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Satellite TVRO Part 2

Sep 2, 2009 3:16 PM, By Russell Brown

Figure 1. Polarities used by satellite transponders
Select figure to enlarge.

Satellite transponders are simple repeaters that downconvert the incoming signal before rebroadcasting it. There are a limited number of satellites and a limited amount of RF spectrum in which to send and receive these signals, so to overcome these limitations, several methods have been implemented to expand the number of signals that a communication satellite can handle.

Polarization

Each transponder’s frequency band is allowed to overlap the next one, because each transponder has a different polarity than the transponders on either side of it. Using crosspolarity reduces the amount of crosstalk between adjacent transponders and reduces the amount of frequency spectrum required.

Broadcast communication satellites use linear polarity, i.e. horizontal and vertical. But in direct-to-home satellite transmissions, as well as others, circular polarity is the standard where there is right-hand circular polarity (RHCP) and left-hand circular polarity (LHCP). (See Figure 1.)

Switching between polarities provides 20db-40dB of isolation, which allows overlapping frequency bands between crosspolarized transponders. Receiving antennas are aligned to receive signals of one or the other polarity, whether linear or circular. The physical design of the antenna determines which polarity is passed through. For linearly polarized antennas, a motor can move the probe of the antenna and shift it from horizontal to vertical and back again, so one output can present either polarity’s signal. Other linear antennas can supply both polarities at the same time to two different ports. The polarity of a linear antenna must be set precisely to be able to receive the intended polarity.

Circular polarity antennas can be designed for LHCP, RHCP or both, but only one polarity can be delivered to one port; to receive both polarities, two ports are required.

Figure 2: Fractionalized transponder plot
Select figure to enlarge.

Circular polarization has two main advantages over linear polarization. First, when setting up a receive antenna, there is no alignment of the polarity because the antenna is designed to accept the correct polarity. Second, circularly polarized signals cannot be depolarized. When polarized signals pass through the atmosphere, they can become depolarized, where the off-axis shift of the polarity of the signals (in linearly polarized signals) causes a reduction in the desired polarity and an increase in the opposite polarity, resulting in interference. This effect is most noticeable in the lower-frequency C-band signals. Due to the nature of circularly polarized waves, a rotation of the signal will not affect its reception.

Fractionalized transponders

Normally, a single uplink facility sends a signal up to (illuminates) a satellite transponder, but with precise control, several uplink facilities can transmit to a single transponder. This cuts down on costs and conserves transponder usage. To do this, all the uplink facilities must coordinate with the satellite’s control center and keep the power levels very close. This is very specialized and is not used for normal TV transmissions. (See Figure 2.)

Frequencies

Figure 3: Transponder frequencies
Select figure to enlarge.

For C-band satellites, the uplink frequencies are 5.925GHz–6.425GHz. Once the satellite’s transponder downconverts the signals, they are downlinked on 3.7MHz–4.2Mhz. Ku-band satellite uplink frequencies are 14GHz–14.5GHz, and the satellite’s transponder downlinks them on 11.7GHz–12.7 GHz. (See Figure 3.)

These signals are received at the antenna and fed into a low-noise block converter where the signal is amplified and mixed with a local oscillator (LO) to produce a lower set of frequencies that will travel a longer distance over coax cable to the receiver or integrated receiver-decoder.

Different LOs are used for different frequency bands, so the resulting frequencies fall into a common band enabling a receiver to use a single intermediate-frequency front-end to receive several different types of satellite signals. That common band for C and Ku is 950MHz-1450Mhz. The LO used for C-band is 5150MHz, and the resulting signals are above the LO. For the Ku-band, the LO is 10750MHz, and the resulting output is below the LO. The stability of the LO can have a great effect on the quality of the signal you receive, or even if you get a signal.