Summer arrived, and with it the yearly itch to do something in the
antenna field. I'm not sure if it's rational or logical, but it does
seem to happen each year!
Most of my previous experiments were VHF, so I thought the time had come to do something more adventureous. An inviting target was the reception of meteorological satellites in the 1700 MHz band. Even though very interesting radio amateur signals exist at 2400 MHz, they don't sit still like the geostationary birds.
As a first step, I didn't want to worry about the tracking system - which surely is a project by itself. A reflector with a directivity of 25 - 28 dB is quite a handfull to point: it only has an aperture of about 6 - 7 degrees, so a precision of around a degree is needed!
And that's how we get to the present project. As simple as it looks, the selection of a parabolic reflector is a compromise between many factors. The feed antenna must be compatible with the reflector. A tradeoff must be selected between directivity ('gain') and size. I finally opted for 25 dB of gain at 1700 MHz (28 dB at 2400 MHz) and a manageable size (1.5 m). 25 dB means a signal gain of around 300.
I've also toyed with the idea of making an asymmetric (off-center) reflector, which has the advantage of a slightly better gain, and a better adaption to the helix antenna I planned to use as a feed. But I plan to add the azimuth-elevation system someday, and the added complication seemed too much.
The ARRL Antenna Book
Design equations, pages 18-18pp. ISBN 0-87259-206-5 (Más datos en Amazon.com)
Microwave Handbook, vol 1
M. W. Dixon G3PFR (editor), Chapter 4, ISBN 0-900612-89-4 (Más datos en Amazon.com)
Reference Data for Radio Engineers
27-38pp, ISBN 0-672-21218-8
Amateur Dish Types
Parabolic Reflecting Antennas
Off-Set Fed Parabolic Reflectors
W2IMU, Dick Turrin, www.nitehawk.com/rasmit/offset.html
SETI Spreadsheet page
Varias hojas de cálculos interesantes para el diseño de antenas
|(c) John Coppens ON6JC/LW3HAZ|