GEM - 2.3 GHz Receiver Development Status

30 November 1994
GEM 2.3 GHz system -- Progress Report
by: Marco Bersanelli and George Smoot modifications in 1997

The present status of the GEM 2.3 GHz system:

1) FEED ANTENNA The 2.3 GHz antenna feed has been constructed and tested in Italy. Measurements of the reflection properties done in Torino show return loss near 2.3 GHz center frequency as low as -38 dB (Fig. 1a). A secondary minimum loss appears near 2.7 GHz, at about -25 dB. The reflection properties are confirmed by the tests done in Berkeley (Fig 1b). In terms of VSWR we get 1.023, which corresponds to -39 dB return loss at 2.3 GHz.

The antenna beam pattern was measured in Italy and is shown in figure 1c and figure 1d for the E and H plane, respectively. The -12 dB beam is about 60 deg, and it's down to ~30 dB beyond 90 deg. Fig 1e shows the span in frequency for the response at 0 deg (reference) and the response at 60 deg.

Figure 1f shows a section view of the feed (in its final version - slightly different from the preliminary design). Figure 1f big shows twice as large version.

2) RF CHAIN The components of the RF chain have been tested in Berkeley.

Fig 2a shows the reflection and transmission characteristics of the tubular filter near the 2.3 GHz center frequency. The 100 MHz band is well defined and rejection at +/- 200 MHz away from 2.3 GHz is down to ~-50 dB. Fig. 2b shows that there are resonance bands near 10-12 and 18-20 GHz with significant transmission. The properties HEMT front end amplifier and MITEQ second amp are shown in figs. 2c, 2d, 2e.

3) RECEIVER AND MOUNT The plan for the receiver design and coupling at the prime focus of the dish is shown in figure 3a, 3b, 3c, and 3d. The interface plate shown in fig. 3a is mounted at dish spider supports of the original secondary reflector, and provides the mechanical interface to the 2.3 GHz receiver box. The 2.3 GHz is coupled to this plate with a repeatable latching mechanism, to make the mounting/dismouting operation as easy as possible (for calibration, etc.). The same interface could be used also for the 5 GHz secondary mount (TBC).

Figures 3b--3d show a possible concept of the mechanical layout of the receiver, using the feed itself (the heaviest and stiffest part) as the support structure of the system. (Note however that these drawings are based on slightly different sizes for the feed and electronics components as were expected before construction - they will be updated).

4) ELECTRONICS The four boards with temperature control, power supply, and DC amplifiers were made by John Gibson in Berkeley and mounted in a single support box (which could be changed). Four temperature sensors have been calibrated. They are placed at 1) detector diode, 2) HEMT amp, 3) feed transition, 4) feed horn aperture.

5) INTEGRATION All the necessary parts were taken to Milano and integrated there. Davide Maino and Marco Bersanelli worked on it, and support was provided by the IFCTR machine shops. Effort was made to have the receiver ready as soon as possible. Final testing and cross checks were made in Berkeley with the test facility.

At completion of the 2.3 GHz system, the receiver was available to take data and did so in Colombia and Tenerife.

Return to the Smoot Group page for a complete description of Dr. Smoot's group's research activities.