CALCULATIONS FOR OPTICAL-INTERFERENCE REGION 



' Lobe pattern factor = 20 log 1/(1- B)- + ID .sin^ (H/2) 



Repeating the process for other vakies of p, a 

 table of 10 log {Pi/ Pi) versus d is obtained. These 

 points have been plotted in Figure 26, together with 

 the points in the diffraction region obtained with the 

 same data in Section 5.7.3. For sketching in the 

 optical part, the value of n is kept in mind, since 

 this indicates on which lobe and where on the lobe 

 the point lies. 



Radar Gain: Two- Way Transmisston 



To change from 20 log A = - 105 to 10 log {P-i/Pi) 

 at the radar receiver, using equation (5) and Gi = G^ 

 = 13.5 db, 



10 log (P2/P1) = 27 + 7.5 + 10 log (7-20 log 1.5 

 + 40 log A, 

 = 27 + 7.5 +10 - 3.5 + 40 log A, 

 = 41 + 401ogA = - 169 db. 



5 <> 5 Type IV. Determination of Con- 

 tours Along Avhich the Gain Factor A 

 Has a Given Value, the Transmitter 

 Height and Wavelength Being Given 



This is the so-called coverage problem which is 

 treated in greater detail in Chapter 6. While the 

 usual coverage diagram is deri'ved on the basis of 

 one-way or communication formulas, the diagram is 

 still useful for radar since a target will return more or 

 less energy to the receiver according to its position 

 on the coverage diagram. The contour gain factor A 



is readily con\'erted to P2/P1 for either one-waj" 

 or radar by means of equations (3) and (5). 



The method described here is more accurate than 

 the graphical methods given in Chapter 6. It is best 

 suited for finding maximum lobe ranges correspond- 

 ing to a given radar gain. If A is given, and h for a 

 given distance d is wanted, a curve can be drawn as 

 in Section 5.6.3 and then hn found for the given 

 value of A. 



Problems 



A radar set operating over the sea has a trans- 

 mitter ^^dth antenna height of 30 meters and a wave- 

 length of 1.5 meters. As in the previous problems, a 

 receiver ^^ ith an antenna of db gain is assumed in 

 place of a target and the gain of the transmitter is 

 again assumed as 13.5 db. The polarization is hori- 

 zontal. The gain factor A, for illustration, is chosen 

 as — 130 db. Positions of the receiver are to be 

 found at which the gain factor takes that value. 



In Chapter 6, purely graphical methods of de- 

 termining the contours (lobes) are given. Here we 

 are concerned with finding individual points on the 

 contour. Thus, for example, n = 1 if the tip of the 

 lowest contour is wanted (as in range determination). 

 Points near the tip require values of n near 1, such as 

 n =1.2 or n = 0.9. For the next higher lobe, the 

 tip of the lobe corresponds to n = 3 and so on. The 

 nulls are at n = 0, 2, 4, ... . However, while the 

 optical formula gives a null at n = 0, that is, near 

 the line of sight, the true value of the field, or of A, 



