The rectangular waveguide components were produced by grinding of the substrate which was Polystyrene 4 lb (comm.). The circular waveguide components, of the same dielectric substrate, were produced by turning. The copper plating of the substrates was performed by using either Shipley or Dynchem process.
Lightweight components and integrated microwave subsystems, whose electrical performance is compatible with their conventional counterparts, can be easily and economically produced. Examples of such components are shown in Fig. 11. A complete TR subsystem, suitable for airborne operation, is shown in Fig. 12.
We introduce new observables of the cosmic microwave background radiation, which can be measured through the detection of high order modes excited within an antenna feed system, coherently combined with those currently detected by space observatories. The use of such observables could potentially further constrain the validity of cosmological theories.
The goal of the present work is to introduce additional observables, primarily for future space observatories, which can also be related to the structure of the temperature field. The approach is to detect high order modes in the feed system, which are excited, together with the dominant mode (the one used already), by the impinging radiation and that typically do not propagate beyond the feed system. Such modes carry information about the radiation, they can be extracted and coherently combined with the dominant mode to form useful observables. Such high order modes are already used in other fields, e.g., for tracking systems used to identify and track the direction of arrival of radio emitters, and the present paper shows how their use can be extended to the characterisation of the CMB.
The paper is organised as follows: In Sect. 2 the current methodology is discussed, with a focus on the main principles and ideas. The objective is to recall few key assumptions which are necessary when interpreting the measured observable, and which are also required for using the newly proposed ones. In Sect. 3 the new observables are introduced. Section 4 describes the approach for a sample antenna system, limiting the discussion to two high order modes and to a single polarisation. Section 5 shows how the important aspect of polarisation is incorporated in the new approach, from an experimental point of view. Implementation challenges due to non-ideal behaviour of the involved components are reported and briefly discussed in Sect. 6, concluding remarks and hints for future work are reported in Sect. 7.
In the above definition \(\overlineT \) is the mean value, measured to be \(2.72548\pm 0.00057 \mathrmK\) [11], and \(\Delta T\) is the fluctuation about the mean. We assume that the antenna feed system has two output ports, reacting to orthogonal polarisation states of the incoming radiation. When random unpolarised radiation is observed, the power is split equally across the two ports, however this is not necessarily the case and it depends upon the physical properties of the source.
The signal available at the power measurement point shown in Fig. 1 is made up of a contribution due to the internal noise from the feed and electronics, and the contribution due to the CMB temperature. We assume for the moment that the CMB temperature is different from zero only at a point source along the direction \(\theta , \phi\) within a small solid angle \(\delta\Omega\), within the antenna reference frame. In such scenario the received power is expressed as follows
The amplitude and phase components in Eq. (3) may be frequency dependent, however here we assume an ideally flat frequency response within the measured bandwidth (furthermore, for the phase we neglect any linear variation in frequency associated to a propagation delay within the system). The power on the left side of Eq. (2) therefore is due to a narrowband noise-like signal being received by the feed, which can be represented, after frequency down-conversion to complex baseband, as follows (time dependency is shown only below, and is omitted in the rest unless it is not obvious)
An important challenge is related to the ability to detect both orthogonal polarisations of the incoming radiation (as conceptually depicted in Fig. 1 and Fig. 2), with proper discrimination. The retrieval of both orthogonal polarisations is feasible for the dominant mode as well as for the higher order modes, as exemplified in [23] and [25], with cross-polar discrimination between 25 and 30 dB over the supported frequency range for the dominant mode. For the high order mode, due to the presence of a null in the boresight direction, the amount of cross-polarisation is more difficult to define. In [25] the simulations show overall good cross-polar performance. The main issue however is that the retrieval of the required orthogonal polarisations for the high order mode may require a complex network of waveguide components, with potential accommodation issues for a space born experiments, maybe less of a concern for ground based systems.
We are not considering here multi-mode feeds like those described e.g. in [14] and [15], in which several modes propagate and are collectively (and incoherently) detected with the main purpose of shaping the antenna power radiation pattern.
A beam waveguide antenna is a particular type of antenna dish, at which waveguides are used to transmit the radio beam between the large steerable dish and the equipment for reception or transmission, like e.g. RF power amplifiers.
Beam waveguide antennas are used in large radio telescopes and satellite communication stations as an alternative to the most common parabolic antenna design, the conventional "front fed" parabolic antenna. In front feed, the antenna feed, the small antenna that transmits or receives the radio waves reflected by the dish, is suspended at the focus, in front of the dish. However, this location causes a number of practical difficulties. In high performance systems, complex transmitter and receiver electronics must be located at the feed antenna. This feed equipment usually requires high maintenance; some examples are water cooling for transmitters and cryogenic cooling for sensitive receivers. With the large dishes used in these systems, the focus is high off the ground, and servicing requires cranes or scaffolds, and outdoor work with delicate equipment high off the ground. Furthermore, the feeds themselves have to be designed to handle outdoor conditions such as rain and large temperature swings, and to work while tipped at any angle.
The beam waveguide antenna addresses these problems by locating the feed antenna in a "feed house" at the base of the antenna, instead of in front of the dish. The radio waves collected by the dish are focused into a beam and reflected by metal surfaces in a path through the supporting structure to the stationary feed antenna at the base. The path is complicated because the beam must pass through both axes of the altazimuth mount of the antenna, so turning the antenna does not disturb the beam.
Beam waveguides, which propagate a microwave beam using a series of reflectors, were proposed as early as 1964.[2] By 1968, there were proposals to handle some of the signal path in pointable antennas by these techniques.[3] By 1970, a fully beam-waveguide approach was proposed for satellite communication antennas.[4] At first, it was believed the complicated signal path with its multiple reflecting surfaces would result in unacceptable signal loss[5] but further analysis showed the waveguide system could be built with very low losses.
The first full scale beam waveguide antenna was the 64 meter antenna at the Usuda Deep Space Center, Japan, built in 1984 by the Japan Aerospace Exploration Agency.[6] After the Jet Propulsion Lab (JPL) tested this antenna and found it better than their conventional 64-meter antennas,[7] they too switched to this method of construction for all subsequent antennas of their Deep Space Network (DSN).
Where in current weather radar systems a slip ring transmits data to the antenna, the SPINNER FORJ brings data rates of several Gbit/s with highest reliablility to weather radar systems around the globe.
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