Since we must not take any thing for granted it will be useful to state what creates radiation and what does not. Fig 1 outlines the basic parts of a normal antenna. The normal antenna is the basic dipole antenna which is of electrical half wave length with symmetric current flow along its length. The current or electrons which flows in the dipole undergoing maximum acceleration after returning from the ends of the dipole which is the main criteria for radiation acceleration of electrons.
Since to use the antenna we must have a transmission line, and any measurement of the antenna in the shack is also including the transmission line contributions which can be varied. FIG 2
The normal picture of the dipole is very symmetrical and is normally displayed for a single frequency, sometimes showing a bit more. FIG 3
The fact is, that we rarely use the antenna at the EXACT design frequency, so the current flow is not so neat. When not on the design frequency, the electrons reach the ends of the physical dipole before it is ready. The point of maximum radiation thus moves at a different place with changes in frequency. So the picture of the dipole is not so neat and tidy until another frequency is found that put the minimum radiation at the ends of the dipole.
It would appear from the above that it would be quite easy to determine all the frequencies where the current flow in a dipole will be symmetrical and of zero value at the physical ends.
With the advent of the Autec Impedance measuring instrument which also measures capacitance, SWR e.t.c. many amateurs are measuring the characteristics of their set up to discover many resonant points on their "antenna" and guided by the multitude of neat dipole current flow diagrams believe that they fully understand what is happening. Those who look deeper at the relationship of the RESONANT points see that things do not really add up. It is at that point that the realization comes about of the contribution of a transmission line that we first begin to understand the underpinnings of the G5RV.
Fig 4 shows a G5RV antenna and we can measure its characteristics with the use of an AUTEC and come up with a graph, Fig 5, and realize that the transmission line has become part of the antenna and contributes in a large way to it's multi band characteristics. Sometimes the current route of flow is symmetrical along the dipole and at other times the flow deviates down the transmission line in an effort to present a symmetrical current flow along the dipole. This tortuous path of electron flow create different radiation patterns, from bi-lobe to multi-lobe form.
If the direction of the resulting lobe or its magnitude is not important, then the antenna can obviously used at many frequencies. Especially if a tuner is used to satisfy the reluctance of a transmitter to tangle with such an 'irregular' radiator. However, it can be seen that for a multi band beam G5RV, the transmission line is a major contributor to it's operation.
