The Parabolic Dish
I tried to follow the SETI League recommendation of using a 3 - 5 metre dish, and bought a 2.7m dish that was the biggest dish I could transport.
This dish seems to be made out of Dural and is black mesh. It cost £150.65 + VAT from Harrison Electronics (01354 651289).
The focus of a parabolic dish can be calculated using the following formula:
Where F is the focal length, d is the dish diameter, and c is the dish depth.
I have calculated the focus of the dish after measuring it and this agrees to within 20mm of the manufacturer's published figures.
The dish support and steering system
The dish is mounted so that it is looking directly up into the sky; i.e. it points at 90 degrees to the ground. When I eventually pair up with another SETI station, I may need to angle my dish a few degrees of this heading. I have built a very simple support that can point the dish accurately, and then leave it fixed in this position.
The dish is supported at three corners by a support built out of a scrap scissor (car) jack welded onto a cheap axle stand. This enables each support to be raised or lowered using the thread of the scissor jack. A spirit level is used to verify that the dish is pointing straight up.
The dish feed support
The dish feed support is necessary to hold the dish feed at the centre of the dish. The assembly should be adjustable to allow the feed position to be easily moved for fine ( or not so fine ) tuning of the focus.
It is also worth noting that as the dish is lying on its back pointing upwards, the dish feed is inaccessible without building some sort of platform over the dish.
The dish feed
This is my dish feed in the version 1 dish feed support.
Low Noise Amplifier
The Low Noise Amplifier consists of a GaAsFET amplifier based of an MGF1403??? built into a copper box. The input to the GaAsFET is a tuned line coupling at high impedance into the FET gate. The output is untuned and terminated in a 50-ohm resistor.
The gain of the GaAsFET amplifier is approximately 15dB and the gain of the MMIC is rated at 23dB. I have not been able to measure the noise figure or noise temperature of my LNA, although I hope to be able to do that soon
Here is the inside of my LNA
The output of this amplifier feeds into an MMIC type number MSA-0885. This adds enough gain to overcome the feeder loss to the receiver.
The receiver is an IC-R7000. This will tune from 25MHz to 2GHz. It is presently used in the USB SSB position, the speaker audio being fed via an attenuator to the computer.
The computer has the following specification
The output from the receiver is taken to the Microphone input of the SB16 via an attenuator. Due to the location of the computer, the audio lead between it and the receiver is about 5m. To reduce baseband noise pickup, the volume out of the receiver is set fairly high and is run into either an 8 ohm load or load/speaker switched at the computer end. A variable resistor wired as a potential divider then sets the level into the SB16.
Two operating systems are installed, Win95 for running existing FFT DOS software, and LINUX for developing new FFT software.
The main software I am using at the moment is a program called specgram running under LINUX. As the program is supplied with source code, I have been able to modify it for SETI use. Here are my latest modifications to the specgram source: specsrc.zip The modifications are as follows :
I'm afraid it’s a bit of a hack, but it works for me, and its something I can modify. I've found the following problems with specgram though.
Finishing this Phase
Focusing the dish has been a major headache. I am now happy that the manufacturer's data of a 36-inch focal length is confirmed from measuring the dish.
The Dish feed support is on design version 2, as version 1 did not have enough adjustment
I have been taking the power data from SPECGRAM and importing it into EXCEL and plotting a graph of the previous 24 hours power readings. I need to compare this plot to a star chart ( or preferably a radio map of the sky ). I can then move the dish feed up and down to optimise the dynamic range of this graph. Using this method, I can optimise the focus.
Then I should be ready to fill in my SETI League participation form and begin monitoring for real.
The next Phases
So far, I have intended to get this system going as quickly as possible. As I am new to this, I would prefer to get something going and see where the weaknesses are so that they can be corrected in future phases
So, here is the plan for the Future
I am located in Cambridge, England. Latitude 52° 13.218N and Longitude 000° 09.160E ( Measured on a Garmin 45 GPS )
Jenny's home page is here
Main J-Squared Web page is here
The photos were taken using a Kodak digital camera DC50 in its highest resolution mode