Bird Technologies 4304A - Manual (Page 20)

9
Impedance Mismatch
In some cases it may be necessary to use the Bird 4304A with a non-50
Ω trans-
mission 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 reflected power levels, the load impedance will change when the wattmeter
is removed.
When the line and load impedances are known, the system’s VSWR equals the
ratio of the two. Always divide the larger impedance by the smaller, since VSWR
must be greater than 1.
Example - Consider using a Bird 4304A to tune a 70Ω
line. If the load impedance is also 70Ω, the wattmeter will
measure a VSWR of 70/50 = 1.4. However, if you remove
the wattmeter, the VSWR will actually be 1.0. Similarly, if
the load impedance is 35.7Ω, the VSWR will be 50/35.7 =
1.4 with the wattmeter and 70/35.7 = 2.0 without it. Cau-
tion 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 chang-
ing the load impedance. When the load impedance is near
70Ω, the Bird 43 will read increasing VSWR as the load
impedance is increased.
Note: When working with non-50
Ω lines, it is especially important to
calculate the exact load power by subtracting the reflected power
from the forward power.