The Australia Project

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 :)

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.

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.

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.

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!

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.

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.

There are loads of online Moxon dimension calculators, a quick Google search should find you one. The one I used was http://ab1jx.1apps.com/ham/calcs/moxon/index.html. 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.

The software used is WsprryPi, https://github.com/JamesP6000/WsprryPi. It's a transmit-only package, no receiving involved.

The antenna will be something like this: http://www.moxonantennaproject.com/construction.htm, 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.

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