A Temperature Controlled Head Amplifier for 408MHz Radio Astronomy
This document describes the design and build of a temperature controlled head amplifier assembly for use in Radio Astronomy at 408MHz. The design is compact so that the amplifier can be mounted close coupled to the Quagi antenna, in this case to a combiner which connects two Quagi antennas together. The system uses a Peltier semiconductor cooling / heating unit driven remotely from a temperature control electronics unit which takes real time temperature readings from the cold plate on which the Peltier element and the head amplifiers are mounted. The system schematic is shown below:
The low noise amplifier is a 408MHz unit with a nominal 18dB gain designed by W5AGO and available from Radio Astronomy Supplies http://www.radioastronomysupplies.com The filter is a 400MHz low pass SMA type VLF-400+ available from Mini Circuits http://www.mincircuits.com. . The line amplifier is a commercial MMIC gain block with a gain of 22dB @ 400MHz mounted in a machined aluminium enclosure fitted with SMA connectors.
The gain of the combined amplifier / filter chain @ 1420MHz is 44dB. The purpose of the temperature stabilisation is to maintain this figure to 0.1dB over long periods of time during observations. This requires that the closed loop temperature control is better than 10C for an outside temperature variation of 200C. In order to achieve this the amplifiers are well thermally bonded to a thick aluminium cold plate with significant thermal capacity. The Peltier cooling element is bonded to the other side of this plate and to the heat sink from which the fan draws waste heat. The unit on the cold side of the plate is surrounded with expanded polystyrene insulation and the whole enclosed in bright aluminised Mylar foil to reflect heat from outside. Finally the entire assembly is fitted inside a white painted plastic rain cover.
The temperature sensor is an LM35DZ IC connected to a CA3140EZ Mosfet Operational Amplifier providing a gain of 10x and able to drive the 30m of twisted pair cable to the control electronics.
Temperature Controlled weather proof 408MHz Head
The construction of the head amplifier is very compact. It needs to be small and as light weight as possible so that it can be mounted alongside and in-between the two Quagi (Yagi) antennas on a connecting boom. The assembly needs to be rain proof in all orientations of the antenna which is achieved by placing the power and RF output connectors on the underside of the assembly. The package is 200mm long by 100mm diameter.
(Above) Compact Head Unit and Casing
(Above) 408MHz Head Amplifier Assembly
The temperature control achieved is better than 0.50C for the normal temperature variations occurring at the observatory site in the . With this level of control it is possible to maintain a stable system gain to around 0.1dB which is sufficient for drift of long term observation baselines not to present a problem. Two of these head amplifiers have been constructed and used in a 408MHz Interferometer arrangement with a 30m antenna separation giving an East – West resolution of ~ 1.50. The dual channel temperature controller unit sits in the receiving room and is based on the same design as the 1420MHz controller. A single multi-core screened cable connects the controller to a distribution box outside, which then feeds each antenna head amplifier. The RF outputs are added in a Wilkinson Combiner and fed via URM 67 coax to the receiver.