1. Field of the Invention
This invention relates to the field of antennas, and more particularly to the field of wideband antennas.
2. Introduction
There is an increasing interest in wideband, low noise receivers for the next generation radio telescopes. Ultra wideband receivers are essential for sweeping over large frequency ranges, frequency agility, detection of short duration pulses, multi-frequency imaging, and simultaneous observation of several spectral lines.
Traditionally, radio telescopes make use of feed horns for illuminating the parabolic aperture because of their simplicity, ease of excitation, versatility, large gain, and preferred overall performance. A feed horn is a horn shaped antenna used to convey radio waves between a waveguide or waveguide-coaxial transition at the transmitter and/or receiver and the antenna reflector. In receiving antennas, incoming radio waves are gathered and focused by the antenna's reflector on the feed horn, which electromagnetically couples them to a waveguide or waveguide-coaxial transition, which, in turn, couples the incoming radio signal to the receiver. Feed horns are used mainly at microwave (SHF) and higher frequencies.
Feed horn bandwidths are limited to less than an octave and, hence, typically a set of feed horns operating at different frequencies is used to observe over a wideband range. A feed for parabola is situated such that its phase center coincides with the focus of the parabola. Different frequency bands can be selected by changing the feed horns. In some cases, it is important to study a scientific phenomenon by observing a source simultaneously at different frequencies.
Typically, in the field of radiometry, smooth, metal surfaces are used to change the path of the energy of interest. Occasionally, two reflecting surfaces (or plates) are positioned such that a standing wave condition at particular frequencies develops between the two surfaces. Such a standing wave condition may be caused by the distance between the surfaces being an odd multiple of half the wavelengths and/or portions of the surfaces being parallel. A standing wave condition causes an undesired frequency dependence, which may affect the transmission path between the surfaces, thereby negatively affecting data obtained through the use of the surfaces.
Due to the sensitive nature of radiometry, it is often necessary to calibrate the receiver. One method of calibration is to use the receiver to measure temperature. Typically, the temperature of objects at different and known temperatures is measured to determine a calibration coefficient. When a receiver is in use on an antenna it can be difficult or impractical to provide enough known temperatures for a proper calibration. The feed horn, itself can be an object of known temperature by attaching a temperature sensor to the feed horn and thus reduce the difficulty of calibration. FIG. 1 schematically depicts a configuration where the feed horn is used to calibrate the radiometer. To measure the temperature of the feed horn 100, for example, a reflective surface 101 placed perpendicular to the axis of propagation is used to reflect the image of the feed horn back into itself in a manner such that the radiometer measures its own feed horn's temperature.
However, if a smooth surface is used to calibrate the receiver using the feed horn's temperature, a standing wave can be created between the feed horn and reflector and the resulting resonance can overwhelm the accuracy of the calibration. For example, standing waves 102, 103 and 104 can be formed at particular frequencies, which resonances may be read by the radiometer as a falsely stronger or weaker signal at those frequencies. Therefore, there is a need for a method of calibrating a receiver without creating a standing wave.