The current transformer TX1 samples the load current, and the voltage transformer TX2 samples the load voltage. In TX1, the load current flows through N1s and induces the load current sample in N1p. Because N1s consists of 1 turn and N1p consists of 10 turns, the current through N1p is 10% of the load current. In TX2, the voltage across N2p is the load voltage and induces the load voltage sample across N2s. Because N2p consists of 10 turns and N2s consists of 1 turn, the voltage across N2s is 10% of the load voltage.
Resistors R1 and R2 configure the circuit so that V_F_0 equals 10% of the forward voltage and V_R_0 equals 10% of the reflected voltage.
FT82-43 ferrite toroids are used for the transformers. Type 43 ferrite material is the optimum material for the HF bands due to its broadband capability in transformers. The inductance per winding decreases with respect to frequency and limits the reactance values of N1s and N2s at high frequencies but allows N1p and N2p to have high reactance values at low frequencies. The FT82 size is larger than that of the toroids used in QRP SWR meter designs due to the need to handle larger amounts of power. Applying excessive power to a toroid results in excessive heating and flux density, either of which may affect toroid properties or even cause damage to the toroid. However, the use of a larger toroid requires a longer length of wire in the primary windings of the transformers. The use of longer wire in the windings results in more stray capacitance between windings and interferes with the proper operation of winding N2p, which is designed to have an impedance of 10,000 ohms.
For the tradeoff between maximizing current-carrying capacity and minimizing stray capacitance between windings, #20 wire is used in the secondary windings that carry the full load current. Because the primary windings need minimal stray capacitance between windings rather than the maximum current-carrying capacity, #30 wire is used.
R1 and R2 both need to be at 50 ohms. In QRP SWR meters, almost any resistors are suitable. Because the QROP Meter must be able to handle more power, I recommend using 51-ohm 2W resistors for R1 and R2. If you prefer the convenience of Radio Shack, you can also use a group of five resistors in parallel to gain the 2W capability: four 220-ohm .5W resistors and one 680-ohm .5W resistor.