 To sum up, the timer first generates a sine wave voltage of the proper frequency, changes it to a peaked wave that synchronizes all circuits of a radar system, then causes the shaping of a rectangular wave that triggers the transmitter. The width of each pulse, or the pulse duration, primarily establishes a set's minimum range. While the number of pulses it transmits per second, the pulse recurrence frequency, or PRF, largely determines the maximum range. Timers, remember, control both pulse width and PRF. They vary greatly in size and construction. In this light portable search set, the ANTPS-1, the unit that does the timing is comparatively small. But this long range early warning set uses a timer that includes a whole rack of complicated equipment. Airborne radar sets naturally must be more compact. For example, this B-17 has a radar, the AN APQ-13, whose timer takes up very little elbow room down there on the bottom shelf. Still it does the same job that the timer in a large ground station does. Once the timer has generated a voltage wave and it's been shaped into the desired rectangular pulse, the transmitter is ready to take over. It uses the voltage energy received from the timer to trigger off a radio frequency oscillator, which may be one or more high-powered tubes. This is the most important unit in the transmitter. It is here that the set generates powerful pulses of RF energy, which are flung into space by the antenna. This oscillator tube handles extremely high voltages and requires a strong source of energy to make it function. The voltage pulse as it now stands, although it's the right shape, isn't strong enough to trigger the oscillator tube. So the transmitter uses a driver and a modulator to amplify the voltage it receives. In the driver, the pulse is reproduced and amplified. The pulse is further amplified by the modulator. Now it's ready to trigger the RF energy that will be transmitted. In other words, it acts as a switch for the RF oscillator. When this oscillator tube is triggered by the amplified voltage pulse, it oscillates at a very high frequency, hundreds of megacycles per second, for the length of time of the pulse duration. The period when the tube is not oscillating is a listening period. Its length depends on the PRF established by the timer. This PRF also determines the exact microsecond when the tube will again begin to oscillate. As in the case of the timer, the transmitter size varies according to use. Here's one belonging to a large land-based early warning set. Here's an anti-aircraft gun laying set, the SCR 584 and its transmitter. Here's the compact transmitter of an airborne set, the APQ 13 in the B-17. Whatever the size or shape of the transmitter, the job it does is essentially the same. Amplifying the voltage pulse until it's strong enough to trigger the oscillator. The oscillator then generates RF energy. Once the high-frequency radio pulses leave the transmitter, they are carried by a system of transmission lines to the antenna. There are many types of transmission lines, parallel to wire conductors, coaxial lines, waveguides, and many arrangements. But their purpose is the same. To carry the RF energy from transmitter to antenna with the least possible loss. The more energy that radiates from the lines, the less there'll be to radiate from the antenna where it's wanted. When properly timed RF pulses pass through the transmission lines and reach the antenna, they're ready for active duty. The simplest radar systems contain two separate antenna arrays. One for transmitting pulses and one for receiving echoes. For example, on the SCR-527, a GCI set, here's the receiving antenna, over here's the transmitting antenna. On the 268, the antennas are on the same mount. All antennas operate alike fundamentally. They concentrate radio-frequency energy into a narrow beam and radiate this beam in a given direction out into space. A pulse goes out. Say, uh, wait a minute. Before you go hunting targets, suppose you show us just what part of the antenna you came from. That's right, each of those dipoles radiates energy and the combination of all dipoles acting together results in a very powerful pulse concentrated into a narrow beam. Some antennas have more dipoles than others, depending on the type of beam desired. The length of the dipole in each set will depend on the wavelength of the RF energy being transmitted. Now let's see how a single one of these dipoles transmits energy. When RF energy is fed to a dipole, it radiates from all sides in the shape of a donut. But since we want the energy to go in a particular direction, we use a reflector, which causes most of the energy to be radiated in one direction. The reflector can be a rod, like the slightly longer one there at the left, tuned to its dipole. Or it can be an untuned wire screen or mesh. Through proper spacing of the dipoles and reflectors, the set focuses the outbound energy into a beam, which, like a narrow beam of light, has its greatest intensity of radiation at the center. Although some radar systems employ two separate antenna arrays, one for sending and one for receiving, most of the newer models use the single antenna system, combining the two functions of sending and receiving. Single antennas are easier to handle and more practical. One type of single antenna employs a parabolic reflector, like the reflector in an automobile headlight. A dipole is located at the focal point of the dish, and a reflector is usually fixed about one-quarter wavelength in front of it. The reflector causes almost all the forward energy from the dipole to be reflected back into the dish. It is then concentrated by the parabola and radiated out in the form of a narrow beam. The returning energy is caught by the dish and focused back to the dipole, now acting as a receiving antenna. An example of this single parabolic type is the antenna of the ANTPS-3, a portable early warning set, a dipole on the left at the focal point of the dish, and the reflector a quarter wavelength in front of it. The gun laying 584 also uses a single antenna for both transmitting and receiving. So does the SCR 720, the aircraft interception or AI set, in the night fighting Black Widow. Underneath this dome in the nose of the ship, the dish scans the area slowly or rapidly, whatever the radar operator desires. Although this single antenna system is obviously more compact, it poses a brand new problem in electronics. That problem is a switch, a switch that will enable one antenna to alternate from transmitting to receiving, as often as 5,000 times a second.