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
The Low Noise Amplifier, or LNA, is sometimes called a preamplifier, or preamp. Its function is to turn an impossibly weak signal into a merely ridiculously weak one. The critical parameters to consider in selecting an LNA are its frequency response, gain, and noise temperature.
Frequency response determines that portion of the electromagnetic spectrum over which a particular LNA will boost the received signal, with minimum distortion or added noise. You should select an LNA with a frequency range consistent with your particular SETI station requirements. For example, C-band Satellite TV LNAs cover the frequency range of 3.7 to 4.2 GHz. Thus they are not suitable for use in SETI stations designed to monitor the 1.4 GHz hydrogen line. Some LNAs incorporate filtering, which reduces the overall range of frequencies amplified, but which can help to reduce out-or-band interference.
Gain, measured in deciBels (dB), is a measure of how much the LNA boosts the incoming signal. Although in many things "if a little is good, a lot is better," this is not the case for preamplifier gain. In fact, excess LNA gain can actually reduce the sensitivity of your SETI receiver. The rule of thumb is that the gain of the LNA should equal the sum of the microwave receiver's noise figure (in dB) plus the RF cable insertion loss (also in dB), plus an additional ten dB. For the average SETI station with a short coaxial cable between the LNA and the receiver, twenty dB of preamp gain is usually about right. If a very long or unusually lossy RF cable is used, a 30 dB gain LNA might be more appropriate.
Noise temperature is a measure of how much additional noise the LNA adds to your SETI system. Since any actual signal has to compete with a variety of natural and artificial noise sources, the lower the noise temperature, the better. The LNAs commonly used for amateur SETI typically have between 35 Kelvin and 100 Kelvin of internal noise. Noise is sometimes expressed not in Kelvins, but as Noise Figure (in dB) or Noise Factor (a unitless power ratio). The SETI League provides a Microsoft Excel ® spreadsheet for conversion between these various noise units. Sometimes you can reduce the noise temperature of an LNA by thermally cooling it. An additional spreadsheet allows you to calculate the improvement acheived by lowering an LNA's ambient temperature.
Many commercial LNAs are provided with a choice of coaxial input and output connectors. Most SETI League members prefer to standardize on the coax connector known as Type N, since this is the connector used on most feedhorns and microwave receivers. To minimize losses, the LNA should be mounted directly on the output connector of the antenna feedhorn, with the appropriate coaxial adapter (probably a Type N male-to-male barrel adapter).
An additional consideration is how to get the appropriate operating potential to the LNA. Most LNAs operate from a DC power supply, typically in the +12 VDC range. Some designs require that this operating voltage be applied via the center-conductor of the RF cable, and some LNA vendors give you a choice between internal and separate DC feed. DC feed via the transmission line requires that the microwave receiver be designed to provide this voltage, or that an accessory called a DC Inserter, or Bias Tee, be connected into the signal path ahead of the receiver, and tied in to an appropriate power supply. Although this is the scheme commonly used to power the antenna-mounted circuitry in commercial satellite TV receivers, many SETI experimenters prefer to run a separate DC cable (such as a telephone cable, speaker cable, or lamp cord) outside to the LNA, and to apply the required DC potential to it inside the SETI station. (Caution: double-check the polarity applied to this cable, as reversing the positive and negative power supply leads can damage the LNA. The center pin of the LNA's power feedthru capacitor is typically positive.)
Although most commercial (and many home-built) LNAs are metal-boxed to provide good shielding against Radio Frequency Interference (RFI), few are provided in weather-proof enclosures. To prevent damage from exposure to the elements, I like to put my LNAs in plastic Tupperware ® sandwich boxes. It is necessary to drill or punch holes in the plastic for the input coax adapter, output cable, and power wiring. Be sure to seal these openings with room-temperature vulcanizing (RTV) silicon rubber, which you can obtain in a tube from most any hardware store.
Information on various commercial LNAs available in kit or assembled form, along with vendor links, may be found in the Preamplifiers and Filters chapter of The SETI League Technical Manual. For the experienced microwave experimenter, schematics, component selection criteria and do-it-yourself information are also provided.
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this page last updated 4 January 2003
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