BIRD Thruline APM-16, Руководство по эксплуатации

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RF Directional Thruline® Wattmeter Model APM–16
10
Impedance Mismatch
There may be cases where it is necessary to use the Bird APM-16 with a non-50
ohm transmission line. If the reflected power is less than 10% and the frequency
is below 200 MHz, the resulting mismatch will not be too serious. At higher
frequencies or higher reflected power levels, the load impedance will change
when the wattmeter is removed from the circuit.
When the line and load impedances are known, the system’s VSWR can be
calculated by dividing the larger impedance by the smaller. Remember that the
VSWR ratio must always be greater than 1.
For example, consider using a Bird APM-16 to tune a 70 ohm line. If the load
impedance is also 70 ohms, the wattmeter will measure a VSWR of 70
/ 50 = 1.4.
However, if you remove the wattmeter, the VSWR will actually be 1.0. If the
load impedance is 35.7 ohms instead, the VSWR will be 50
/ 35.7 = 1.4 with the
wattmeter and 70 / 35.7 = 2.0 without it. Caution must therefore be used, since
both good and bad matches can have the same measured VSWR. In this case,
the correct impedance can be determined by slightly changing the load
impedance. When the load impedance is near 70 ohms, the Bird 43 will read
increasing VSWR as the load impedance is increased.
Note:
When working with non-50 ohm lines, it is especially
important to calculate the load power by subtracting the reflected
power from the forward power.