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SATELLITE TVRO

Satellite Television Recieve Only

[My Technical Notebook]


A VHF folded dipole construction Coaxial cable Velocity test Stacking Colinear 3 x 3/4 A Duplicated Boomer Yagi antenna for 2 m Transmission lines problems This Page
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A Brief overview about Satellite Television

Sine mankind started to explored the outer space in early 60's up to now there are many radio telecommunication Repeaters placed in the outer space called Telecommunication Satellites. Almost all telecommunication satellites are a Geosynchronous orbital types that are placed ± 36,000 km right above the equator, if it's viewed from the earth it'll remain at the same location, Actually it moves in its orbit in step with the earth rotation, for this reason it's called also as Geostationair satellite. They are divided into several classes in conjunction with it's operating frequency, there are several classification type, such S Band, C Band, KU Band etc. The well-known classes for satellite television broadcasting are the C band and KU band type. KU Band is popular for Direct TV broadcasting. At the beginning most Satellite TVRO were using the C Band. The C Band Satellites has a down link frequency of 3.7 ~ 4.2 GHz, (500 MHz width). This 500Mhz width frequency area is divided into smaller bandwidth base band frequency called Transponder. Since the Transmission polarity in the UHF and above is usually utilized to get more transmission spaces, this 500MHz width frequency can at the same time be used for 2 transmission polarization without interfering each other, vertical and horizontal or left circular and right circular are commonly used, so with this technique it's likewise that the bandwidth is expanded with an extra 500 MHz width. By this a total 24 transponder of ± 40 Mhz width base band can be allocated (see Table below)

Sejak manusia mulai merambah angkasa luar diawal tahun 60 han sampai sekarang telah ditempatkan banyak Stasiun radio pancar ulang diangkasa luar yang disebut Satelit Telekomunikasi. Kebanyakan satelit komunikasi adalah jenis orbit Geosinkron yang diletakan tepat ± 36.000 Km diatas khatulistiwa, sehingga ditinjau dari permukaan bumi ia akan tetap berada di tempat yang sama. Sesungguhnya ia bergerak dalam lintasan orbit setapak dengan perputaran bumi dan karena itu disebut juga satelit geostationair. Satelit ini terbagi atas beberapa kelas lingkup frekwensi kerjanya, sehingga terdapat beberapa jenis seperti Ban S, Ban C, Ban Ku dsb. Yang lazim digunakan untuk pancar ulang siaran televisi adalah Ban C dan Ban Ku, Ban Ku dikenal untuk Pancaran langsung Siaran televisi. Pada awalnya Siaran pancar ulang Televisi mengunakan Ban C. Satelit Ban C memakai Frekwensi pancaran kebumi (down link) antara 3,7 ~ 4,2 GHz. (Lebar 500 MHz). Lebar ban 500MHz ini akan dibagi dalam beberapa daerah dengan lebar yang lebih kecil yang disebut transponder. Dilingkup frekwensi keja UHF keatas lazim dipakai tehnik polarisasi pancaran untuk memperoleh tempat yang lebih luas. Jadi lebar ban 500Mhz ini bisa digunakan secara bersama untuk 2 macam polarisai pancaran tanpa saling mengganggu, Umumnya Polarisasi pancaran vertikal dan horisontal atau pancaran sirkuler kiri dan kanan. Dengan mengunakan tehnik ini seakan diperoleh tambahan lebar ban sebesar 500 MHz. dan keseluruhan bisa diperoleh 24 buah transponder selebar ± 40 MHz. (lihat tabel dibawah ini)


C Band Transponder Frequency allocation

Vertical ODD Transponder center Frequency
T 1 T 3T 5T7T9T 11T 13T15T 17T 19T 21T23Telemetry
down link
  3720 37603800384038803920396040004040408041204160
Horizontal EVEN Transponder center Frequency
        T 2T 4T 6T8T10T 12T 14T16T 18T 20T 22T24        
 
          3740
3780382038603900394039804020406041004140
4180        
The effective transponder bandwidth is 36 MHz so that in between the transponder there is 4 MHz spacing

Television broadcast signals usually utilized 1 full transponder, But for economical reasons there are some TV Broadcast using a half transponder only or 2 TV broadcast shared 1 transponder divided in the FDM (Frequency Division Multiplexing) or TDM (Time Division Multiplexing) mode. If a FDM mode is used, usually the sound channel is separated using a SCPC (Single Channel Per Carrier) this mode resemble of a VSAT.

Untuk keperluan siaran TV biasanya digunakan 1 transponder penuh. Namun ada karena alasan ekonomis hanya digunakan setengah transponder atau menyewa satu transponder yang digunakan secara bersama dengan cara pembagian FDM ataupun TDM. Biasanya bila cara FDM yang dipakai maka saluran suaranya dipisah memakai saluran SCPC mirip dengan sistim VSAT

THE C BAND SATELLITE TVRO SYSTEM

The C Band satellite Television system.
  • The television studio delivered its program signal to a satellite earth station that processed the signal in into a suitable transponder base band format like adding a ± 30 Hz dithering triangle waveform at the wideo signals (This dithering technique is use to eleminate interference to terrestrial microwave system), determined the pre emphasis time constant etc, further modulate it to the uplink frequency of around 6 Giga Hertz band in FM Mode, send it up to the geosynchromous communication satellite, the satellite amplifies the received signal and convert the frequency to 3.7 Giga Hertz thereafter retransmit it back downward to earth.

  • The Down link signal from the satellite will be collected by an outdoor Parabolic antenna bundle it into a beam and reflect it to another antenna called Feed horn that is located right at the focus point. An LNA (Low Noise Amplifier) amplify the signal and delivered it to a Down converter unit that convert the frequency to a much lower frequency. The down converter delivered the signal via a cable to the indoor receiver unit. The first generation Satellite TVRO used 70 MHz 36 MHz banwidth (1 Transponder) the transponder is selected here.

  • The Receiver unit received the signal, Demodulate it into a baseband signal that is further fed to three circuits that is to

    1. The audio circuits that demodulate the FM audio carrier into audio signals,
    2. AFC Down Converter transponder tunig voltage
    3. Video processing unit that consist of Video amplifier De emphasis circuit, video low pass filter, video polarity selector, Video clamping unit that removes the triangle signal. and Video amplifier.

  • Stasiun Televisi menyalurkan isyarat programanya ke stasiun bumi, yang kemudian mengolahnya menjadi isyarat frekuensi Base Band transponder yang sesuai. Seperti menambah getaran gelombang segitiga berdenjut ± 30 Hz pada isyarat video (Technik denyutan ini adalah untuk mengurangi gangguan terhadap peralatan microwave terestrial), menentukan jenis pre emphasisnya dll.Kemudian dimodulasikan secara FM ke frekwensi Uplink sekitar 6 Giga Hertz. dan dipancarkan ke satelit geosinkron. Satelit menguatkan isyarat yang diterima dan mengubah frekwensinya ke frekwensi downlink 3,7 Giga Hertz . Lalu dipancarkan ulang kebumi.

  • Isyarat down link dari satelit dikumpulkan oleh sebuah antena parabola yang diberkas kearah sebuah antena lain bernama Feed Horn pada titik fokus. Isyarat ini diperkuat oleh LNA dan disalurkan ke Down Converter yang mengubah frekwensinya menjadi lebih kecil, Pada generasi awal TVRO memakai 70 Mhz selebar 36 Mhz dan disini pula dilakukan pemilihan transponder.

  • Perangkat penerima menerima isyarat dari antenna, me didemodulasinya menjadi isyarat base band yang kemudian disalurkan ke tiga unit rangkain.

    1. Rangkaian suara yang me demodulasi isyarat gelombang FM suara menjadi suara.
    2. Rangkaian AFC untuk tegangan pemilih transponder
    3. Rangkaian pengolah Video yang terdiri atas penguat video, rangkaian deimphasis, Low pass filter, pemilih polarisasi video, rangkai Clamping yang memotong isyarat denyut gelombang segitiga dan akhirnya penguat akhir video
.

A VHF folded dipole construction Coaxial cable Velocity test Stacking Colinear 3 x 3/4 A Duplicated Boomer Yagi antenna for 2 m Transmission lines problems This Page
  Page 1 of 4


Updated March 2000.-

Still Under Construction