1. 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)

2. 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

3. 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

4. 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

5. 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

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

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

8. Ideal Transformers N1=N2 Identical Windings Lenz and FaradayB1
Lossless
1 mA
B1+B2 =0
1 mA ZL
Lenz B2

9. 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)

10. 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, Ohm Load and
a Short CKT
Record Data on supplied form
Does the data make sense?

11. 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