Compact self-directional antenna based on a helical ring

In portable wireless devices, antennas have to respect several severe criteria. They must be compact, have a low sensitivity to their surroundings, and thus have controlled directional radiating characteristics. Circular polarization may also be an important concern. Such criteria exist for RFID antennas [1], mobiles handsets that must respect SAR constraints [2], or positioning systems (GPS, GALILEO, GLONASS). Few designs achieve to meet all these criteria. For instance, a well-known technique to achieve directivity is to use antennas with reflectors of size at least a quarter of wavelength. But in addition to their large sizes, such antennas are sensitive to their environments.