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
Most low noise amplifiers used for amateur SETI and radio astronomy 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. 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.
The bias tee is typically a small box with two coaxial connectors, and a place to hook up the LNA's required DC operating voltage from a power supply. One of the connectors is marked "from LNA" or "RF + DC", and will have DC appearing on its center pin. The other connector is marked "to receiver" or simply "RF", and will not have DC on it. Caution: It is essential that you hook the bias tee into your system the right way around, or damage to your microwave receiver could result.
The internal circuitry of the typical bias tee is rather simple. There's usually a 50 ohm microstrip transmission line connecting the two coaxial connectors, with a chip capacitor mounted in the middle to serve as a DC block. A radio frequency coil (rf choke) carries DC from a feedthru capacitor to one side (the LNA side) of this microstrip. To the feedthru capacitor, the user connects the LNA's operating potential (typically +12 VDC) from a lab power supply, or a "wall wart" type power adapter such as may be found at Radio Shack ® as their Catalog Number 273-1652 ($12.99 US list). Caution: It is essential that you observe proper polarity when connecting a power supply to the bias tee, or damage to your LNA could result.
A commercial bias tee favored in many amateur SETI stations is manufactured by Down East Microwave as their Model BT, and sells for $35 US (plus appropriate postage). The unit uses type N female connectors. It is compatible with LNAs from Down East Microwave, and other vendors, and is relatively foolproof -- as long as attention is paid to correct power supply polarity, and to the direction in which the unit is inserted into the transmission line.
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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