whatever.)
When a line is terminated in a resistance equal to its characteristic impedance (the matched condition) the voltage on it and current in it are in phase.
In the meter we arrange that the two voltages, one is the voltage sample and the other is a voltage derived from the current sample, are equal in magnitude under matched conditions.
One of the diodes produces a d.c. from the sum of the voltage and current samples. The other diode produces a d.c. from the difference between them.
The sum represents the forward power, the difference represents the reflected.
Since the two voltages are equal under matched conditions, the difference between them is zero.
With the switch in the forward position the meter reads full scale. With it in the reflected position it reads zero.
The next cases to consider are with the line open or shorted (infinite s.w.r.)
Taking the open case first.
There is no current in the line, so we just have the voltage. The meter will read full scale in both the forward and reflected positions of the switch as there is no voltage proportional to current to subtract. This means "all the power is reflected". The difference between the forward and reflected will be zero, therefore zero power is dissipated in the termination.
Taking the case of a shorted line.
There will be no voltage on the line, just current in it. There will be no voltage sample to subtract and the meter will again read full scale in both forward and reflected positions.
Under intermediate mismatched conditions the magnitudes (and phases) of both incident and reflected voltages and currents will add and subtract to give intermdiate power readings. Their difference, again, being the power expended in the termination.