SWR MeterSession 3

Learn about Transformers and SWR Meters

Forward Waves on Transmission Lines ZL = Zo (Characteristic Impedance)

ZLV+

-

I

• Zload = 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

SWR Meter

Reverse Waves on Transmission Lines

ZL = Zo (Characteristic Impedance)

ZLV+

-

I

• Here, ZL = Zo

• There is only a reverse wave V=-IZo• SWR = 1.0

TransmitterR

Port 1 Port 2

Current now reversed

SWR Meter

Normally ZL not Equal to Zo

Forward and Reverse Waves on the Line

ZLV+

-

I

• ZL not equal to Zo

• Standing Waves exist

Transmitter F LoadR

Port 1 Port 2

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

• Set the other dots by inspection• This is not arbitrary

Marking the Transformers

Ideal Transformers N1=N2Identical WindingsLenz and Faraday

1 mA

1 mA

B2

B1

B1+B2 =0

Lossless

LenzZL

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