Vectronics SWR-584C - Manual (Page 23)

SWR-584C Instruction Manual HF/VHF/220MHz SWR Analyzer
23
180 0
j+X
j-X
R1 (Low) R2 (High)
Transmission
Z = R1 x R2
° °
25 Ohms 100 Ohms
50 Ohm
Line
SWR = 2:1
Load = 100 Ohms
Z = 50 Ohms
Z = 25 x 100
Viewed on a Smith Chart, the transformed load impedance (R ±jX) literally traces a circle
around the characteristic impedance of the transmission line with each 360-degrees rotation.
The greater the mismatch, the larger the diameter of the circle. We can use this behavior in
two different ways to determine the impedance of an unknown transmission line.
One way is to intentionally introduce a resistive mismatch at one end of the line and measure
the amount of impedance transformation it causes at frequencies where X=0. As shown above,
for each full rotation there will be two Zero-Crossing Points where X=0 and the load becomes
purely resistive. One occurs below the line's characteristic impedance and the other above.
Using the high and low resistive values (R1, R2), we can calculate the impedance of the line.
Or, alternatively, we can connect a series of trial loads at the far end of the cable until we find a
value where the impedance excursions are reduced to zero (the line's characteristic
impedance).
When running these test, coax may be piled or coiled on the floor, but balanced lines must be
isolated as described in Chapter 5.5. Connect Beverage antennas directly to the analyzer.
Intentional Mismatch Method:
To ensure accuracy, use a non-inductive load and choose a
value that holds the high and low impedance excursions well within the analyzer's accurate
measurement range (7-650 Ohms).
[ ] Connect the cable to the analyzer and terminate the far end with the load you have chosen.
[ ] Tune VFO for the lowest frequency where Impedance and Resistance indicators both dip.
[ ] Fine tune to locate the point where X=0 and R are both at their minimum value.
[ ] Press Gate to confirm θ = 0°. Write down the Resistance (R) reading as R1.
[ ] Tune up in frequency to find a distinct Impedance peak.
[ ] Fine tune to locate the point where X=0 and Resistance (R) peaks at maximum value.
[ ] Press Gate to confirm θ = 0°. Write down this Resistance (R) reading as R2.
[ ] Multiply R1 x R2 and find the square root of the product.
[ ] The result is the characteristic impedance of your line.
Example:
R1=36 ohms and R2 = 71 ohms, (36 x 71 = 2556), square root = 50.6 ohms.
If you wish to confirm the result, try other load values.
Load Substitution Method:
At HF, use non-inductive fixed-value resistors, a physically small
carbon potentiometer or trimpot, a (compact) decade box, or a broadband transformer of
known accuracy (to extend range). Above HF, avoid any load that could introduce stray