Direct Energy, 2018a

4 ANTENNAS 81
[
Pfront
F/B ratio=
P back
]dB
⎡
∣ −→ ∣ ∣∣
⎤
E front
=2⎣
∣ −→ ∣ ⎦ ∣∣
E back
dB
(4.12)
The F/B ratio for the example of Fig. 4.4 can be calculated from the
azimuth plot. The strength of the eld in front direction is 9 dB stronger
than the strength of the eld in the back direction.
⎡
∣ −→ ∣ ∣∣
⎤
E front
⎣
∣ −→ ∣ ⎦ ∣∣
=9dB (4.13)
E back
From this information, we can calculate the strength of the eld in the
front direction to the strength of the eld on a linear scale.
⎡
∣ −→ ∣ ∣∣
⎤
⎡
E front ∣ −→ ∣ ∣∣
⎤
E front
⎣
∣ −→ ∣ ⎦ ∣∣
=10log 10
⎣
E back ∣ −→ ∣ ⎦ ∣∣
(4.14)
E back
dB
dB
⎡
∣ −→ ∣ ∣∣
⎤
[
1
E |
−→
]
E front|
front 10
⎣
∣ −→ ∣ ⎦
·
∣∣
=10 | −→ E back|
dB (4.15)
E back
lin
⎡
∣ −→ ∣ ∣∣
⎤
E front
⎣
∣ −→ ∣ ⎦ ∣∣
=10 9
10 =7.94 (4.16)
E back
lin
If this antenna is being used as a transmitter, signal in the front direction
is 7.9 times as strong as the signal in the back direction. The front to back
ratio species the power ratio, and for this antenna, it is 18 dB.
[
Pfront
F/B ratio=
P back
]dB
⎡
∣ −→ ∣ ∣∣
⎤
E front
=2⎣
∣ −→ ∣ ⎦ ∣∣
E back
dB
lin
=18dB. (4.17)
When selecting an antenna, many decisions related to the antenna directivity
are needed. A particular application may require an isotropic or
a directional antenna. If a directional antenna is needed, the magnitude
of the directivity must be decided. Additionally, the orientation of the
antenna must be decided so that nodes and nulls are in the appropriate
directions. Both the azimuth angle and the elevation angle of the nodes
and nulls should be considered [50, p. 22-1].