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Editor's Note: The following product has been discontinued. The data sheet is retained here for historical reasons.
A Radio Astronomy Down Converter Assembly
According to discussions on The SETI League's Argus technical email list, there exists the need for a low cost microwave assembly to facilitate the construction of hydrogen-line radio telescopes. Olde Antenna Lab of Denver is pleased to respond to this need by introducing the Hydrogen LNBF.
The term LNBF comes to us from the direct-broadcast satellite industry. It refers to a combination of a low-noise preamplifier, a block frequency downconverter, and a feedhorn assembly, all in a single, weatherproof package designed for mounting directly to a parabolic dish antenna. Our LNBF converts the 1420 MHz precession frequency of interstellar hydrogen (a popular spot on the dial for radio astronomy and SETI research) to an intermediate frequency (IF) of 144 MHz. The user's existing 2-meter Ham (Amateur Radio) receiver or VHF communications receiver can be used to demodulate the resulting signal. The components of the block will cover the entire SETI 'water hole' (1420 - 1660 MHz) when used with a corresponding tunable IF (144 - 384 MHz receiver).
The receiver which you choose to use following the LNBF should contain a single-sideband (SSB) detector. Either upper sideband (USB) or lower sideband (LSB) may be used. We do not contemplate that extraterrestrial civilizations will necessarily employ SSB modulation. Rather, an SSB receiver will convert whatever signal it happens to receive, with whatever modulation -- or even no modulation at all -- into audio tones. These tones can be digitized by a computer's sound card, and analyzed in software for the hallmarks of artificiality, those characteristics which confirm their extraterrestrial intelligent origin. The output from an FM or AM detector, on the other hand, may not contain audio components (especially in the case of an unmodulated carrier, which is a likely scheme for interstellar beacons), and thus would produce no useful output from a computer's sound card. VHF receivers containing SSB detectors (sometimes called Multi-Mode or All-Mode receivers) are readily available through ham radio retailers, and commonly used by serious VHF/UHF amateurs. Check with your local ham radio club for specific recommendations.
The Olde Antenna Lab Hydrogen LNBF starts at the "F", with an OAL 2.25 turn (element) right-hand circularly polarized Helix feed antenna. The Helix was designed to adequately illuminate a wide range of parabolic dish sizes having an F/D of 0.25 to 0.35. The polarization, RHCP, was selected to minimize any manmade noise, terrestrial or extra-terrestrial, that might cause interference to the hydrogen line signals from outer space.
Because mirrors reverse the image, an RHCP feed working in conjunction with a parabolic dish will be most sensitive to incoming LHCP energy. Among the strongest sources of potential interference in the radio astronomy band is the extensive constellation of Global Positioning Satellites. Since GPS downlinks are all right-hand circularly polarized, using a left-hand polarized antenna system for radio astronomy and SETI will significantly reduce troublesome GPS interference.
The Helix is connected to a Down East Microwave, Inc., (DEMI) Ultra Low Noise PHEMT preamplifier offering greater than 16dB gain and a noise figure of less than 0.4dB. The output of this preamplifier is then fed into a (DEMI) SETI Receive Converter that shifts the 1420 MHz hydrogen-line signals to an intermediate frequency of 144 MHz, for recovery by a two-meter band ham radio receiver. The downconverter offers a conversion gain of greater than 32dB. The combination of preamplifier and converter thus exhibits an overall gain of 48 dB at the antenna, and will drive moderate coaxial cable runs, providing a minimum discernible signal (MDS) of -135dBm at 144 MHz.
This LNBF is supplied fully assembled and tested. All connections between units of the block are done using hardline (UT-141 coaxial tubing) and Omni Spectra hardline connectors. The hardware is enclosed in a weatherproof housing or radome of PVC. On the rear of the radome there are only two connections; one for 13.8 VDC (+/-10%) at ~400 ma via an RCA connector (center is positive), the other a BNC(f) coaxial connector that outputs the I.F. at 144 MHz. An aluminum girth strap is provided that will attach to a tripod of dish arms and assist in the positioning of the Helix antenna at the focus of the dish used.
Overall size of the assembly is 14.25" long by 5.5" in diameter. Total weight is approximately 7 pounds. List price of the Hydrogen LNBF is $499.95 US, plus postage and insurance. A 20% discount coupon being provided to SETI League members in good standing reduces their final price to $399.95.
If the focal length of the intended dish is unknown or the user is not familiar with the math to find the focal length, please use the following equation:
F = D^2/(16c)
Once the radome is loosely secured to the girth strap using the tripod arms, place the front of the radome at the focal point. Using a known 1420 MHz source, slowly move the radome toward the dish until the maximum signal strength is obtained; then secure the radome in place at that point.
There are no provisions for making adjustments to the block hardware as the radome is completely sealed. Should the unit fail it is recommended that it be returned to the Olde Antenna Lab of Denver for repair, and only after receiving a return confirmation. Please address requests for return confirmation, and questions/concerns to:
Olde Antenna Lab of Denver
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this page last updated 23 October 2004
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