In a previous article, we explained how an antenna (a passive device) has gain. The answer, of course, is that gain is measured relative to an antenna that radiates evenly in all directions. If the beam of the antenna is narrowed, the antenna has “gain,” simply because the power is being focused into a narrow area. In terms of peak signal, this means that for a given power entering the antenna it appears that there is more power than there would be for an omnidirectional antenna. However, the power is not truly being amplified; it is merely being focused. This focusing property is called directivity.
Directivity is a measure of how directional an antenna is or the width of the beam in which it radiates. The more directive the antenna, the narrower the beam and the higher the antenna gain.
Gain is a function of directivity and loss; our theoretical isotropic antenna that radiates evenly in all directions has a gain of one and no loss. Obviously, it is impossible to have negative antenna gain without loss.
Pros and Cons of Directivity
So, what are the benefits and drawbacks of directivity? The answer depends on the application.
Increased antenna gain means more sensitivity. In most cases, this is a good thing. However, increased gain comes at the cost of the antenna operating in a narrower beam. This narrow beam may be a bad thing.
For example, consider the case of a cellphone. While a highly directional cellphone antenna would mean greater range, it would also mean that the cellphone would only pick up signal if it was pointing in the correct direction. A cellphone is intended to be portable. If it has to point in a specific direction to receive signal, it is no longer portable.
On the other hand, point-to-point communications where the transmit and receive points are stationary, such as point-to-point wireless applications, are ideally suited to directional antennas. The directional antennas focus the power where it is needed rather than throwing power in all directions.
Other Benefits of Directional Antennas
Thus far we have discussed directivity in terms of increased antenna gain, but there can be another benefit to directional antennas: less noise reception.
By focusing the antenna in the direction from which we want to receive, the narrow beam prevents receiving interfering signals outside of the beam. Therefore, by pointing the antenna in the correct direction, we can “ignore” a powerful interfering signal in order to pick up a weak one, provided the interfering signal is coming from a different direction.
Directional Antennas
Almost all antennas are directional, at least to some degree.
Even a monopole is somewhat directional; it can’t “see” signals below its ground plane. But when we usually think of directional antennas, we think of very narrow beam antennas such as the Yagi or the parabolic dish. While the basic standalone dipole will have a max gain of around 2 db, antennas with high directivity have a very narrow beam resulting in very high gains, potentially over 10 db.
ARSI 
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