 This switch is called a transmit receive or TR switch. It's a combination of spark gaps and transmission lines tuned by stubs to present an easy path for RF energy in one direction, a hard path in another. One spark gap is located in a stub of the transmission line between the transmitter and the antenna. The other is in the stub of the line between antenna and receiver. The system is normally tuned to block off the line from the transmitter and leave the line open to the receiver, like this. The strong RF pulse from the transmitter causes the gaps to spark, changing the values of the tuning stubs, making the path from transmitter to antenna the easy one and at the same time blocking the path from transmitter to receiver. After the transmitter fires and shuts down, the gap stops sparking and the path from antenna to receiver is again open. Now when the echo returns, the normal action of the lines sends all the energy to the receiver. Once the echo signal returns, it must be picked up and amplified by a suitable receiver. Basically, a radar receiver is the same as the average radio receiver. Both are super heterodines. The parts and even circuits are a good deal alike. To understand radio, you'll find the mechanics of the radar receiver pretty easy to grasp. In some sets, when the weak signal enters the receiver, it's amplified immediately. But in all cases, its high frequency has to be lowered before we can work with it any further. It gets lowered in the mixer. Another frequency generated in the local oscillator is mixed with it, and the result is a lower intermediate frequency, or IF signal. But this has to be amplified still further. It may go through as many as seven stages of IF amplification before being fed into the detector. The detector filters out the intermediate frequency, and what's left is like the square pulse that originally triggered the transmitter. But this isn't strong enough yet to register on the indicator, so a final buildup takes place in the video amplifier. The signal is now ready for the indicator. We'll take up the whole subject of indicators in the next film of this series. What you've seen and heard today is a very simple explanation of the different components of any radar set. An illustration of what takes place inside a timer, transmitter, and antenna when they send a pulse of RF energy into space, and in the antenna, receiver, and indicator, when the weak returning echo is picked up. In a single second, this cycle of interconnected actions can occur 5,000 times. So you see, the simplest radar is still a complicated piece of equipment. That's why you've got to know what makes each unit tick.