Electromagnetic Interference from Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 569

Backward-zone interference is simulated by introducing a time delay between the direct
and multipath signals and suitably alternating the latter. For the forward zone, where the
actual time delay is negligible, the direct line shown in Figure 9-17 is eliminated and only
the modulated signal is applied to the receiver and the spectrum analyzer.
Figure 9-17. Block diagram showing an experimental arrangement for simulating TV
interference by a wind turbine, with which to measure the perception of multi-path
interference by test subjects. [Sengupta and Senior 1979a]
HAWT TVI Perception and Tolerance Levels
Figure 9-18 illustrates thresholds of perception of interference on Channels 7 and 50
in the form of the modulation index, m P, T as a function of the strength of the carrier signal.
Two types of color receivers were used during the measurement of these responses:
Receiver A is typical of older TV technology and Receiver B represents newer receivers.
For the interference simulating the backward zone thresholds of perception on any given
channel are reasonably independent of the ambient signal level for either receiver, but
smaller modulations can be perceived with the newer receiver. Modulation thresholds are
higher for forward-zone interference ( i.e ., without time lag), for both receivers. With the
newer receiver the threshold was quite large at strong ambient signal levels.