“Sólo se tiene un derecho cuando alguien debe someterse a respetarlo”
ACTOS DE COMERCIO
As previously stated, for multiport arrays that are not designed for MIMO applications, a distinction should perhaps be created, as in their practical operation they are typically used in an “all excited” manner - that is, we are not necessarily interested in separate „embedded‟ channels. The rest of this chapter is devoted to the efficiency characterisation of larger sized (conventional) antenna arrays designed for radio astronomy applications.
In this chapter, a new equation is developed to characterise the efficiency performance of conventional arrays, taking into account the realistic “all excited” nature of the arrays‟ practical operation. This sub-section is concerned with an introduction of the antenna array under test. Again, this work results from a collaboration between two parties; the antenna array (denoted the Octagonal Ring Antenna or ORA for short), was developed by A. K. Brown and Y. Zhang at the University of Manchester [22]. Therefore, in this sub-section it will only be briefly described.
The antenna under test in this study is a five-element compact dual polarised aperture array prototype based on an octagonal ring configuration as proposed in [22]. The basic design premise of the array has been formed from the conceptual theory of current sheet arrays (CSA) introduced by Wheeler [23]. Thus, the design aims to make use of, instead of reducing the mutual coupling between the array elements. The reason why the coupling is required in the design strategy is because in order to keep the radiation pattern side lobes under control, the array must be operated in a region where the electrical separation between elements is small; hence the mutual coupling is high. This also necessitates why the array must be used in an „all-excited‟ manner.
The spacing between successive elements in the prototype is configured in a triangulated manner with the horizontal distances set at 112 mm between each consecutive feed and vertical feedlines of 55 mm in length. Each antenna element includes an integrated stripline transition which was known to introduce some loss. However due to its integrated nature this was treated as part of the antenna.
The desired operational frequencies of the prototype are from 400 MHz to 1400 MHz in which the aforementioned coupling effects are used to obtain a good level of impedance matching throughout the entire range. The overall dimensions of the array are width = 540 mm, height = 215 mm and the depth of the array from the front to the metallic ground at the back is 105 mm.
The front and side view of the array prototype can be viewed in Figs. 4.18 (a) and (b) respectively.
Fig. 4.18 (a): Front view of five element ORA prototype
4.10 Measurement Parameters
The measurement parameters and procedures will be split into two sections; one to detail the all-excited efficiency measurements in the RC, the other details the power loss deduction of the power dividers that were employed for this study.
4.10.1 RC ‘All-Excited’ Measurement Parameters
The stirring sequences were configured to encompass 1˚ mechanical stirring intervals and polarisation stirring. A total of 718 measurement samples thus existed per frequency point. The measured frequency ranges in this instance were selected from 400 MHz to 1000 MHz owing to the upper frequency limit of the power divider employed. A total of 801 frequency data points was utilised to ensure a sufficiently large number of modes would be excited in the chamber throughout the measurement range. As consistent with standard efficiency measurement procedures in the RC, a reference measurement first took place for calibration purposes; a log periodic antenna (Rohde & Schwarz HL223) was used having known performance values (> 90%). The transmitting antenna was a homemade Vivaldi antenna working from 400 MHz to 2 GHz. The set-up used in the all-excited array measurement is depicted in Figs. 4.19 (a) and (b).
(a) (b)
4.10.2 Power Divider Measurement Procedures
For the determination of the power divider loss values, an open air test site was selected. An 8:1 power divider (Mini Circuits model no. ZC8PD1-10-S+) with a voltage standing wave ratio (VSWR) < 1.21 from 300 MHz to 1 GHz was employed in this study. The power divider had rigid coaxial cables connected from the output ports of which five in total were used to connect to the array feeds with equal weights. This left three ports on the power divider obviously unused; hence throughout all measured sequences these ports were terminated in impedance matched loads (50 ohm in this case). Fig. 4.20 depicts the measurement set-up to determine the power divider loss.
Fig. 4.20: Power divider measurement To deduce the loss of the power divider accurately, the same impedance matched loads on the unused ports needed to remain in place during the loss deduction measurement to ensure the power dissipated by the divider would be consistent.
To determine the power loss, S-Parameters were employed in conjunction with a VNA. All outputs bar one were terminated in impedance matched loads and the transmission coefficient
S
21 was measured between the common input and one output as a function of frequency. This gave rise to five separate transmission coefficient measurements; one for each output port, and a total insertion loss T
IL was defined according to (4.23).
6 2 2 1 , 10 log 10 ) ( m m IL dB S T (4.23) Received voltageUnused ports terminated in 50 ohms
where: T m R m V V S , 1 , , for m = 2, 3, 4, 5, 6 respectively.