SWR Meter Session 3 Learn about Transformers and SWR Meters Forward Waves on Transmission Lines ZL = Zo (Characteristic Impedance) I Port 1 Port 2 F + V Transmitter ZL Load - Zload = Zo (No reflected wave) Zo = 50 Ohms There is only a forward wave. V=+IZo SWR = 1.0 (No standing Wave)

Reverse Waves on Transmission Lines ZL = Zo (Characteristic Impedance) SWR Meter Reverse Waves on Transmission Lines ZL = Zo (Characteristic Impedance) I Port 1 Port 2 R + V ZL Transmitter - Here, ZL = Zo There is only a reverse wave V=-IZo SWR = 1.0 Current now reversed

Normally ZL not Equal to Zo Forward and Reverse Waves on the Line SWR Meter Normally ZL not Equal to Zo Forward and Reverse Waves on the Line I Port 1 Port 2 + F R V Transmitter ZL Load - ZL not equal to Zo Standing Waves exist Forward wave reflects at load

SWR Meter Things to note in tests Forward wave power always greater than reverse wave power Forward wave power fixed for constant Tx Power Reverse wave power = 0 for ZL = Zo SWR 2:1 for ZL = 100 or 25 Ohms Meter actual measures Voltage and Current on the line SWR must be found from VFWD and VREV on the board

Kits and Parts “meter” T1 samples current T2 samples voltage RF appears at Diodes Diodes act as detectors Vrev and Vfwd measured SWR Calculated or Meters Calibrated

Marking the Transformers Two Transformers Mark the red dots as shown This is arbitrary I set the dots on the single turn

Marking the Transformers Set the other dots by inspection This is not arbitrary

Ideal Transformers N1=N2 Identical Windings Lenz and Faraday B1 Lossless 1 mA B1+B2 =0 1 mA ZL Lenz B2

SWR Meter Circuit…modified Current in T1 Opposite direction for Forward and Reverse R4, R3 balance the bridge Add 33K resistors to J1,1 and J1,3 to J1,2 (ground)

Measurements Apply 2.5 Watts to RFin Connect RFout to an open ckt Note VFWD and VREV Adjust R3 or R4 to set VFWD= VREV Now measure VFWD and VREV For a 50,100 25 Ohm Load and a Short CKT Record Data on supplied form Does the data make sense?

Summary Meter measures voltages and currents SWR calculated Good example of Ferrite Transformers Simple circuit has many limitations Many examples on the WEB sma connector with jumper