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Hydrogen Line Weak Signal Source

Among the accessories most useful to a SETIphile setting up a receiving station is a calibrated Weak Signal Source. The circuit described here is a very low power transmitter putting out a stable signal on the 21 cm neutral Hydrogen line (1420.40575 MHz). It can be used for frequency calibration, antenna optimization, receiver tweaking, sensitivity comparison between preamps, and testing of Digital Signal Processing software. If you have experience working with microstripline printed circuit assemblies, it should be easy for you to construct this crystal-controlled Hydrogen Line test oscillator. It is commercially available in kit and assembled form from our friends at Down East Microwave Inc.

Completed hydrogen-line weak signal source, with a quarter-wave (5 cm) antenna of brass hobby tubing soldered to the output microstripline. The test signal can be heard in sensitive SETI receivers at a distance of several hundred meters. Construction details appear below. Click on the thumbnail to view full-sized image.
SETI League photo

The Weak Signal Source is based upon a design by Paul Wade, N1BWT, and built on a printed circuit board of his devising, which is supplied by Down East Microwave as their part number WSS-1296K. The original circuit was optimized for use by radio amateurs as a 1296.1 MHz weak signal source. Minor circuit modification is required to put it on the Hydrogen Line frequency. The operating frequency is determined by an HC-18/U piezoelectric overtone crystal (Y1) which is cut for one twelfth the operating frequency. As supplied in the 1296.1 MHz kits, the frequency of Y1 is 108.00300 MHz. For this application, I used a crystal at 118.36715 MHz. This custom crystal costs $20, and can be ordered from Down East Microwave.

Down East Microwave provides an etched, drilled and plated circuit board for this project for $20, and a complete kit of parts for $60 US. The board must be modified, as discussed below. Down East now offers a full kit for the 1420 MHz version of this oscillator, with the required custom crystal included. Their kit bears the part number WSS-1420K.

The commercial version of the SETI League weak signal source is sold by noted equipment supplier Down East Microwave, as their Model 1420-WSSK for $60 in kit form. Here is a packaged version, using an enclosure supplied by Down East Microwave. (Note the homebrew querter wave whip antenna at upper right.) A discount is offered to SETI League members in good standing.
SETI League photo

The printed circuit board contains no-tune microstrip bandpass filter elements, which must be modified for the intended operating frequency. Figure 1 depicts the swept frequency response of one of the filters as originally designed. By removing metallization from the circuit traces with a hobby knife, the filters can be tuned up to resonate as depicted in Figure 2. The process involves trimming 0.1 inches of metallization off the ends of each filter pole (12 places total) so that each end falls even with the inside edge of the adjacent perpendicular microstrip, as shown in Figure 3. The portions of PC trace to be removed are marked with red asterisks in the figure. Making straight cuts across the strips, and then peeling off the excess metallization, seems to be the easiest way to modify these boards. A detailed photo of the modified board, with the xacto-removed metallization (in)visible in 12 places, may be seen here.

I built my weak signal source according to the plans supplied by Down East Microwave, except that the resonant frequency of the crystal oscillator tank circuit needed to be raised to the new crystal frequency. This is done by reducing the number of turns of inductor L1 from 10 to 9. The design includes an optional output multiplier (harmonic generator) consisting of a diode and inductor on the output microstripline. These must be left off.

The weak signal source kit was designed with an SMA coaxial connector at its output port. I left mine off, and instead mounted a quarter-wavelength antenna directly to the output microstrip. This is simply a piece of 1/8 inch diameter brass hobby tubing, filed flat at one end for soldering down to the output microstripline, and extending out 5 cm beyond the edge of the circuit board.

I run my weak signal source from a 9 volt battery. The bias resistors were optimized for a 12 volt supply, and may be changed to raise active device current back to the original values at the lower operating potential. However, I did not make this modification. The only ill effect of the lower battery voltage is reduced output power, which is hardly a factor in this application. I place my weak signal source out on the back fence, aim my parabolic antenna more or less in its general direction, and have a half-scale S-meter indication on my ICOM IC-7000 receiver. We hardly need more power. The spectral output of my test oscillator is seen in Figure 4. Its spectral purity and output amplitude are within the limitations established by FCC Part 15 regulations. I suspect that using the previously mentioned output frequency multiplier, running the unit at its rated 12 volts, or attaching the oscillator to a gain-type antenna, could easily result in electromagnetic interference to other radio-astronomers, as well as violation of the Part 15 limits. It is the responsibility of the user to avoid interfering with other users of this protected radioastronomy spectrum.

Please Note: Successful completion of the above Weak Signal Source kit demands precision microwave assembly technique and, in some cases, the use of complex electronic test equipment. This kit is considered appropriate to advanced microwave experimenters. SETI League members lacking extensive experience building and troubleshooting microstrip assemblies should purchase an assembled and tested unit, since a nonfunctional or improperly assembled kit probably can't be economically repaired, even if the vendor is willing to try.

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