Copyright © 1999 by H. Paul Shuch, Ph.D.
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @ setileague.org
from Proceedings of Microwave Update 1999:359, American Radio Relay League, October 1999.
The typical amateur hydrogen-line radio telescope or SETI system uses an antenna-mounted low noise amplifier, or LNA, which is often located some distance away from the station's microwave receiver. To get power to the LNA, and add enough gain to overcome feedline losses, most of us use three rather costly components. This article shows how one inexpensive UHF TV accessory found at Radio Shack® can do the job of those three specialized parts, at about forty percent of their price.
My radio telescope uses a Skyvision #02-3000018 satellite TV line amplifier just behind the LNA. This $24.95 unit exhibits flat gain from 950 to 2250 MHz, and is a natural for 1420 MHz radio astronomy. Both the line amp and my LNA require 12 to 24 volts of DC fed through the center conductor of the coax, which I apply via a Down East Microwave bias tee, inside the SETI station. The required operating voltage comes from a Radio Shack "wall-wart" style DC power supply. Total cost for these three items was around $73 US.
Since the existing lineup works well, and doesn't exactly break the bank, I had no strong incentive to make system changes. However, while browsing the local Radio Shack ® store for a UHF TV antenna project, I came across a perfectly acceptable replacement for the three items listed above. The replacement is their Catalog Number 15-1115B UHF Coaxial In-line Amplifier, and consists of a line amp, DC bias tee, and wall-wart power supply, all for just $29.95 US. The line amp is rated at 430 to 1430 MHz, exhibiting 20 dB gain and a 5 dB noise figure at the 1420 MHz hydrogen line. The bias tee (the spec sheet calls it a Power Injector) has 2 dB maximum insertion loss, and the attached power supply puts out 18 VDC at 50 mA.
Using the standard rule of thumb for required preamp gain (downstream noise figure plus ten dB), this line amp is ideal behind any LNA with at least 15 dB of gain. Since my LNA has 20 dB of gain, its noise temperature dominates system performance, and line amp noise contribution is negligible. Similarly, the gain in the line amp is adequate to override the insertion loss of the DC block, plus the additional loss from a 100 foot run of RG-8 coax. And since the line amp's current draw is only 20 mA, the power supply marginally but adequately provides the additional 25 mA required by my LNA, with a few electrons to spare.
As it comes out of the package, the line amplifier has a DC block screwed onto its input connector. This is to keep the UHF TV antenna (if using the amp in its advertised application) from shorting out the DC power supply. If you want to pass DC through the coax to your LNA, don't forget to remove this clearly marked subassembly. The line amp itself does pass DC from its output to its input connector.
I haven't changed over my L-band radio telescope yet, but the numbers work, and this hardware checks out on the bench. I've used this combination on my Ku-band radio telescope (behind a standard Direct Broadcast Satellite low-noise block downconverter) with good results, and find the power supply adequate to run both the LNB and the line amp (with not quite so many electrons to spare).
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