Why can a feedline be cut randomly when the antenna needs a specific length?

Question

With a mobile two-way radio you have the radio, an antenna mount kit and the actual whip antenna.

The antenna needs to be trimed to the correct length so to match the freequency you are Tx/Rx on.

When you install the antenna mount kit though, you cut the feedline length without regard to electrical length. Why can you do that?

I would think that the feedline and base were just an extension of the antenna whip and that any change in the length of the cable would have huge impact to the matched frequency.

What is in the base of the antenna mount that allows for this?

Answer 1

The reason you trim the antenna itself is so that the impedance at its base matches that of the transmission line (usually 50Ω for most RF applications).

It is a property of transmission lines in general that if it is terminated at both ends by its characteristic impedance, then the current and voltage measured anywhere along it will be the same. As long as this is true, it doesn't matter what the actual length of the line is; its properties won't vary.

However, if the antenna is not properly matched to the line, then you'll create "standing waves" on the line (current and voltage will vary along its length, depending on the wavelength of the signal), and the overall length of the line will have an effect on the operation of the system.

Answer 2

The antenna feed-line is usually coaxial cable and it intentionally does not radiate significant amounts of RF energy by design. A propeller shaft on a boat wouldn't work without the propeller and the coax cable is equivalent to the shaft. Speaker cables don't make audible sounds (well maybe very tiny ones if they come close to a magnet).

The antenna base will either be a very simple impedance matching network or just a protective cover for the the exposed wires being joined together.

Answer 3

Typically, the impedance of the radio and the coax feedline are the same and constant (e.g 50 Ohm), so the length of the line does not impact the signal quality (aside from small attenuation). In order to deliver the most power to the antenna, the impedance of the antenna has to be the same as the line and the radio (matched). The impedance of the antenna depends not only on the shape of the antenna itself but on its surroundings. Therefore, it is often necessary to adjust the impedance (trip the antenna) once it has been installed.

In some rare instances, the line may be used as part of a matching circuit itself. For example, if the antenna impedance is 100 Ohm and the radio is 50 Ohm, using a 70 Ohm coax line exactly 1/4 of the wavelength long will match the antenna to the radio. In this situation, the length of the line is crucially important to the performance of the antenna.

Answer 4

Consider the traditional view of the current and voltage relative to ground at each point on the antenna as a sine/cosine waveform starting at the antenna feedpoint.

Voltage is not absolute; it is always measured relative to something else. So you can pick any arbitrary point to call "zero".

If you decide to call the voltage at the feedpoint zero, then at which exact point in the waveform from the transmitter this happens to be makes no difference. A quarter wavelength out, voltage (relative to the feedpoint) will be at a maximum and current at a minimum; the antenna will present a purely resistive load, which is the definition of antenna resonance (though it won't necessarily be perfectly matched to a 50 ohm feedline - an antenna at resonance and a 1:1 SWR to a particular feedline impedance are two completely different things!).

If we needed to consider the feedline, then we would also need to consider any distance the signals have travelled inside the transmitter as well. Especially in UHF and up, these distances can potentially be significant fractions of a wavelength or more. So we should be thankful that we only need to concern ourselves with the voltage differences along the length of the antenna proper.

The antenna mount itself generally only provides a sturdy electrical connection from the feedline to the antenna. You'll get the same results if you splice the feedline and simply let it continue in two directions from the feedpoint (barring differences in ground-plane coupling).