The characteristic impedance of the sample lines was also measured using the following mathematical expression:
Z_o = √( √(R1^² + X1^²) x √ (R2^² + X2^²))

With R1 + X1 equal to the measured impedance at the +45 degree offset frequency and R2 + X2 equal to the measured impedance at the -45 degree offset frequency.

The results of the sample line characteristic impedance measurements are shown in Table 2.

In addition Crain conducted measurements of the sample line electrical lengths that were derived by measuring the open circuited 90 degree and 450 degree resonance frequencies. The results are shown in Table 3.

The measured characteristic impedance and electrical length variations relative to the average are less than ±1 percent and ±0.5 degrees. Based on these measured data, Crain returned to his office to develop a computer model for this complex array using MININEC Broadcast Professional V14.5 software. Each of the four towers were modeled using 19 segments with the tower 1 and 2 segments having a uniform length and radius and towers 3 and 4 being modeled in a graduated diameter stacked cylinder approach where the bottom segments had longer segments and larger radii with uniform segment and radius at the top fixed cross section portion of the tower. The tower spacings and orientations from the FCC database were identical in the self-impedance and DA models. The segment lengths and radii were varied while maintaining the tower height as constant to converge on modeled tower impedances with the other towers shorted that matched the corresponding measured impedances. A table of the final modeled segments for towers 3 and 4 are shown in Table 4.

- continued on page 4