Tipe Antenna
Sebelum kita masuk ke ilmu hitam yaitu perambatan sinyal wireless, kita perlu memahami bagian penting dari sistem wireless yaitu antena. Antena adalah sarana untuk menyambungkan pemancar ke media, dalam hal ini, ruang kosong. Antena adalah radiator elektromagnetik; itu menciptakan medan elektromagnetik yang keluar dari antena pemancar ke antena penerima, yang kemudian mengubah gelombang elektromagnetik menjadi sinyal listrik yang diterapkan ke tahap input penerima.
Ada beberapa jenis antena dalam tiga kategori besar: omni-directional, directional, dan semi-directional.
- Antena omni-directional merambat ke segala arah.
- Antena semi-arah menyebar secara terbatas, ditentukan oleh sudut tertentu.
- Antena directional memiliki "beam" sempit yang memungkinkan perambatan yang sangat terarah; jenis akrab adalah parabola dan Yagi. Masing-masing memiliki karakteristik dan aplikasi yang unik.
Penguatan pasif memperkuat sinyal
Semua antena menunjukkan penguatan pasif, yang berfungsi untuk memperkuat sinyal. Gain pasif diukur dengan kuantitas dBi, yang merupakan gain yang dirujuk ke antena isotropik teoretis; antena isotropik mentransmisikan energi secara merata ke segala arah, dan tidak ada di alam. Gain antena dipole setengah gelombang yang ideal adalah 2,15 dBi. Perlu juga dicatat bahwa ketika directionality meningkat, demikian juga gain.
EIRP, atau setara (atau effective) isotropic radiated power, adalah ukuran daya maksimum yang akan dipancarkan antena isotropik teoretis ke arah penguatan antena maksimum. EIRP memperhitungkan kerugian dari saluran transmisi dan konektor, dan termasuk penguatan antena yang sebenarnya. EIRP memungkinkan perhitungan keluaran daya nyata dan nilai kekuatan medan, jika penguatan antena aktual dan daya keluaran pemancar diketahui.
Dipole antennas, rubber ducky
Dipole antennas are the most common type of antenna used and are omni-directional, propagating radio frequency (RF) energy 360 degrees in the horizontal plane. These devices are constructed to be resonant at a half or quarter wavelength of the frequency being applied. This antenna can be as simple as two pieces of wire cut to the proper length or can be encapsulated as shown in the illustration; this configuration is commonly referred to as a “rubber ducky” antenna. The dipole is used in many enterprise and small office and home office (SOHO) Wi-Fi deployments.
An antenna exhibits a typical impedance, allowing for matching of the antenna to the transmitter for maximum power transfer. If the antenna and transmitter are not matched, reflections will occur on the transmission line which will degrade the signal or even damage the transmitter. These reflections are described by the term standing wave ratio (SWR) and indicate the efficiency of the transmission line. SWR of 1:1 would indicate that no power is reflected and lost; 5:1 would indicate a reflection and loss of 44%. SWR is commonly used as a voltage ratio and referred to as VSWR.
Antena dipole, rubber ducky
Antena dipole adalah jenis antena yang paling umum digunakan dan bersifat omni-directional, menyebarkan energi frekuensi radio (RF) 360 derajat pada bidang horizontal. Perangkat ini dibangun untuk beresonansi pada setengah atau seperempat panjang gelombang dari frekuensi yang diterapkan. Antena ini bisa sesederhana dua potongan kawat yang dipotong dengan panjang yang sesuai; konfigurasi ini biasa disebut sebagai antena “rubber ducky”. Dipol digunakan di banyak perusahaan dan penyebaran Wi-Fi kantor kecil dan rumah (SOHO).
Sebuah antena menunjukkan impedansi yang khas, memungkinkan pencocokan antena ke pemancar untuk transfer daya maksimum. Jika antena dan pemancar tidak cocok, maka akan terjadi pantulan pada saluran transmisi yang akan menurunkan sinyal atau bahkan merusak pemancar. Refleksi ini dijelaskan dengan istilah rasio gelombang berdiri (SWR) dan menunjukkan efisiensi saluran transmisi. SWR 1:1 menunjukkan bahwa tidak ada daya yang dipantulkan dan hilang; 5:1 akan menunjukkan refleksi dan kehilangan 44%. SWR umumnya digunakan sebagai rasio tegangan dan disebut sebagai VSWR.
Directional antenna
Directional and semi-directional antennas focus radiated power into narrow beams, adding a significant amount of gain in the process. Antenna properties are also reciprocal. The characteristics of a transmitting antenna, such as impedance and gain, are also applicable to a receiving antenna. This is why the same antenna can be used for both sending and receiving. The gain of a highly directional parabolic antenna serves to amplify a weak signal; this is one reason why this type of antenna is frequently used for long distance links.
Patch antenna, microstrip antenna
A patch antenna is a semi-directional radiator using a flat metal strip mounted above a ground plane. Radiation from the back of the antenna is effectively cut off by the ground plane, enhancing forward directionality. This type of antenna is also known as a microstrip antenna. It is typically rectangular and enclosed in a plastic enclosure. This type of antenna lends itself to being manufactured by standard printed circuit board methods. Patch antennas are widely used semi-directionals; a patch antenna can have a beamwidth of between 30 to 180 degrees and a typical gain of 9 dB.
Sector antenna
Sector antennas are another type of semi-directional antenna. Sector antennas provide a pie-shaped (sector) radiation pattern and are usually installed in what is known as a sectorized array. Beamwidth for a sector antenna can be between 60 to 180 degrees, with 120 degrees being typical. In a sectorized array, antennas are mounted back-to-back to provide full 360-degree coverage. Sector antennas are used extensively for cellular communication.
Yagi antenna
A commonly used directional antenna is the Yagi-Uda Array, usually just called a Yagi. It was invented by Shintaro Uda and his colleague, Hidetsugu Yagi, in 1926. A Yagi antenna uses several elements to form a directional array. A single driven element, typically a dipole, propagates RF energy; elements placed immediately in front of and behind the driven element re-radiate RF energy in phase and out of phase, enhancing and retarding the signal, respectively. The elements are called parasitic elements; the element behind the driven element is called the reflector, while the elements in front of the driven element are called directors. Yagi antennas have beamwidths in the range of 30 to 80 degrees and can provide well in excess of 10 dBi passive gain. A multi-element high-gain Yagi is shown in Figure 4.
Parabolic or dish antenna
Parabolic, or dish, antennas are the most familiar type of directional antenna. A parabola is a symmetric curve; a parabolic reflector is a surface that describes that curve throughout a 360-degree rotation—a dish or, to use the technical term, a paraboloid. A parabolic reflector has a high degree of directivity and has the ability to focus RF energy into a beam, much like a flashlight. Parabolic antennas have a very narrow beamwidth, usually not exceeding 25 degrees. Gain is dependent on diameter and frequency; at 2.4 GHz, a 1 meter dish will provide about 26 dBi gain, while a 10 meter antenna will provide 46 dBi gain at the same frequency. The antenna is “fed” by either a half wave dipole antenna or a feed horn. Parabolic antennas are used for long distance communication links between buildings or over large geographic areas. Very large parabolic antennas are used for radio astronomy and can provide gain of 10 million or about 70 dBi.
Grid antenna
A variation of the dish is the grid antenna. Given that a parabolic reflector will present a large solid surface to the wind, it follows that high or even moderate wind conditions will cause the dish to move out of alignment or deform. To prevent this from happening, the reflector is perforated into a grid. The spacing of the grid elements is frequency dependent; it is inversely proportional to the frequency. Gain and beamwidth are similar to the parabolic antenna.