MEDIUM FREQUENCY AND BELOW
Most of the receivers and transmitters that you will see used in the mf portions of the rf spectrum and
below are very similar in design. In chapter 1 we discussed the operational uses of the equipment; now
let's look at the equipment itself.
Equipment items covered in this and other chapters are meant to be merely representative of
equipment that may be encountered in naval communications. No attempt will be made to include all of
the possible equipment or equipment configurations.
You should realize the transmitters used in bands of medium frequency and below are similar to
those you studied in chapter 2. In other words, a transmitter used in one frequency range is basically the
same as one used in another range. However, there are some differences. Two of the differences are
component size and the use of a technique called DOUBLING UP.
The components used in bands of medium frequency and below are much larger physically than the
ones previously discussed. This is because of the higher operating voltage and current levels required to
produce the very high-powered rf outputs needed for the uses covered in chapter 1. A given resistor used
in an hf application may be rated at 1/2 watt, whereas the same resistor used in a lower frequency
application would probably be rated in tens or even hundreds of watts.
A block diagram of a doubled-up transmitter is shown in figure 5-1. Remember, bands of medium
frequencies and below are used almost exclusively for broadcast and are on the air continuously.
Doubling up increases reliability. As you can see, two transmitters are located in the same equipment
cabinet. This allows you to quickly transfer circuits if one should fail. This dual installation also allows
both amplifiers to be used together to double the output power. When you use this application, you
sacrifice the doubling-up capability of only the power amplifier. All the other components are still
available as backups. Let's go through figure 5-1 and describe the block functions.