Talk:Image antenna

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Possible error[edit]

I'm really not sure but I think there is an error in the article here. I don't think the image antenna has to do with reflections. For instance think of a ground plane that is angled downwards at 45 degrees. If it was about reflections this would really change things. Instead I think it's about how the electric field can form around the antenna. The ground plane gives the field something to form to. The shape of the field is identical to if a dipole was used, it is identical to if the image antenna was real. Without the ground plane the field would have to terminate maybe at the antenna connector and it wouldn't have a good shape anymore.

In a radio antenna the electric fields can be divided into two types: the near fields, also called induction fields, which just store oscillating energy in the space near the antenna, and the far fields, the electromagnetic waves or radio waves, which carry energy permanently away from the antenna. In a monopole antenna using a ground plane, the type you describe above, the ground plane actually performs two related functions:

For the near fields it serves as a capacitor plate, receiving the oscillating electric field from the antenna element (called the displacement current) and returning it to the ground side of the feedline. As you say the plane gives the near fields something to "form to". Without it, there would be currents on the ground shield conductor of the feedline, which would act as an additional antenna.
For the far fields (the radio waves) it serves as a reflector, reflecting the downwardly directed radio waves upward, thus increasing the signal power density above the plane. This is the 'reflection' function you mention above. As you probably know, a perfect infinite ground plane doubles the power density and gain of a monopole over that of an equivalent dipole.

This applies to a monopole antenna, where the ground plane is connected to one side of the feedline. Other types of antennas can also use conductive planes, such as reflective array antennas, which just function strictly as reflectors. --Chetvorno^TALK 20:53, 24 June 2024 (UTC)[reply]