SDR diversity and Ah-Ha!

Foundations of Amateur Radio

In every new technology there is an ah-ha moment, the single one insight that defines for you personally what this technology is all about. No doubt this happened when Amateurs first used valves, when they started using transistors and so on. For me that moment happened during the week.

You've heard me talk about the absurd noise floor, that is, the incredible amount of local radio noise that I experience at my shack. I've been working my station portable to get away from the racket.

During the week I came across something that is likely to change that.

You've no doubt heard about diversity reception. You can use two different antennas, do some fancy phase switching and make the noise go away. Now I should clarify, at least briefly what that looks like. Imagine throwing a stone into a lake, it makes waves. If you throw two stones into the lake at the same time, the waves get bigger, but if you were to time it just right, you could throw in one stone, then the next. If you timed it just so the first stone would make a wave top whilst the other stone made a wave trough, the two would cancel each other out.

You can do the same with light, shine a torch onto a piece of cardboard with two slits cut into it. Behind the cardboard you'll see light and dark patches where the frequencies line up and cancel each other out.

Light and radio waves are part of the same spectrum, so you can do the same with radio waves. You could use this technique to cancel out, or rather filter, local noise.

So far I've not said anything particular worthy of ah-ha, but stick around.

There are devices made that you can use to create the equivalent of two slits, by changing inductance and capacitance within a specific circuit, you can align two signals from two antennas and make them cancel each other out. The way that works best is if one of the antennas is really good at hearing noise and the other is responsible for hearing the required station.

You can then mix the two signals, I don't have such a device, but I'm told there is an art to making this work, and out pops the station you care about.

In software defined radio or SDR, you can do this exact same thing. Only you don't need a circuit to do it, you can show the results in real-time and you can create a user interface that makes it really easy to try different things. The example I saw is PowerSDR, an open source project that allows you to control many different radios.

Picture a circle with a line that is attached to the centre of the circle and the end is attached to your mouse pointer. You can move the mouse anywhere in the circle and as you do this, you're controlling two parameters, the phasing angle and the gain. The gain is the length of the line, the angle is the direction in which the line is pointed.

While you're moving your mouse about, the signals from both antennas are mixed together according to the position of the mouse at the time.

The end result is a completely interactive direct feedback loop where you can see and hear the effect of the mouse location. You can move it around very simply, and immediately, continuously see the result.

The outcome of all this is that you can bring your noise floor down by 30dB or more, and hear stations that were completely inaudible within the racket.

I'd heard it being described, but seeing it in action was a show stopping moment for me and right there and then I knew that the landscape in radio has changed forever.

I'm Onno VK6FLAB

This and other episodes of Foundations of Amateur Radio can be heard via podcast or download at podcasts.itmaze.com.au.