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How does the signal know where the cable ends and the antenna starts?

  An excellent question! Without diving too deep into the theory, let’s start with a few basic terms.

  The “signal” that an antenna is receiving or transmitting is called an electromagnetic wave. This is exactly the same type of wave as light. It is just that our eyes are sensitive to a narrow range of frequencies that we call light. Electromagnetic waves that are lower in frequency behave exactly the same but we cannot see them. These lower frequencies are generally called RF (radio frequencies). Antennas are used to radiate (transmit) and receive electromagnetic RF signals.

  The cable or wire that goes between the antenna and a receiver or transmitter is called a transmission line. We use this term even if the cable is simply used for receiving purposes. A transmission line is specifically designed to not radiate an electromagnetic wave but only transport it from one end of the transmission line to the other. There are several types of construction that can meet this requirement but you are probably most familiar with coaxial cable – the same type of cable that is used for “cable TV”.

  

  In the case of the coaxial cable, the outer shield keeps the electromagnetic wave contained between the outer shield and the inner conductor. And as it does so, the electromagnetic wave moves from one end of the cable to the other.

  The device that generates the electromagnetic wave is called the source. This could be a transmitter or, when receiving a signal, it is the antenna. At the other end of the transmission line is the load. When transmitting, the antenna is considered the load and when receiving it is the receiver that is the load. By convention, the electromagnetic wave travels from the source end to the load end.

  So now armed with these few terms, we can restate your question as: Why isn’t the transmission line included as part of the antenna when accounting for the length of the antenna?

  The simple answer is to realize that an antenna is specifically designed to receive or radiate electromagnetic waves. And you are correct that the dimensions of the antenna play a key role in its overall performance. But now contrast this to the transmission line that is specifically designed to not radiate electromagnetic waves.

  We can draw a comparison here to a garden hose and a sprinkler. The garden hose is specifically designed to transport the water from one end to the other without leaking any of it along the way (although I have a few hoses that seem not to have gotten that memo). The sprinkler, on the other hand, is specifically designed to “leak” water in a very specific pattern and volume.

  I hope that is the level of answer you were expecting. Feel free to use the comments to ask for any clarifications or additional depth.

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