ANTENNA DISTRIBUTION SYSTEMS
Receiving antenna distribution systems operate at low power levels and are built to fit a standard 19-inch rack. Each piece of distribution equipment is fitted with termination or patch fittings designed for ease of connecting and disconnecting. A basic patch panel is shown in figure 3-11. Even a fundamental distribution system has several antenna transmission lines and several receivers. Normally a patch panel consists of two basic patch panels. One panel is used to terminate the antenna transmission lines and the other the lines leading to the receivers. Any antenna can be patched to any receiver through the use of patch cords.
Figure 3-11. - Basic rf receive patch panel.
Many distribution systems are more complex. A complex distribution system to cover most situations is illustrated in figure 3-12. In this system you can patch four antennas to four receivers, or you can patch one antenna to more than one receiver via the multicouplers (multicouplers are covered later in this chapter). You can also patch rf and audio from one compartment to another. A frequency standard is connected (through a distribution amplifier not shown) to the receivers.
Figure 3-12. - Complex distribution system.
Transmitting antenna distribution systems perform the same functions as receiving systems. However, because of the higher power levels, design and fabrication problems are more difficult. The ideal design would be to have all the transmission lines designed for the highest power level. But because high-power patch cords are expensive, large, and difficult to handle, this approach is seldom followed.
In practice, the basic patch panel we just looked at in figure 3-11 is practical for low power levels. Another type of transmitter patch panel is shown in figure 3-13.
Figure 3-13. - Transmitting antenna patch panel.
This type of transmitting antenna patch panel is interlocked with the transmitter so that no open jack connection can be energized and no energized patch cord can be removed. This provides you with a greater degree of personnel and equipment safety.
Figure 3-14 is a filter assembly multicoupler that provides seven radio frequency channels in the 14-kilohertz to 32-megahertz range. Any or all of these channels may be used independently of any of the other channels, or they may operate simultaneously. You can make connections to the receiver by means of coaxial patch cords, which are short lengths of cable with plugs attached to each end.
Figure 3-14. - Electrical filter assembly.
A set of nine plug-in type filter assemblies is furnished with the equipment and covers the entire vlf, lf, mf, and hf bands. Only seven of the assemblies may be installed at one time, and you have the option of selecting those you need to cover the most used frequency bands.
Figure 3-12 illustrates how the filter assembly is used in combination with other units to pass an rf signal from an antenna to one or more receivers.
Most multicouplers for the hf range are designed for use with either transmitters or receivers, although some are used with both. There are a large number of channels in a multicoupler so that many transmitters can be used at the same time on one antenna. This is especially true in the 2- to 12-megahertz range.
Figure 3-15 shows you an antenna coupler group designed primarily for shipboard use. Each coupler group permits several transmitters to operate simultaneously into a single, associated, broadband antenna. You can see this reduces the total number of antennas required in the limited space aboard ship.
Figure 3-15. - Antenna coupler group.
These antenna coupler groups provide a coupling path of prescribed efficiency between each transmitter and its associated antenna. They also provide isolation between transmitters, tunable bandpass filters, and matching networks.
In previous areas we have discussed different methods of voice communications. At times, however, the message is too long for practical transmission by voice. To get information or an idea across to another person far away, you may also need a chart, map, or photograph. Teletypewriter (tty) and facsimile equipment allow us to do just that, with ease. Let's see how this is done.
To give you an idea of how intelligence is sent via teletypewriter, let's take a look at the manual telegraph circuit. This circuit, shown in figure 3-16, includes a telegraph key, a source of power (battery), a sounder, and a movable sounder armature. If the key is closed, current flows through the circuit and the armature is attracted to the sounder by magnetism. When the key is opened, the armature is retracted by a spring. With these two electrical conditions of the circuit, intelligence can be transmitted by means of a teletypewriter code. These two conditions of the circuit are referred to as MARKING and SPACING. The marking condition occurs when the circuit is closed and a current flows; the spacing condition occurs when it is open and no current flows.
Figure 3-16. - Manual telegraph circuit.
If the key at station A is replaced by a transmitting teletypewriter and the sounder arrangement at station B is replaced by a receiving teletypewriter, the basic teletypewriter circuit (loop) shown in figure 3-17 is formed.
Figure 3-17. - Simple teletypewriter circuit.
If a teletypewriter signal could be drawn on paper, it would resemble figure 3-18. This is the code combination for the letter R. Shaded areas show intervals during which the circuit is closed, and the blank areas show the intervals during which the circuit is open. The signal has a total of seven units. Five of these are numbered and are called INTELLIGENCE units. The first and last units of the signal are labeled START and STOP. They are named after their functions: the first starts the signal, and the last stops it. These are a part of every teletypewriter code signal: the START unit is always spacing, and the STOP unit is always marking.
Figure 3-18. - Mark and space signals.
The teletypewriter signal is theoretically a perfect signal. The time between each unit remains the same during transmission of the signal. The shift from mark to space (and vice versa) is called a TRANSITION. A transition occurs at the beginning and end of each unit when it shifts from mark to space or space to mark; a character may have two, four, or six transitions.
When figuring the time duration of a signal character, no allowance for transition time is made since the transition is instantaneous and is considered to have zero time duration. The time duration for each unit is measured in milliseconds.
Q.10 What is the function of an antenna patch panel?