For those of you who have never been involved in the world of radio before, it would be a good idea to read amateur radio books such as the ARRL Antenna Handbook ( http://www.arrl.org ), ARRL Technical Information Pages ( http://www2.arrl.org/tis/tismenu.html ) and the W1GHZ online microwave antenna handbook at ( http://www.qsl.net/n1bwt/preface.htm ).
Surfing to various amateur radio sites will be very helpful, especially to obtain information & software to help build a homemade antenna. There are many things to consider in antenna installation, such as grounding system, good coax cable and various types of antennas. The key to your survival is in the antenna system used.
Some Types of Antennas
Basically, there are several types of antennas that are commonly used for wireless Internet network operations, including:
- Omnidirectional Antenna, usually used on Access Points to provide Internet access to WARNET within a 360 degree radius.
- Sectoral Antenna, usually used on Access Points to provide Internet access to WARNET or customers within a certain radius, usually 90 degrees, 120 degrees and 180 degrees.
- Directional antenna, usually placed in WARNET to direct direct connection to Access Point.
Shown in the picture is a 19 dBi antenna used by the author at home to connect to the Internet 24 hours a day. The antenna is installed on a 1.5 inch plumbing pipe. With a height of about 1.5 meters from the roof. Not far from the antenna is installed a lightning rod.
The installation of the WLAN antenna is not like that widely used by WARNET colleagues who use high towers that cost a lot of money. Line Of Sight (LOS) calculations are important to determine whether the Antenna position is good or not. Frankly, this height is a bit reckless because according to calculations it should be placed on a tower with a height of around 10 meters.
Antenna Connecting Coax Cable
Coax cable is a cable used to connect an antenna to a transmitter or receiver. This cable has a specific impedance, which is used in wireless LAN is a coax cable that has an impedance of 50 ohms.
Unfortunately, the most deadly component in a wireless Internet installation is the coax cable because it has attenuation. The table below shows the attenuation of several types of coax at 2.4GHz.
| Tipe Kabel | Redaman / 10 meter (pada frekuensi 2.4GHz). |
|-------------|---------------------------------------------|
| RG 8 | 3.3 dB |
| LMR 400 | 2.2 dB |
| Heliax 3/8” | 1.76 dB |
| LMR 600 | 1.7 dB |
| Heliax ½” | 1.2 dB |
| Heliax 5/8” | 0.71 dB |Field experience, it is better to use heliax or LMR cable. Indeed, the price is quite expensive, around Rp. 250,000 / meter if you buy new, sometimes we can get it at a cheap price if we are lucky. It is better not to use RG-8 cable at all if you want to be safe.
Antenna Connector
The connection between WLAN equipment, coax & antenna becomes very important because the connector is a power damper if the installation is not good. At least a good connector will consume about 0.3-0.5 dB of power. N & SMA connectors are designed to work at high frequencies. There are several types of connectors used for WLAN installations, namely:
N-Female is usually used on the antenna or lightning protection side.
N-male connection on the coax cable that will connect to the antenna.
SMA male Right Hand Polarization connectors are usually connected to a small coax cable (pig tail) to connect to the connector on the WLAN card.
SMA female right hand polarization connectors are usually installed on WLAN cards.
To connect a WLAN card that has an SMA connector for small coax, to a larger diameter LMR or Heliax coax cable. Usually a connecting cable is made with a different connector (N & SMA). This cable is known as a pig tail.
Grounding System
Basically there are three (3) types of grounding systems, namely:
- Safety ground, this is for high voltage electrical power (PLN)
- Lightning ground, this is to channel lightning to the ground.
- RF ground, this is for grounding RF (radio) signals.
What we need in WLAN operation is Lightning Ground and RF Ground, both must be placed in separate places, do not put them together.
RF Grounding system is mainly needed for omnidirectional or sectoral antennas. For directional antennas it is usually not needed, because one part of the directional antenna has become the RF ground itself. RF ground can be made from several radial cables on the ground connected to the ground coax.
Lightning Ground
Lightning Ground in detail can be seen at http://www.arrl.org/tis/info/lightning.html . Some colleagues usually use water in a drilled well as a good ground. The point is, we must provide a place with the lowest resistance for lightning to enter the path we provide. Of course, with this lightning ground, anti-lightning equipment is needed to be connected to the coax cable we use so that it can channel lightning to the lightning ground.
Antenna Polarization
In the world of electromagnetics, radio signals/waves can propagate in the air with two perpendicular polarizations depending on the type of field they are traveling on, an electric field or a magnetic field.
Using the correct antenna polarization will allow us to:
- Improves isolation from unwanted signal sources. Discrimination by cross polarization (x-pol) is typically around 25 dB.
- Reduce interference.
- Defines the area/region covered (serviced).
Horizontal polarization
The antenna on the side is in a horizontal polarization position. Note the dipole antenna in front of the parabolic reflector is in a horizontal position. Horizontal polarization is usually used for point to point (P2P) communication connections.
Point To Multi Point
On the side image is a sectoral antenna that is usually installed on the base station (BTS) where the Access Point is placed. Because this sectoral antenna must provide service to several stations at once or Point To Multi Point (P2MP) then it usually uses vertical polarization. Omnidirectional antenna is another example of an antenna that uses vertical polarization.
Antenna Omnidirectional
Antenna omnidirectional
Omnidirectional antennas are designed to provide service within a 360-degree radius from the location point. Very suitable for Access Points to provide service to the surrounding WARNET within a short distance of 1-4 km. This type of antenna usually has a low Gain of 3-10 dBi.
The radiation pattern of an omnidirectional antenna can be seen in the image below. The horizontal field section shows radiation that is almost 360 degrees circular.
The vertical field section shows a cross-section with a very thin field on the vertical axis. This means that only stations located in front of the antenna will receive a strong signal, stations located above the antenna will have difficulty receiving a signal.
radiation that is almost 360 degrees circular
Sectoral Antenna
Sectoral antenna
Sectoral antennas, like Omnidirectional Antennas, have vertical polarization and are designed for use at base stations (BTS) where Access Points are located.
Unlike omnidirectional antennas that can provide service in a 360-degree range. Sectoral antennas only provide service in a limited area/sector. Usually 45-180 degrees only. The advantage gained by limiting the service area, sectoral antennas have a greater gain than omnidirectional antennas. Usually sectoral antennas have a gain between 10-19 dBi.
It can be seen in the image that the horizontal field section of the sectoral antenna is only widened on one side. While the vertical field section is very flat like an omnidirectional antenna.
Sectoral Antenna Radiation
Antenna Directional
Antenna Directional
Directional antennas are generally of two types, namely:
- Yagi
- Parabola
Yagi antenna has a smaller gain (7-15 dBi) as shown in the side image of some examples. The more radiators used, the higher the gain of the antenna. The radiation pattern is shown in the image below. Its shape is roughly like a ball both in horizontal and vertical field sections.
Horizontal Pattern For Vertical Polarization
Satellite dish
Parabolic Antennas usually have a minimum gain of around 18-28 dBi. The side image shows a parabolic antenna with horizontal polarization that is commonly used for point to point (P2P) communication.
It can be seen in the image that the polarization field of the antenna, both horizontal and vertical, is very sharp. Indeed, this parabolic antenna has a large gain, but it must be compensated with a very narrow beam width. This means that if the direction of the antenna is shifted slightly, the signal will be lost at the other end.
Using a 24dBi antenna for short distances is not effective. Advice for WLAN users, it is better to use an antenna with sufficient gain so that reliability remains high from mechanical disturbances, such as antenna shifting, etc.
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