The Australia Project

Somebody must be able to hear a Pi Zero on the other side of the world!

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The Raspberry Pi SoC has an on-board clock generator which can be used as a 10mW QRP (low power) radio transmitter when paired with a suitable filter. This project aims to push the limits of how far away a Pi transmitter can be heard.

WSPR is an amateur radio mode designed to test the limits of radio propagation. Using a simple dipole antenna a Raspberry Pi running a WSPR transmitter can be heard across the Atlantic when the atmospheric conditions are right. The idea of this project is to minimise feeder loss and provide some antenna gain to extend its range as far as possible. Australia is almost as far away as it is possible to go from the UK, hence the name: the Australia Project.

This was a Pi Zero competition entry but since I've become a Hackaday contributor I'm no longer eligible to win any Hackaday contests. No matter, I still enjoy the challenge they present even if my entry is in spirit only, and the breadth of entries from other people never ceases to amaze.

There are three components to this project, aside from a Pi Zero.

1: A low-pass filter. The Pi produces a square wave which contains a lot of harmonics. If I'm transmitting on 14 MHz then I'm also transmitting something on 28 MHz, 42 MHz, 56 MHz, and so on. This has the potential to cause interference, and bring the UK radio regulator OFCOM to my door. I therefore need a low-pass filter, an LC network designed to reject these higher frequency harmonics. To that end I've produced some prototype boards for a tiny 15mmx20mm Raspberry Pi expansion board containing a 7-pole LPF and an SMA socket. Incidentally I think this may be one of the smallest Raspberry Pi add-on boards produced. See this project log for more details of the LPF. I will eventually offer this tiny LPF board as a kit alongside my larger LPF kits.

2: A feeder and balun. A typical transmitter will send its RF to the antenna along a length of co-axial cable. Co-ax is good stuff, but even the best has a loss associated with it. For every metre of cable I lose a little bit of power, and when I only have 10mW I need all of it. So in this case I won't have a feeder, I'll put my Pi Zero up on the antenna at the feedpoint so none of my RF is lost. There's another component involved in getting the RF to the antenna, a balun. The antenna feed is balanced, it expects to see an electrically symmetrical feed. Sadly what comes out of the filter isn't balanced, it's just one wire with the RF on it, and earth. So I need a balun, a transformer to ensure correct matching between filter and antenna with no loss of RF. I originally expected my balun to be wound on a small ferrite ring, but as you can see in this project log showing the balun design and this project log showing my completed balun, I went for an air cored design.

3: An antenna. I will be using the 20m (14 MHz) amateur band, as it is one of the bands most suited to long distance work. 20m refers to the wavelength, so an antenna with two 1/4 wavelength elements can be in the region of 10 metres wide. I will be creating a beam antenna to try to focus my RF power and provide a bit of gain, so in addition my antenna can be several metres long. At the moment the design which has most favour for me is the Moxon, a two-element beam with the ends of the elements folded at right angles to save some space. There are Moxon designs that use fibreglass fishing poles for a reasonable price and weight. You can see the calculated dimensions of my 20m Moxon in this project log.

When I have these three components assembled, I will put my antenna as high up as I can, select the right bearing for Australia from a Great Circle map, run the WSPR software, and wait for reception reports on the internet. The propagation should pick up as we go into spring, so by summer I should know whether my project has achieved its aim.

  • Why no progress?

    Jenny List05/01/2016 at 13:19 1 comment

    I got a contract in. Which is great, I get money for doing work and all that, but means my Pi-to-Oz thing has had to be put on one side for a short while. Apologies, followers, but you gotta eat :)

  • Still awful British weather

    Jenny List03/09/2016 at 08:54 0 comments

    Looking out of the window, the field I'll be putting the Moxon up in has flooded. Never mind, the HF bands are pretty poor at the moment propagation wise. So if you're following this project don't give up hope. Any minute now the foul weather will pass, and antenna work can resume.

  • Awful British winter weather

    Jenny List02/21/2016 at 20:08 1 comment

    It's not what you'd call great antenna-erecting weather out there at the moment. More from this project when the current storms settle down a little.

  • And lo! A balun appeared!

    Jenny List02/11/2016 at 11:37 0 comments

    In my previous update I posted details of my balun design, a trifiliar air-cored transformer.

    Here is the resulting balun, a length of 3-core UK mains flex stripped of its outer insulation and wound round a length of plastic water pipe. It'll happily take kilowatts so it's a bit of overkill for my 10mW, however it was exceedingly cheap and quick to make.

    I have yet to wire it up, I'll do that when I build the antenna. But if the blue and brown wire are my input/output connections at each end, the green/yellow wires will both be taken back the length of the balun to the other end, and connected to blue at one end and brown at the other.

  • Balun design

    Jenny List02/09/2016 at 15:34 0 comments

    This project needs a balun between filter and antenna, to ensure maximum efficiency in transferring the RF from the transmitter(unbalanced) to the antenna(balanced). There are many types of balun, here are some details of the one I'll be using for this project.

    I would normally not take a balun too seriously when I am using a co-ax feeder. Just put some turns of co-ax round a ferrite ring to stop any RF going back down the outside of the feeder and call it good. A simple balun, but one that does the job.

    But in this case I don't have any feeder, I want to minimise loss. So resonant transmission line baluns are out too. I'm left with transformer baluns, and my choice is between an air cored one or a ferrite one. My gut feeling is that a ferrite one will have more loss than air cored, so I'll go for air core.

    The classic air cored 1:1 balun is a trifiliar transformer as below:

    Three wires are wound side by side on a former and connected as shown, with appropriate connectors. I may forgo the co-ax connector and wire straight into my filter.

    I will use plastic pipe about 1" diameter, and wind about 15 or 20 trifiliar - three wires together - turns, that should be enough.

    There you are, balun design, sorted!

    (Edit) See the resulting balun in this project log!

  • LPF design

    Jenny List02/06/2016 at 09:59 0 comments

    As you can all see from the photo, I've already made the LPF. In time I will make this available as a kit, I'm only held back temporarily by sourcing connectors, I have another device needing them that will make up the rest of an order.

    It's a 7-pole Chebyshev design. Circuit diagram of LC network below.

    The component values were chosen as the closest preferred values to those derived from a QUCS simulation.

    Band C1,C4C2,C3L1,L3L2

    The inductors are wirewound ones, I suspect multilayer parts would not have enough Q.

    And it all fits with 1206 SMD parts neatly on a 15mmx20mm board with a 10-way socket on the underside to mate with the first 10 Raspberry Pi pins.

    The connector is an edge-launched SMA - it's quite small, and not impossible to solder by hand.

    I think this is one of the smallest Raspberry Pi add-on boards made, if not the smallest.

  • Moxon calculations

    Jenny List02/05/2016 at 09:42 0 comments

    There are loads of online Moxon dimension calculators, a quick Google search should find you one. The one I used was They provide this diagram showing the Moxon antenna dimensions, the calculated values for the 20m WSPR frequency 14.0956MHz are below.

    A: 7.7594 M

    B: 1.1882 M

    C: 0.1892 M

    D: 1.4385 M

    E: 2.8159 M

    For my Moxon I intend to follow a well-trodden path of making a support in the form of a large X supporting each corner.

  • Software and fishing poles

    Jenny List02/04/2016 at 13:16 0 comments

    The software used is WsprryPi, It's a transmit-only package, no receiving involved.

    The antenna will be something like this:, built with wire and four telescopic fibreglass fishing poles. Since the feedpoint for this design is in free space I'll either have to use a short length of co-ax and mount the Pi at the centre, or more likely construct a lightweight waterproof enclosure and mount it at the feedpoint.

  • LPF

    Jenny List02/03/2016 at 09:15 0 comments

    Low pass filter built. I have a regular-sized LPF board available as a kit, but I thought the Pi Zero deserved something tinier.

View all 9 project logs

  • 1
    Step 1

    This project is an amalgamation of three subprojects: LPF, balun, and antenna. More build instructions to follow.

View all instructions

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frustum wrote 08/01/2016 at 05:34 point

Neat idea.  At the risk of being pedantic, square waves are made up of the odd harmonics of the fundamental, which means the filter requirements are relaxed somewhat as the first harmonic with energy is the 3x harmonic, not the 2x harmonic.

  Are you sure? yes | no

bfesser wrote 07/06/2016 at 19:50 point

I would love to see a 'FM band' version of this, for use with PiFM:

I'd buy at least one of these, as an SMD soldering kit.

  Are you sure? yes | no

Untitled wrote 02/06/2016 at 18:37 point

From HackADay's article, "The filter keeps the Pi’s harmonic filled square waves from messing up every band from DC to light"

....So without a filter you could be interacting with light? Please explain.

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Jenny List wrote 02/06/2016 at 18:49 point

I think that's a bit of poetic licence from the Hackaday writer, certainly not from me :)

A 14MHz squarewave is a mixture of multiple frequencies all at multiples of the 14 MHz fundamental. So 14MHz, 28MHz etc. etc. As the multiple grows, the intensity drops. So while in theory there might be a component every 14MHz way up the spectrum, in practice they will drop away way, way before light.

I'm most worried about any output in the used-by-humans radio spectrum that might interfere with other users, so think in terms of VHF and UHF. The filter removes those very effectively.

  Are you sure? yes | no

Mark Jessop wrote 02/03/2016 at 11:14 point

Hi Jenny,

I'm an amateur radio operator in South Australia.

Your project should certainly be doable. From experience with 20m between Europe and Australia, you will want to be beaming 'long path', that is, beaming to your South-West, instead of 'short path' (south-east). We find that long path to europe tends to open around 5pm-7pm local time (UTC+9.5), so adjust accordingly.

To better give you a chance to get a WSPR packet through, we may be able to set up a portable hexbeam antenna in a low-noise antenna one afternoon/night - just let me know when you have something on the air!


Mark VK5QI

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Jenny List wrote 02/03/2016 at 13:19 point

Hi Mark, and thanks for that. I think it will be do-able, but with 10mW it's certainly pushing it. And thanks for the long-path advice too. I had wondered about that. I will keep posting and I will give you a shout when it is live. If anyone is prepared to look specifically for it that would be the cherry on the cake!

Jenny, G7CKF

  Are you sure? yes | no

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