Composition of Basic HF Equipment

A HF installation for two-way communication consists of one or more transmitters and one or more receivers. The most common type of modulation/waveform is single-sideband suppressed carrier (SSBSC). The operation of this equipment may be half- or full-duplex. We define half-duplex as the operation of a link in one direction at a time. In this case, the near-end transmitter transmits and the far-end receiver receives; then the far-end transmitter transmits and the near-end receiver receives. There are several advantages with this type of operation. A common antenna may be shared by a transmitter and receiver. Under many circumstances, both ends of the link use the same frequency. A simplified diagram of half-duplex operation with a shared antenna is shown below.

Half Duplex Block Diagram

Full-duplex is when there is simultaneous two-way operation. At the same time the near end is transmitting to the far end, it is receiving from the far end. Usually, a different antenna is used for transmission than for reception. Likewise, the transmit and receive frequencies must be different. This is to prevent the near-end transmitter from interfering with its own receive frequency. Typical full-duplex operation is shown below.

Being interfered with by one's own transmitter is called co-site interference. Care must be taken to assure sufficient isolation. There are many measures to be taken to mitigate co-site interference. Among those that should be considered are frequency separation, receiver selectivity, use of separate antennas and their sufficient separation, shielding, filtering transmit output, power amplifier linearity and grounding, bonding, and shielding. The worst environments for co-site interference and other forms of electromagnetic interference (EMI) include airborne and shipboard situations, particularly on military platforms.

When there is multiple transmitter and receiver operation required from a common geographical point, we may have to resort to two- or three-site operation. With this approach, isolation is achieved by physical separation of transmitters from receivers, generally by 2 km or more. However, such separations may even reach 20-30 km. At more complex installations, a third site may serve as an operational center, well isolated from the other two sites. In addition, we may purposely search for a "quiet" site to locate the receiver installation; "quiet" in this context means quiet regarding man-made noise. Both the transmitter site and receiver site may require many acres of cleared land for antennas.

The sites in question are interconnected by microwave LOS links and/or by coaxial cable or fiberoptic cable. Where survivability and reliability are very important, such as at key military installations, at least two distinct transmission media may be required. Generally, one is LOS microwave and the other some type of cable.

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