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Tutorial
Tutorial
on Wireless LAN Antennas
(continued)
So,
with a directional antenna, you can experience a great
advantage. Let’s say that you have a directional antenna
with a gain of 9 dB. With a unidirectional antenna, you would
be required to reduce your TFO by 3 dB, but with a directional
antenna, you only have to reduce your TFO by 1 dB, bringing
your EIRP to 29 + 9, or 38 dBm.
Table 3 below lists some examples of gains and TFO with a
directional antenna.
| TFO |
Directional
Antenna Gain |
EIRP |
| 30
dBm |
6
dB |
36
dBm |
| 29
dBm |
9
dB |
38
dBm |
| 28
dBm |
12
dB |
40
dBm |
| 27
dBm |
15
dB |
42
dBm |
Table
3: Sample TFO and EIRP for Directional Antennas
Antenna Selection
To gain the performance needed over great distances, you could either add repeaters or use special directional antennas.
Different antenna designs render different gain factors. A single-element antenna may render a gain of 6 dB, whereas a microwave antenna may render a gain of 12 dB. A parabolic antenna could render a gain of over 60 dB.
In the LAN world, single-pole
Omni-directional antenna with limited gain and directional array antenna have received particular attention lately do to the release of products by companies such as
Vivato.
Building an array antenna from a single-element antenna demonstrates how different patterns are created with an array (Figures 2a-e). A single antenna (e.g., a car radio antenna) has a gain of 3 dB, with a transmission pattern similar to the manner in which waves propagate in water when a rock is thrown into a lake (Figure 2a).
If a single element produces gain of 3dB, would two elements give a gain of 6dB? Not exactly. The signals would be both added and subtracted to produce a pattern as shown in Figure 2b.
|
Figure
2a: Single Element Pattern |
|
Figure
2b: Two
Element Pattern |
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