Transponders and Antenna Subsystems.pptx

Transponders

1. Telem e try, Tracking, Co m mand & Monitoring(TTC&M)

2. Transponder Antenna Subsystems 🞇 Transponders- A transponder consists of BPF to select the particular channel’s band of frequency, a downconverter & an output amplifier.

3. 🞇 Redundancy is provided for high-power amplifiers in each transponder by including a spare TWTA(travelling Wave tube amplifier) or solid-state amplifier(SSPA) that can be switched if primary power amp l i fier s fai l . 🞇 The lifetime of high power amplifier’s is limited and they represent least reliable component in most transponders. 🞇 Providing a spare HPA in each transponder incr eas e s the pr o b a bil i t y t h a t t h e s a t el l i te wi l l r e a ch the end of its working life with all its transponders working.

4. 🞇 Transponders for use in the 14/11-GHz bands normally employ a double frequency conversion scheme 🞇 Incoming 14-GHz carrier is translated to an IF around 1GHz. 🞇 The amplification & filtering are performed at 1 GHz & a relatively high-level carrier is translated bac k t o 1 1 GH z fo r amp l i f ic a tion by HPA .

5. Satellit e Ante n nas: 🞇 W i re Antenn a s : M onopole & D i pole 🞇 Horn Antennas 🞇 Refle c tor A ntennas 🞇 Array Antenn a s

6. Wire Antennas: 🞇 These are used primarily at VHF & UHF to provide communications for the TTC&M systems. 🞇 They are positioned with great care on the body of the satellite in an attempt to provide omnidirectional coverage. 🞇 An antenna pattern is a plot of the field s t rengt h i n f a r f i eld on an t enn a . 🞇 It is usually measured in decibels(dB).

7. Hor n Ante n nas: 🞇 These are used at microwave frequencies when relatively wide beams are required, as for glob a l co v erage . 🞇 A horn is a flared section of waveguide that provides an w i de an d a aper t u re good s e v eral wa v e l e n g t hs ma t ch be t we e n t h e wa v egu i de i mpe danc e an d free spac e . 🞇 Horns are also used as feeds for reflectors

8. Reflector Antennas: 🞇 The most often used antenna for satellite systems, particularly for those operating above 10 GHz, is the parabolic reflector antenna . 🞇 The s e ar e u s ual l y il l u m inate d b y one or mo r e ho r ns and provide a lager aperture compared to horn antenna. 🞇 Parab o l i c refl e ctors o f fer a m u ch hi g he r ga i n t han t hat achie v ab l e b y t he ho r n an t en na al one.

9. Arr a y Ante n nas: 🞇 Array antennas are also used on satellites to create multiple beams from a single aperture. 🞇 Multiple beams are formed by combining the ra d ia ti o n f r om se v eral s m al l e l e m ents made up of d i pol e s , ho r ns e t c. 🞇 I r i d iu m an d G l o bals t a r use d t hese kind o f an t en nas t o gene ra t e up t o 1 6 be am s f r om a single aperture for their LEO mobile telephone systems.