Generally bridge type SWR Meters are used for Antenna tunning for low power applications. The thru-line or in-line SWR meters are not sensetive enough for QRP applications.

Given below is a modified circuit for a sensetive thru-line SWR meter for HF.

 Notes:

• LM324 is a single supply Quad Op-amp. Use any two op-amps.
• Any Ge diode will work fine for complete HF range.
• Any VU type audio u-meter having 100 to 400 u-amps full scale deflection will work fine.
• Coupler is made of about 8" long RG-58 50 ohms coaxial cable. Take about 10" piece of RG-58 cable. Strip-off the rubber jacket. Push braid inside from both ends to loosen it. Remove the braid carefully from the cable. Put two thin coupler wires, 180 degrees apart and tape them on the cable as shown. Put back the braid and cover the cable with heat-shrink tubing to complete the antenna coupler.

Micro-meter calibration for SWR markings can be done in the following way:

Assuming that full scale deflection current of micro-meter is A,

• SWR                             Meter current
• 1.0                                0.0A
• 1.5                                0.2A
• 2.0                                0.33A
• 3.0                                0.5A
• 4.0                                0.6A
• 10.0                              0.818A
• Infinity                           1.0A

For example, for 400 micro-amp full deflection meter, SWR 2.0 point on the meter scale will be .33 X 400 = 132 u-amp. For SWR 3.0, this will be 0.5 X 400 = 200 u-amp. etc.