OPEN É.N0 Hf^UECrOR
OPEN É.N0 Hf^UECrOR
Fig. 9. Feedhom mounting defa//s.
You will find that most of the transistors th£ article recommends run from $17.50 to $70.00 each. As you can see, this could get expensive, especially if you need two or three pre-amps in series for more gain, What we have done is change the transistor to a MRF901, which costs $1,44. This device has less than a 3-dB noise figure at 2 GHz, but only about 6 dB of gain, Now, 6 dB of gain when you have a snowy picture does not help much. For a location that is marginal, we have built two of these preamps in the same box. That gives about 12 dB of gain and also eliminates the need for two connectors. At this frequency, too many connectors in the line can add a lot of loss and is expensive, I am sure it would be possible to build more stages into one box if needed, but we have not yet tried this. We have had up to three of these double preamps together with short cables. This worked well and gave a lot of gain.
We built most of our preamps using double-sided printed circuit board. This is faster and much easier than brass. Be sure to solder together intersecting edges on the inside where possible. At this frequency, a good rf-tight box is desirable, WA9HUV's article gives two possible circuits depending on what kind of transistor is used. When using a MRF901, use the circuit on page 8 of the article, which is labeled
Fig, 1. We show the method of coupling two stages together in Photo G here,
The prearnp should be mounted as close to the feedhorn as possible, and it also will need to be enclosed in a weatherproof box. Power tapped from the downconverter can be i^f j.7 ."Il-tgiS"- . » .-, A.-- " " ¡^-p.1 .Ti1
used to power as many preamps as are found to be necessary.
In this article we have shown you details for building a cheap and easily-reproducible MDS receive system. Most of the circuits and ideas have been gleaned from previous construction articles and manufacturers' applications notes, so no originality is claimed. Although the system as described performs very well, if you find that you can make any significant improvements to the systems, we certainly would be interested in hearing of them.
Since the original design of the system, printed circuit boards have been laid out and are now being made. These boards, along with some of the harder-to-find components and complete antennas, are available from the authors. Send an SASE for details. ■
We would like to thank John Fox W0LER and Ernie Simon W9JCE for the help and encouragement they gave in completing this project.
Photo }. Here are snow saucers before modifications. Shown are the steel 69-cm (27-inch) model made by Flexible Flyer and the 64-cm (25-inch) aluminum Sno Coaster made by Mirro* Remove the handles before using as a reflector.
Fig. 70. Final construction of saucer sled antenna; The open end of the feedhorn should end up 28 cm (11 inches) from the surface of the reflector for proper focus,
1. "MDS; What Is it?ls, James Edwards, 73, November, 1978.
2. A Vidiot's Guide to Microwave TV, H. Paul ShuchT Chief Engineer, Micro Comm, 14908 Sandy Lanes San Jose CA 95124. (Send Paul an SASE and $1.00-)
3. nterdigita! Converters for 1296 and 2304 MHz," R, E, Fisher WA2CGH, GS7, January, 1974,
4. "High-Performance Balanced Mixer for 2304 MHz," Paul Wade WA2ZZF, Ham RadioT October, 1975.
5. "Solid-State 2304-MHz Converter," Walter Stanton K2JNG; Douglas Moser WA2LTM; Dolph Vilardi WA2VTR, Ham Radio, March, 1972,
6. "Twin-Diode Mixer," Jim Dietrich WAGRDX, Ham Radio, October, 1978.
7. "Finding the Focal Length of Surplus Microwave Dish Antennas:' John Franke WA4WDL, Ham Radio, March, 1974
8. "1220 MHz—Use It or Lose lt!'\ Bill Hoisington K1CLL, 731 March, 1978.
9. "Down Converter Receives Satellite Video," Percy Rouse, Jr., Microwaves, November, 1978.
10. "improved Grounding for the 1296-MHz Microstnp Filter," H. Paul Shuch N6TX, Ham Radio, August, 1978.
1 V'Clean Local Oscillator Chain for 1296," Paul Wade WA2ZZF, Ham Radiot October, 1978.
12. "A Twelve-Foot Stressed Parabolic Dish:' Dick Knadle K2RIW, QSTt August, 1972.
13. "Cylindrical Feed Horn for Parabolic Reflectors/' Norman Foot WA9HUV, Ham Radio, May, 1976.
14. RSGB VHFUHF Manual, Third Edition, D. S. Evans G3RPEt G. R. Jessop G6JP.
15. "Narrow-Band Solid-State 2304-MHz Preamplifiers," Norman Foot WA9HUV, Ham Radio, July, 1974.
16. "Parabolic Reflector Gain/' Walter Pfiester, Jr. W2TQK, Ham Radio, JulyT 1975.
Was this article helpful?