74Mhz Interferometer
I started out to build a project based along similar lines to the 21Mhz interferometer receivers used by the Fringe Dwellers..
However, I have found HF frequencies below 40Mhz are very noisy and I suspect this is the same in most suburban areas. However using a WinRadio and tuning through the bands suggested for radio astronomy observations I found that the 74Mhz band was the lowest frequency I could find that was reasonably quite.
The 74Mhz band was heavily used by commercial users in previous years, but with the advent of mobile phones and other digital technologies, I assume the band now is used to a lesser degree. This was also confirmed by investigating who uses the band by visiting the ACMA website. As a result I have found what I think is a sweet spot at 73.5Mhz for the Adelaide metropolitan area, but this will be different for other areas of Australia.
The only down side in using this frequency is that it is not suitable for Jupiter observations as the natural signal emitted signals peak at around 8 to 11Mhz and reduce at higher frequencies rarely emitting above 30Mhz.
| Attachment | Size |
|---|---|
| 74Mhz_Interferometer_Planning_Ideas_Draft.pdf | 762.5 KB |
74Mhz 16 Element Collinear Broadside Array
In selecting an antenna for Radio Astronomy it is important to achieve a high degree of gain, low noise and have a a reasonable bandwidth for the frequency chosen.
After a review of many antenna designs, I have settled on a 16 Element Collinear Broadside Array as the most cost effective and high gain system to deploy on the 74Mhz band.
Element dimensions for 74Mhz
Each 1/2 wave feeder element = 1.948M
Each 1/2 wave reflector element = 2.064M
Spacing between feeder and Reflector = 0.771M
Overall dimensions = 6M X 4.2M X .771M
This is the basic electrical layout of the antenna it has balanced feed-line with an output impedance of between 300 to 400 ohms and an expected gain of 16dbi and a beam width of about 30 degrease.
Each element is made of aluminium tubing and can be mounted directly using a simple U-clamp to welded steel frame without insulators. Affixed in the centre of the 1/2 wave element the impedance is at its lowest, so an insulator is not required.
Collinear Broadside Arrays can also be connected in pairs simply, without the need of complex impedance matching networks.
Interferometer Downconverter Components
Still finalising the circuit diagrams but here are some of the core component datasheets I am reviewing to include in the design.
| Attachment | Size |
|---|---|
| SA630.pdf | 291.67 KB |
| ERA-3.pdf | 213.69 KB |
| SA602A.pdf | 117.39 KB |
| an1982.pdf | 53.44 KB |
| AD8307.pdf | 395.16 KB |
| powermeter_circuit.pdf | 1.97 MB |
| LTC5508fa_dn335f_notes.pdf | 92.51 KB |
| LTC5508fa.pdf | 246.74 KB |
| AD630.pdf | 275.71 KB |
| Attachment | Size |
|---|---|
| SA630.pdf | 291.67 KB |
| ERA-3.pdf | 213.69 KB |
| SA602A.pdf | 117.39 KB |
| an1982.pdf | 53.44 KB |
| AD8307.pdf | 395.16 KB |
| powermeter_circuit.pdf | 1.97 MB |
| LTC5508fa_dn335f_notes.pdf | 92.51 KB |
| LTC5508fa.pdf | 246.74 KB |
| AD630.pdf | 275.71 KB |