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Published byNathan Barker Modified over 2 years ago

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

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

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

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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 Z L = Zo SWR 2:1 for Z L = 100 or 25 Ohms Meter actual measures Voltage and Current on the line SWR must be found from V FWD and V REV on the board

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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

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

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

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

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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)

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

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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

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