Chapter 8 
ANGLE-OF-ARRIVAL MEASUREMENTS 
ANGLE-OF-ARRIVAL MEASUREMENTS 
IN THE X BAND? 
HE purrosn of this work was to observe the varia- 
tion in angle of arrival of waves in the X band. No 
simultaneous air sounding data were taken although 
general weather observations were made. 
‘The method of measuring the angle of arrival makes 
use of a very sharp-beamed antenna (Figure 1) 
mounted so that it may be mechanically tilted back 
6 INCHES WIDE (I5° HORIZONTAL BEAM) 
20 FEET 
ANTENNA SWINGS 
+.75° 
Ficure 1. Sharp-beamed antenna used for measuring 
the angle of arrival. 
and forth about its center thus sweeping the beam of 
the antenna through an arc which may be set to include 
the expected angle of arrival of the incoming signal. 
The sharp-beamed antenna has been used to measure 
the angle of arrival of waves from a distant trans- 
mitter over an optical path where both a direct wave 
and a water-reflected wave are present. If the output 
A_ONLY (REGORD 1) 
SIGNAL AMPLITUDE 
A+B (RECORD 2) 
Ficure 2. Variation in signal intensity during scan. 
A, direct ray only. B, direct and reflected rays super- 
posed, 
KIO SEC+ TIME ——> 
of the receiving antenna is fed to a receiver and this 
receiver is fitted with a recording type output meter, 
records of the type shown in Figure 2 will be obtained 
as the antenna scans. Record 1 will be obtained if only 
a single, direct wave is arriving at an angle correspond- 
ing to the mid-point of the antenna swing. The dis- 
tances between peaks of maximum amplitude along 
the record will then be equal. A shift in the angle of 
arrival of the wave would appear on the record as a 
change in the spacing between the peaks. If two 
separate waves, direct and reflected, are arriving 
sBy W. M. Sharpless, Bell Telephone Laboratories. 
318 
simultaneously, the record will appear something 
like record 2. 
The actual antenna used for the measurement is a 
section of a parabolic cylinder arranged so that its 
beam at the center of swing is pointed directly at the 
transmitter. This is also the angle at which waves 
arrive on a normal day. A normal day has been taken 
as one when the angle of arrival is the same (within 
the accuracy of the measurements) as that calculated 
from actual earth geometry and when free space field 
is received from the direct wave. 
The physical position of the antenna may be held to 
approximately 1/100 degree by the use of a plum-bob 
line dropped from the top of the 20-ft antenna to the 
base. Possible errors in reading the records, however, 
limit the expected relative accuracy to about 1/60 
degree. Slight errors in the actual building of the 
antennas and in the locating of the feed limit the 
final accuracy to what is believed to be 1/25 degree. 
The horizontal angle of arrival is measured with a 
duplicate antenna turned 90 degrees from the vertical 
with its flat side toward the ground. The accuracy of 
measurement is the same as in the vertical plane case. 
The entire equipment, including the two scanning 
antennas, other reference antennas, the receiving 
equipment, and the receiver building are located on 
a rotatable platform which is 25 ft in diameter. This 
equipment, located on top of Beer’s Hill, New Jersey, 
may thus be pointed toward any of several transmitters 
and comparisons made of the angle of arrival from 
each transmitter. 
Observations during the summer of 1944 have been 
made on two optical paths shown in Figure 3: (a) A 
24.1-mile path partly over land and partly over water 
between New York City and Beer’s Hill, New Jersey. 
The normal reflecting point for the reflected ray on 
this path is in the salt water of Raritan Bay; (b) a 
12.6-mile path between Beer’s Hill and Deal, New 
Jersey. This path is all over gently rolling land. The 
transmitters at both Deal and New York radiate waves 
polarized at 45 degrees so that either vertical or hori- 
zontal polarization may be used at the receivers. 
Results of angle-of-arrival measurements made dur- 
ing the summer of 1944 indicate that on both the Deal 
and New York circuits the greatest variation of angle 
of arrival in the horizontal plane was +1/10 degree. 
Times were found when the angle of arrival remained 
«s much as 1/10 degree east for short periods on the 
New York circuit, but for the most part the horizontal 
angle of arrival normally fluctuated +1/10 degree 
from the normal day direction on both the Deal and 
