354 PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
(+) 
50 
So 
30° 
<\ 
\ 20 
QIN DEGREES 
Figure 21. Major lobes (relative field strength) for traveling-wave antenna. 
metry, as in Figure 18. In this case the radiation is 
horizontally polarized. It can also be used as an 
inverted V in a vertical plane with the point of the 
V directed upwards; the radiation is then vertically 
polarized. This antenna, also called a semi-rhombic, 
is represented by the upper half of Figure 20. 
Om IN DEGREES 
L/d 
Ficure 22. Angles for major lobes for traveling-wave 
antenna. ; 
Rhombic Antenna 
This type of antenna is based on the same prin- 
ciple as the traveling-wave V antenna. The rhombic 
antenna consists of four wires arranged in the form 
of a rhomboid or diamond (Figure 23). The reflec- 
CHARACTERISTIC 
Ni 
DIRECTION OF 
= x! 
IMPEDANCE Han 
MAX RADIATION 
Ficure 23. Rhombic antenna. 
tionless termination of the wires is achieved by 
connecting the two wires at the end opposite the 
input to a resistance equal to their characteristic 
impedance. 
As in the case of the V antenna, the rhombic 
antenna can be used both horizontally and vertically; 
at the longer waves the horizontal arrangement is 
usually more practical. The optimum tilt angle of 
the rhombic (angle @ of Figure 23) is not very 
critical provided the legs are not less than two wave- 
lengths long. The radiation pattern is not very 
sensitive to frequency and the rhombic antenna can 
therefore be used over a fairly wide frequency range 
(of the order of 2 to 1). Rhombic antennas have 
appreciably higher gains than V antennas. 
The field in the axial direction is equal to 
= NOt _ 08 bs iE ee | 
Ergxial d = mae d el sin i) 2 (20) 
Effect of Perfectly Conducting Ground. If the 
rhombic is placed in a horizontal plane,- height H 
above ground, the effect of the image rhombic 
must also be considered. The net result is that the 
direction of the resultant lobe maximum is tilted 
up by an angle e. It can be shown that, for a given 
angle « and wavelength d, to point the major lobe 
at vertical angle « the following relations for H, 
L, and ¢ must hold: 
gs 
4 sine 
we Bes 
sin? ¢€ 
@ = 90° —.« 
