Final answer:
The first step before sending a radio message is to check the frequency to ensure clear communication. Understanding radio set symbols and frequencies is crucial, as is knowing the correct frequency to identify and interpret signals, whether terrestrial or extraterrestrial.
Step-by-step explanation:
The first thing you do before sending a radio message is to check the frequency. This is important because you need to ensure that the frequency you intend to use is clear and is the correct one for your intended recipient. When reading the face of your radio set, you would typically find information such as the radio's dial, which indicates the frequencies to which the radio can tune, as well as various symbols that represent different functionalities of the radio. Understanding these can help you to correctly set your radio frequency for effective communication.
For a cellular phone signal in the United States at 850 MHz, you can calculate the wavelength of the radio waves using the formula λ = c/f, where λ is the wavelength, c is the speed of light (approximately 3×108 meters per second), and f is the frequency. Knowing the correct frequency is also crucial when attempting to interpret or listen for signals from a direction, such as an extraterrestrial source, as you would need to know which frequencies or channels the message might be coming from to detect it.