SETI League Technical Manual -- Block DiagramCopyright © 1999 by H. Paul Shuch, Ph.D.
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @ setileague.org
The low noise amplifier mounted at your SETI antenna feed operates from a DC power source, typically in the +12 VDC range. Whether you apply operating potential to the LNA via a bias tee through the center conductor of the coaxial cable, or directly via a separate power cable, you will need to obtain an appropriate power supply.
Although regulated laboratory power supplies are indeed suitable (and in fact are used in The SETI League labs to test LNAs), they are rather expensive. An alternative is to use a "wall wart" type power supply, which plugs directly into the household AC mains.
One such unit which was recently tested at The SETI League laboratory is the Radio Shack ® catalog #273-1652D AC to 12 VDC adapter, which is rated at 500 mA output current. Detailed measurements are shown below. The ripple is higher than we'd like, but all the LNAs which we recommend have on-board 3-terminal voltage regulators (12 VDC in, down to 5 VDC for the FET) which brings power-line hum down to negligible levels. The bottom line: this is the cheapest ($12.99 retail) wall wart we know about which has adequate load regulation and filtering to do the job of powering up a SETI LNA.
| RL (ohms) | VL (VDC) | IL (mA) | P (W) | ripple (mv p-p) | ripple (%) |
|---|---|---|---|---|---|
| 220 | 16 | 73 | 1.2 | 250 | 1.6 |
| 110 | 15 | 136 | 2 | 400 | 2.7 |
| 68 | 14 | 205 | 2.9 | 600 | 4.3 |
| 27 | 13 | 481 | 6.3 | 1200 | 9.2 |
Graham Vincent, the volunteer SETI League Regional Coordinator for New Zealand, contributes this alternative solution for powering an LNA:
"I've got a small 7 Ah sealed lead acid battery running mine. At 15 mA it lasts for weeks between recharges, and no noise either. Cost was about US$12 and everyone has a battery charger, don't they?"
Further information on the care and feeding of LNAs, along with vendor links, may be found in the Preamplifiers and Filters chapter of The SETI League Technical Manual.
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