Monday, February 18, 2013

Geek Post: Frequency-Scanned Linear Array Antenna Designed by Yours Truly

Back in 1999, Ol' Backwoods was really into radar.  I designed a 5.7 GHz radar to track the oversized model rockets known as high-power amateur rockets.  Some of these go to altitudes over 20 miles, although the ones I tracked never went over about 35,000 ft.  This weekend, while digging through some old stuff in one of my storage sheds, I found one of my radar antennas.

There were several iterations of this system, known as DARTS, for Digital Amateur Rocket Tracking System.   In one of them, I used a frequency-scanned linear array to point the transmit beam in the left-right (azimuth) direction.  The frequency of the transmit signal, over about a 50 MHz range, literally controlled the beam direction from left to right.

(UPDATE: I found the original web pages describing the system on the Internet Archive/ Wayback Machine).

The antenna itself is about 7" wide.  The  coax would be connected at the left edge.  The antenna was milled on a T-Tech Quick Circuit circuit board milling machine, just like the one in my lab off to the left of me now.

The antenna is fabricated in 10-mil RT-Duroid material.

Frequency-Steered Linear Array Beam Antenna, designed by yours truly.
It was designed using software from a book,  CAD of Microstrip Antennas for Wireless Applications by Robert A. Sainati.  I actually corresponded by email with Dr. Sainati during the design.

Theoretically, by the time the signal went through all the elements in the array, there would be no signal left; it would all be radiated outward.  But since the phases never lined up exactly right except when the beam was pointed straight ahead, I had a small surface-mount resistor to get rid of the rest of the signal.

Since the rocket had a transponder on it to bounce the signal back, I didn't have to transmit much power.  Plus, as I recall, this array had over 20 dB of gain when pointed straight ahead.

Anyway, I thought y'all might find this interesting.

1 comment:

  1. how to design the antenna?to calculate the size of patch?