Bird Technologies 4391A - Manual (Page 17)

5
ary, the coupling currents produced from the waves of one direction will add in
phase, and those produced from waves of the opposite direction will accordingly
subtract in phase. The additive or “arrow” direction is assigned to the forward
wave.
The electrical values of the element circuits are carefully balanced and designed
so that current from the reverse wave will cancel the other almost completely.
The result is directivity always higher than 25 dB, which means that the element
is highly insensitive (nulled) to the REVERSE direction wave. Being highly direc-
tional, the Thruline element is sensitive (at one setting) only to one of the travel-
ling waves that produce standing waves by interference. Thruline Wattmeter
measurements are, therefore, independent of position along standing waves.
Figure 4 Readings with Various Envelopes