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Choosing your Antenna
                                            by Joemel Caballero

Going for a wireless connectivity might be complicated for an average people like you but looking in a different perspective will give
you a more refined understanding of connectivity.

Segregating each part of the wireless gadget can lead you to the technology used in catering the data to the end-user. The antenna is one of the basic block of wireless connectivity.

Your wireless requirements will dictate what you need in choosing the antenna for your system. Various antennae will give various result which could affect your data rate in relation to the environment (distance, blocking object, interference, power of the unit where the antenna is connected, Sensitivity, etc.)

Below is the summary of the types of antenna which could help you funnelized (I’m not quite sure if this word exist but it’s good to the ear) your option in your system.

 
Monopole

This antenna has been on the RF field since the discovery of radio wave. It’s the simplest of them all. This is easy to assemble and doesn’t require in-depth understanding of antenna. Most of these are terminated in 50Ω and 75Ω impedance.
 
A very popular broadband wireless connection is the WIFI and most of these will take the 50Ω impedance of terminating port. A very common type of antenna with monopole configuration is the meader line antenna. It has some matching turns of coil on the middle or bottom part of the antenna or maybe a series of coil to form the antenna itself. This is the most cost efficient monopole antenna. Other forms are Franklin, COCO antenna, an OMA, whip and etc.
The radiation pattern of these antennae is omni or spread equally on all the direction forming like a donut shape looking at the top of the antenna and hence the name “Omni directional antenna” This is very ideal for a Point-to-Multipoint (PTM) distribution to reach more end-user.
 
Dipole Antenna

From the name itself, it’s a product of two monopole antenna. Basically a monopole has an electrical image of the actual physical pole to form a dipole when referred to an RF ground. The dipole has  two physical monopole. The other pole was added to produce the electrical image of the monopole when the antenna is far away from an RF ground. A very good example of this dipole is the YAGI-UDA antenna.
 
It is using a dipole as the radiator together with other element to improve antenna gain and directivity. It has a good F/B ratio and its most commonly used in a PTP (Point-to-Point) signal transmission. If you only want few people to receive your signal or you don’t want other signal interfering with your desired signal then dipole is the answer. The drawback of this antenna is that it has a limited frequency coverage since the bandwidth is just dictated by the size of the dipole.
 
The radiation pattern of dipole is near identical on opposite side. The side lobes have lesser level as compared with the main lobes (two opposite side). Some antenna designer prefer to reduce one opposite side of the radiation pattern to give more directivity and as on the case of the Yagi-Uda, a reflector and director were use to reduce one opposite side radiation to form the Back lobe and increasing front pattern to form the front lobe or the main lobe and hence the F/B (front-to-back) ratio was derived.
 
Cascaded Dipole

With the limited frequency range of the dipole antenna, designers were able to cover most of the radio frequency band by changing the size of the dipole. The setback here is that the end user won’t be changing back and forth his antenna whenever he wants a change of frequency band. The solution was to cascade a different size of dipole to cover most of the band without any physical manipulation of the end user to the antenna.
 
A very good example here is the log-periodic antenna. Each dipole is arranged and connected in a manner wherein a 180º shift is present on the cascaded connected dipole element.
 
Stacked array Dipole

Another method used to connect dipoles is by stacking. A very good candidate for this antenna is the end-fire array. It is physically a dipole of equal length but stack together.

Increasing the gain of your antenna

Antenna has a fixed radiation pattern on an ideal space (vacuum and no obstruction). Various method where applied to the antenna so that the signal can be focus on a narrow area thereby increasing its magnitude (gain). Different method has different form of directivity and this is called the beam width. This is basically the region of the radiation pattern where the signal is maximum and this has been the basis of calculating the “reach” or distance coverage of the antenna.

The yagi-uda is using a reflector and director as previously mentioned. It is using a simple segment of reflector and directors to focus the signal. Increasing the directors would decrease the beam width which means the particular point of focus is getting small. The use of corner reflector would further concentrate the incident signal to the director. This has a 90º angle and the director is mounted on the bore sight of the antenna.  A rectangular flat surface reflector is also a simple gain enhancement which basically reduces the back radiation and reflects some of the incident signals. A flat panel type is an example of these antennae.
 
A parabolic reflector is another form of highly develop reflector. The three dimensional parabola has a focal point on the center of the reflector and has slightly longer length than the depth of the parabola. Most PTP is using a parabolic reflector due to its high gain and narrow angle of beam width. Another form of reflector is the Offset parabolic reflector. It’s basically a parabolic reflector but a leaf segment of the parabola is taken. The feed point is away from the bore sight of the antenna and the whole area of the parabola is used to reflect the incident signal.

There are many more types of antenna but the foundation of these is the monopole. It only tend to complicate due to the added gain increasing add-on installed on the antenna system. These manipulate the direction of radiation and reflection of signals. It will minimize the interference of unwanted signal coming from any direction depending on the implementation.

 

 
 
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