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Ok, it is time for another blog post. I have taken a vow to share more of my projects with the world, partly in case someone is interested, partly to motivate myself to do more cool stuff! I figure neither of these are bad.

I’ve had a bit of an itch to try out radio astronomy for a few years, ever since I discovered that it was possible without spending thousands of dollars. Whilst the best performing antenna for this work is a parabolic dish (the bigger the better!), I wanted to start with a smaller, simpler, easier, antenna. I chose the helical antenna. My logic was that I should be able to see the hydrogen line on at least our closest star, hopefully enough to spurn further interest. It would also act as an ideal test platform for other related electronics such as low noise amplifiers and filters.  Suffice to say, the decision was made.

After reviewing a number of the online calculators for helical antennas, I decided upon about 10 turns. This is very approximately 600mm long at 1.42GHz. The single hardest part seemed to be the coil form, so a plan was hatched to use my laser cutter and some perspex to make this easy.  It turned into a fun project to parametrically generate helical antennas in Autodesk Inventor. Suffice to say, tell it the frequency, the desired plastic diameter, wire size, perspex thickness and the number of turns and it produces a set of files that can be lasercut to produce the requested antenna.

Anyway, enough talking – to the photos!  If more information is required, please contact me and I will create another post!

The almost completed helical antenna.

The almost completed helical antenna.

Helical antennas have a characteristic impedance around 140 ohms. To match to 50 ohm cable and connectors, some form of matching is required. When I designed the antenna, I intentionally brought the first turn in close to increase the capacitance to the ground plane, on the hope this would help lower the impedance.  There are a number of other techniques that can be used, but I wanted to start with this one!

The feed connection to the SMA connector.

The feed connection to the SMA connector.

Luckily I have access to a VNA (Vector Network Analyser) at work that makes impedance measurements a breeze, unfortunately the easiest way to get an image is to use a camera phone. As can be seen, without any extra matching work the impedance at my desired frequency is about 62 ohms, corresponding to a VSWR of less than 1.2. To say that I am pretty happy with this is an understatement! Given that it took absolutely no effort to get it to this point!  I may try to improve the match in future, but for now it is fine with me!

A plot of the antenna impedance.

A plot of the antenna impedance.

Finally, here is an image showing how it all fits together. The slots are used with nylon screws to hold it all together.

The base of the antenna, showing room for the matching wire, cutouts for SMA connector and the fixing method between parts.

The base of the antenna, showing room for the matching wire, cutouts for SMA connector and the fixing method between parts.


PCB Manufacture

People need PCB’s. It is just that simple. Sure, most people are happy to use them in their off-the-shelf devices, but some of us like to make our own.  Whilst laying out my own board is something that I find soothing and relaxing in a weird, strangely Zen like way – the manufacture of them is a separate matter.

I have played the games of making my own boards: from using etch-resistant pens, through to toner transfer and most lately the UV exposure technique. They all work to a point. Whilst the UV exposure technique canbe great , when you start playing around with 8mil traces and 300 via’s on a board it can get a little tedious. That’s why these days I use Mitch!

This board uses internal cutouts, no extra charge here!

After starting with Futurlec and migrating to Seeed Studio for board manufacture, I found out about Mitch Davis from  Mitch is an Aussie that is now living in Shenzen, and get’s his kicks from helping people get PCB’s manufactured at the typical bargain basement prices you would expect out of Shenzhen.  His website is not quite as snappy as Seeed’s, you need to email him for a quote for your board. But the great thing is that you have to email him for a quote! He is very personable, easy to get along with and the entire process is very smooth. His minimum quantity is only 5 boards. Fancy using an internal cutout? No worries! Are you a bit unsure of something, just ask! How about black boards for only an extra couple bucks? Yep, no worries! He is also able to 0.4mm thick boards, great for low weight or trying to get 50 ohm traces.  Even 4+ layers are no issue. A full list of his specs are at: My advice would be to contact him:, you won’t regret it.

This 0.4mm board was manufactured and in my hand in 1 week.

Note: I have no affiliation with him at all, except being a very happy customer of about 5 orders and approximately 20 boards. Yes, I know the review is a bit gushing – but what can I say, he really is that good!

Just a small collection of boards that Mitch has organised for me.

Yesterday was the transit of Venus here on Earth.  If you haven’t already heard about it, then you for sure you don’t know the people I know!

Just a quick post to share a couple of photos that I took.

An earlier shot of Venus transitting the sun.

Later in the day I managed to setup the gear in the office and sneak this shot:

A very late shot of the transit - look for the small bump in the bottom left.

The photos were taken through a Skywatcher ED80 Telescope with a Canon 40D Camera mounted at prime focus.  They have had almost no processing as I don’t have access to Photoshop at work.  The second photo is a bit fuzzier as it was taken through the dirty office windows!

That’s all folks!

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