An Introduction to Software Defined Antennas
Software defined radios or SDR monitor many frequencies for a signal. The same techniques can be used to create a software defined antenna or SDA that can generate a wide array of signals. How do you create a software defined antenna? How are software defined antennas used with software defined radios?
Creating Software Defined Antennas
You can electrically create phase shifts in a circuit to create an SDR/SDA. We’ll ignore these for the sake of simplicity.
You can use a phase shifter such as those used in phased array radar. It uses switching in different transmission lines to vary the electrical length of a line. You can even combine phase shifters with several antennas to steer the beam.
To steer the beam, you start with multiple phasing lines or phase shifters, all of the same length. The top lines are set to have a longer length while the bottom lines are set to a shorter length of coax. This setting causes the beam to shift up. If you set the bottom lines as longer length of coax and the top lines as short lengths of coax, the beam will shift down. (Unless you live at a high elevation, this is counter-productive.). If the network has more antennas, the signal has greater gain and you get finer control. There are sixteen way power dividers for software defined antennas in public use today, combined with ultrawide band antenna (UWB) arrays.
The UWB antennas allow the software defined radio to receive a wide range of frequencies using just one antenna / element per reception site. When there are many antennas, you could steer the beam by turning on and off individual elements. The more common layout, though, is the four square antenna configuration.
In the figure here, we see the comparison of a standard four square coax layout with signal generation and reception controlled by a power divider. (Thank you, Kent Britain, WA5VJB, for creating the diagram and giving me permission to use it.)
The main difference in the software defined antenna arrangement is the software defined radios placed at each of the receiving UWB antennas connected via a data cable to a central computer, instead of relaying the signal through a power divider to an amateur radio rig.
The antennas are usually located one quarter of a wavelength apart. If you’re picking up 10 meter signals, the antennas need to be at least 2.5 meters apart. Any closer, and they can’t receive the signal. Farther apart, and they won’t receive it as well though half and full wavelength distances between the antennas are acceptable. The benefit of the four square antenna layout is that you can switch the beam between any four directions with 360° coverage with a minimum of antennas. This configuration also nullifies a lot of signal noise.
The layout shown with software defined radios connected to each antenna allows them to digitally process the signals and peak them, and the differences in reception time between each antenna allow the radios to determine the signal’s exact direction. You can also determine the received beam’s pattern, which would be visible on a waterfall display. The only limitation is the SDR resolution and processing capability.
Practical Applications for SDR and Software Defined Antennas
Software defined radios or SDR are in use by the military and a growing number of commercial applications. They are starting to be used in amateur radio.
A practical application of software defined antennas and SDRs is in cell sites. A multi-function cell site can use the same technology to provide a directional beam to each user. This reduces the occurrence of dropped calls and dead areas.
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