FIGURE 2. How increases in effective radiated power (ERP) of a transmitting radio dish are related to wavelength and the size and distance of the target.

The increase in ERP is plotted on the vertical axis for two antennas as a function of wavelength. For a given antenna size, the power is increasingly concentrated into a smaller region as the wavelength decreases. When the antenna surface becomes comparable to the wavelength, this dependence breaks down (indicated by the dashed line). The 100-m diameter antenna has 16 times the effective power of a 25-m antenna for the same wavelength and target size. This advantage also holds when the telescope is used for receiving: the size of the region contained within the telescope beam at a distance of 100 light years is shown on the right-hand scale. For example, the 100-m radio telescope will include a region 0.14 light years across. Because this is about 4,000 times the diameter of the Earth's orbit around the Sun, we would certainly receive a transmission from an Earth-like planet when pointing towards a star at 100 light years.