A multiband antenna composed of helical elements for sensor networks

we have proposed a very compact 400MHz/900MHz dual frequency bands antenna composed of two normal mode helical elements, which are self resonant at 400MHz band and 900MHz band respectively, and a single loop which is fed and electromagnetically coupled to the two normal mode helical elements. The helixes and the loop are arranged with their axes in parallel. The size of the 400MHz band helix is with 18mm (less than one thirtieth of the wavelength at 445MHz) diameter, 18mm length and five turns and the 900MHz band helix has the same turns and their sizes are about a half of those of 400MHz ones. Fundamental antenna characteristics such as input impedance and radiation patterns were studied and the possibility for sensor network application was ensured.