[TenTec] OmniVI Plus Over Drive Amp

[Ken Brown]

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