Bird Technologies 4527 - Manual (Page 22)

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These curves are typical for all element types (H, A, B, C, D, ...) at their respective
frequencies. Since a C type element has a frequency range of 100 - 250 MHz,
response curves for other element types can be approximated by replacing the
100 and 250 MHz points on the chart with the extremes of the element’s fre-
quency range, and recalc-ulating the other frequency points accordingly.
Example - For a B element (range 50 - 125 MHz) simply
divide all frequencies by two. For an E element (range 400 -
1000 MHz) multiply all frequencies by four.
Harmonics or subharmonics that lie outside of the frequency range of the ele-
ment may exist in the circuit under test. A rough approximation of the element’s
response to harmonics can be made with these curves. Using an element for
measurements outside of its frequency range is not recommended. The
response curves presented are only typical, and not guaranteed.
Impedance Mismatch
There may be cases where it is necessary to use the Bird 43 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 test
frequencies and/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 equals the
ratio of the two. Always divide the larger impedance by the smaller, since VSWR
must always be greater than 1.
Example - Consider using a Bird 43 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 the wattmeter
is removed, the VSWR will actually be 1.0. Similarly, if the
load impedance is 35.7 ohms, 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 impor-
tant to calculate the load power by subtracting the reflected power
from the forward power.