The Bird Thruline RF Directional Wattmeter model 43 accuracy is
specified as +/- 5% of full scale reading.
That means that if you have a 500 watt element (model 500H) measuring
the output Ten-Tec Hercules II amplifier, and the reading is at exactly
full scale, the actual power could be anywhere from 475 watts to 525
watts. It also means that if you rotate the element to read reflected
power, and you see a reading of two and a half divisions (which is 25
watts on the 500 watt scale) the actual reflected power is somewhere
between 0 and 50 watts.
That corresponds to a reflected power percentage between 0 and 50/475 =
10.5%, or expressed in terms of VSWR 1.0:1 to 1.95:1.
Even if the reflected power indication is zero, 5% of full scale is 25
watts,
so the percent reflected power could be as high as 25/475 = 5.3% or an
SWR of almost 1.6:1.
You say you have an SWR of exactly 1.0:1? Yea, sure you do. You're going
to trim your antenna because 1.5:1 SWR is too high? Go ahead, I'll just
operate with it as it is.
Using the smallest standard HF element, the 50H, a zero reading of
reverse power could be as high as 0.05 * 50 = 2.5 watts. If the forward
power using the 500H element was indicating exactly full scale, a zero
reading with the 50H element for reverse power could be as much as
2.5/475 = 0.53%, expressed in VSWR almost 1.16:1.
This will make it very difficult to determine the VSWR of one Bird 43
Thruline meter, using another one cascaded to measure the reflected
power from the second meter, since the insertion VSWR of the Bird 43 is
rated at less than 1.05.
DE N6KB
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