# 10.2 Satellite Transponders

A communication satellite functions as a distant repeater; it receives uplink signals from Earth stations, processes the signals, and then downlinks (retransmits) them to their intended Earth destinations. The International Telecommunication Union has allocated specific bands for satellite communications (Table 10-1). Of these, the bands used by the majority of U.S. commercial satellites for domestic communications are the 4/6 GHz band (3.7 to 4.2 GHz downlink and 5.925 to 6.425 GHz uplink) and the 12/14 GHz band (11.7 to 12.2 GHz downlink and 14.0 to 14.5 GHz uplink). Each uplink and downlink segment has been allocated 500 MHz of bandwidth. By using different frequency bands for Earth-to-satellite uplink segment and for satellite-to-Earth downlink segments, the same antennas can be used for both functions while simultaneously guarding against interference between the two signals. The downlink segment commonly uses a lower-frequency carrier than the uplink segment because lower frequencies suffer lower attenuation by Earth’s atmosphere, thereby easing the requirement on satellite output power.

# Table 10-1 Communications satellite frequency allocations.

Table 10 1 Full Alternative Text

We shall use the 4/6 GHz band as a model to discuss the satellite-repeater operation, while keeping in mind that the functional configuration of the repeater is basically the same regardless of which specific communication band is used.

Figure 10-4 shows a generalized block diagram of a typical 12-channel repeater. The path of each channel—from the point of reception by the antenna, transfer through the repeater, and final retransmission through the antenna—is called a transponder. The available 500 MHz bandwidth is allocated to 12 channels (transponders) of 36 MHz bandwidth per channel and 4 MHz separation between channels. The basic functions of a transponder are (a) isolation of neighboring radio frequency (RF) channels, (b) frequency translation, and (c) amplification. With frequency-division multiple access (FDMA)—one of the schemes commonly used for information transmission—each transponder can accommodate thousands of individual telephone channels within its 36 MHz of bandwidth (telephone speech signals require a minimum bandwidth of 3 kHz, so frequency spacing is nominally 4 kHz per telephone channel), several TV channels (each requiring a bandwidth of 6 MHz), millions of bits of digital data, or combinations of all three.

# Figure 10-4 Elements of a 12-channel (transponder) communications system.

Figure 10 4 Full Alternative Text

When the same antenna is used for both transmission and reception, a duplexer is used to perform the signal separation. Many types of duplexers are available, but among the simplest to understand is the circulator shown in Fig. 10-5. A circulator is a three-port device that uses a ferrite material placed in a magnetic field induced by a permanent magnet to achieve power flow from ports 1 to 2, 2 to 3, and 3 to 1, but not in the reverse directions. With the antenna connected to port 1, the received signal is channeled only to port 2; if port 2 is properly matched to the band-pass filter, no part of the received signal is reflected from port 2 to 3. Similarly, the transmitted signal connected to port 3 is channeled by the circulator to port 1 for transmission by the antenna.

# Figure 10-5 Basic operation of a ferrite circulator.

Figure 10 5 Full Alternative Text

Following the duplexer shown in Fig. 10-4, the received signal passes through a receiver bandpass filter that ensures isolation of the received signal from the transmitted signal. The receiver filter covers the bandwidth from 5.925 to 6.425 GHz, which encompasses the cumulative bandwidths of all 12 channels; the first received channel extends from 5,927 to 5,963 MHz, the second one from 5,967 to 6,003 MHz, and so on until the twelfth channel, which covers the range from 6,367 to 6,403 MHz. Tracing the signal path, the next subsystem is the wideband receiver, which consists of three elements: a low-noise wideband amplifier, a frequency translator, and an output amplifier. The frequency translator consists of a stable local oscillator, which generates a signal at frequency f 0= 10,105 MHz, connected to a nonlinear microwave mixer. The mixer serves to convert the frequency f r of the received signal (which covers the range from 5,927 to 6,403 MHz) to a lower-frequency signal f t=f 0−f r. Thus, the lower end of the received signal frequency band gets converted from 5,927 to 4,178 MHz and the upper end gets converted from 6,403 to 3,702 MHz. This translation results in 12 channels with new frequency ranges, but these signals carry the same information (modulation) that was present in the received signals. In principle, the receiver output signal can now be further amplified and then channeled to the antenna through the duplexer for transmission back to Earth. Instead, the receiver output signal is separated into the 12 transponder channels through a multiplexer followed by a bank of narrow band-pass filters with each covering the bandwidth of one transponder channel. Each of the 12 channels is amplified by its own high-power amplifier (HPA), and then the 12 channels are combined by another multiplexer that feeds the combined spectrum into the duplexer. This channel separation and recombination process is used as a safety measure against losing all 12 channels should a high-power amplifier experience total failure or degradation in performance.

The information carrying capacity of a satellite repeater can be doubled from 12 to 24 channels over the same 500 MHz bandwidth by using polarization diversity. Instead of transmitting one channel of information over channel 1 (5,927 to 5,963 MHz), for example, the ground station transmits to the satellite two signals carrying different information and covering the same frequency band, but with different antenna polarization configurations, such as right-hand circular (RHC) and left-hand circular (LHC) polarizations. The satellite antenna is equipped with a feed arrangement that can receive each of the two circular polarization signals individually with negligible interference between them. Two duplexers are used in this case: one connected to the RHC polarization feed and another connected to the LHC polarization feed, as illustrated in Fig. 10-6.

# Figure 10-6 Polarization diversity is used to increase the number of channels from 12 to 24.

Figure 10 6 Full Alternative Text