This also means that anything left out can disappear. Don't risk the loss of a vintage Baldwin headset for the sake of a more favorable consideration from the judges. When your child's teacher blithely chirps how wonderful it would be for people to be able to listen to the set, it is merely a display of enthusiastic ignorance. Don't fall for it. I won't go into the whys and wherefores here, but will be happy to discuss it if you contact me.
Some projects lend themselves well to working models at the fair.
Even these usually require an attendant. A picture and an electronic schematic of the set are always appreciated on the display board, along with a Polaroid of you clinging to the top of an extension ladder rigging the antenna. A good display has an eye catching title across the top, a clear statement of the purpose of the project, a one or two sentence description of the experiment, some experimental data that makes sense, either tabular of graphed, a quick summary and a statement as to whether or not the hypothesis was supported. Back to the original problem. You can access most of it from my links page.