Steerable antennas

Steerable directional antennas offer advantages that justify their difficult fabrication.

My old 75-meter parasitic array

When designers need to improve the performance of any radio system, they turn their attention to improving the antennas, which improves system transmit AND receive performance. When I was a teenager, I was a ham radio nut. Most of my radio adventures took place in the HF (high frequency) amateur radio bands between 2 and 30 MHz — a fraction of the 2.4 and 5 GHz (GigaHertz) frequencies where today’s WiFi action is. A wavelength at 4 MHz (MegaHertz) is about 75 meters — antennas in this range are large. I strung a simple 75 meter band half-wave horizontal dipole antenna atop my parent’s house; it radiated in the east-west directions, was about 120 feet long, and worked fine. I had no money to invest, but wanted better antenna performance. I settled on constructing a 2-element parasitic array, which I did by adding a second horizontal dipole more or less in parallel, spaced about 1/8 wavelength apart from the first dipole. I ran a half-wavelength open-wire transmission line from my station in the basement to the center of the second dipole. I was able to electrically shorten and lengthen the second dipole by placing a variable capacitor across the end of the transmission line. The second (“parasitic”) antenna element’s radiation would either reinforce or subtract from the driven element’s signal, depending upon what reactance I added at the far end of that 1/2 wavelength transmission line.

How well did it work?

Because of physical constraints, none of this array’s dimensions was ideal, but it did yield about a 2 to 3 dB (decibel) front-to-back ratio, in either the East or West direction as I chose. Not great numbers, but I knew nobody else who could “steer” his antenna at such a long wavelength. I noticed the improvement when receiving: the signal to noise ratio (SNR) would improve slightly when I steered the antenna in the direction of the transmitting station, which was usually hundreds of miles away.

In theory, the array’s vertical angle of radiation was lower than that from a single dipole. I never measured this, but the array did seem to work better on long skip propagation than did the dipole.

Steerable Parasitic Antenna Array, 5 GHz
Illustration: L. Petit, L. Dusopt and J. Leheurte, “MEMS-Switched Parasitic-Antenna Array for Radiation Pattern Diversity” IEEE Transactions on Antennas and Propagation, vol, 54, M. 9, Sep 2006.

Steerable WiFi Antenna Arrays

Today the same principle is being applied to electronically steerable parasitic element antennas in the GHz (GigaHertz) range. At 2.5 GHz, a wavelength is only about 5 inches, so construction is relatively easy. They’re often constructed as illustrated: etched from a copper laminate on a non-conductive substrate, so dimensions are tightly controlled. They’re steered under program control, not by a kid sitting in the basement. I’ve not tested their performance, but I’ll bet that they beat my old 75-meter band parasitic array by a wide margin.

Electronically steerable antenna arrays are now used in some new MIMO (multiple-input and multiple-output) wireless WiFi access points. The relatively new WiFi standard known as IEEE 802.11n specifies MIMO, which allows greater throughput with less signal fade. Each antenna is dynamically steered as required by each frame: at 2.5 GHz, that’s just a hundredth of a microsecond or so. The exact steering of the antenna may be varied from frame to frame. IEEE 802.11n provides for four steerable antennas per transceiver. That requires a ton of high-speed computing power just to steer the antennas!

AWACS antenna photo by Daderot
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© Russ Bellew · Fort Lauderdale, Florida, USA · phone 954 873-4695

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