Category: Radio


Continuing on from my previous post where I laser cut a helical antenna for basic radio astronomy, I wanted a way to mount this. Whilst most people that are into these hobbies seem to have  a mast that they can dump another antenna on, I didn’t. Also, I rent so adding a mast wasn’t really a great idea. Also, I’m impatient and have been known to be a bit lazy.  Finally, because I am not receiving terrestrial signals, the antenna does not need a clear view of the horizon, just a clear view of the sky. An idea was thus born.

My logic was along the lines of “take antenna, put on post, attach giant paperweight”. Nice and simple, as long as I could lift said paperweight – it would be portable and easy to work with. Questions then included, what sort of post and what sort of paperweight? The post I wanted to be galvanised so it could handle a considerable time in the outdoors, and the paperweight I wanted to be easy and cheap – hence, premix concrete!

A quick visit to Fyshwick (the home of all things useful industrial in the Canberra region) and I returned with:

  1. ~800mm long, 2″ square, galvanised fence post ($4,  an offcut – check out MetalMart)
  2. a 2″ end cap to make it look swish ($2.50, MetalMart)
  3. a collection of M8 galvanised bolts for reinforcing (about $4, Bunnings)
  4. a 79c plastic bucket ($0.79, Bunnings)
  5. and a bag of premix concrete ($5.50, Bunnings)

Not bad for less than $20.

The bucket, and a drill for scale.

The bucket, and a drill for scale.

The plan was to fill the bucket with concrete, put the post in after  mounting the bolts for reinforcing, let the concrete go off and finally have a portable antenna mount! My calculations based on typical concrete densities were in the ballpark of 24kg for a 10L bucket, so I bought 2 bags just in case. The final product weighs in at about 22kg including the post – pretty good in my opinion, given how much concrete densities vary.

The post and bolts, with holes drilled.

The post and bolts, with holes drilled.

The post with bolts mounted, held in position to demonstrate the "reinforcing".

The post with bolts mounted, held in position to demonstrate the “reinforcing”.

Anyway, I’m happy with the results – I now need to mount the antenna and start looking for a signal!

The post in place, professionally supported and awaiting the concrete to harden.

The post in place, professionally supported and awaiting the concrete to harden.

Finally, a photo of the finished product:

The finished product, awaiting antenna.

The finished product, awaiting antenna.

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.

 

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