First study transformer theory. Particularly look at air core transformers at high frequencies. An transmitting antenna may be considered as a single turn primary of an air core transformer. The magnetic flux spreads out losing strength as it goes. There will be a current induced in any conductors influenced by the field. This current wills cause a field that will re-radiate the signal. In many instances this will be negligible. However large areas such as conductive roofs and buildings may "reflect" a radio signal quite efficiently.
The effect may be enhanced by several techniques such as length, spacing, orientation, multiplication and directionality.
A yagi is an arrangement of passive elements which are cut and spaced so as to effectively re-radiate the radio signal.
Now that we have enhanced the current in the antenna, this current may be detected and amplified.
In some instances the current may be enough to directly power very low drain and/or efficient devices.
A dish reflects a radio signal into a tight beam making transmission and reception much more efficient.
Anecdote: In the early days of satellite TV some small fiber-glass dishes hit the market. They didn't work. The builder had looked at a dish and said;
"I can do that."
He bought a sample dish for a pattern and starting building dishes. He was not aware that there is an internal conductive layer in plastic and fiber-glass dishes. Without the conductive layer the dishes were useless.
Bill
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"Why not the best?"
Jimmy Carter
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