Quarter Wave Stub Demo

One of the trick that you can perform to your friend (not a Ham) is this quarter wave stub. Looking at the DC (direct current) point of view, the bulb should not light since there is a shorting wire (stub) that will ground any voltage present across the 12V bulb. But strange things happen when a strong rf source is brought near the dipole antenna. The bulb should light brightly! This phenomenon can be easily explained when we understand how the quarter wave stub behaves when excited at its resonant frequency.

In my prototype, I use a 435MHz  3w transceiver to excite the dipole antenna and the 12V pilot bulb was used as an indicator for rf current present across the feed point of the antenna. The antenna itself is approximately 13 inches in total length while the stub length is 6.5 inches. Spacing of the stub was merely approximated to around 1/16 of an inch. Part of the trick is that you can also cut open the far end of the stub and show to your friend how the bulb immediately off at this time.   ---73 de du1vss