Bird Technologies APM-16 - Manual (Page 22)

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These curves are typical for all element types (H, A, B, C, D, ...) at their respec-
tive frequencies. Since C elements have 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 recalculating the other frequency points accordingly. For
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 ele-
ment’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 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 fre-
quencies 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 cal-
culated 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 imped-
ance. When the load impedance is near 70 ohms, the Bird 43 will read increas-
ing VSWR as the load impedance is increased.
 Note: When working with non-50 ohm lines, it is especially import-
ant to calculate the load power by subtracting the reflected power
from the forward power.