Antenna

The type of antenna we use is called a phased dipole. That might sound a bit complicated but it's a very simple antenna design that delivers good performance, nice and cheaply. The most expensive part of the antenna is usually the coax cable used to join it together and back to the receiver!

As mentioned earlier, the nice thing about the frequency we use is that you don't need millimetre accuracy. For the moment I'll also spare you the math involved in calculating the various lengths. <I might add it later>

The type of wire you use for the antenna elements isn't critical. In theory it could be fencing wire! The practical limitation is that you should really solder the connections and that tends to make copper wire the best option. <type of wire etc>

 

The centre baulin can be made of anything that doesn't conduct. It should also be weather proof and tough enough to take a bit of a beating in windy conditions.

The weight of the coax that hangs from the baulin needs to have some mechanical strain relief. The team at Narrabri initially suffered quite a few failures. Not only did they have wind to contend with but also the occasional kangaroo strike! Their reliability improved when they solved the problem (thanks to some work experience students at the Compact Array.)

I made my baulins out of acrylic (perspex). Believe it or not, the cheapest source I could find was a $2 ruler! I cut it in half and glued the two pieces together to get the required thickness. I'd decided to include a BNC connector on the baulin to take the weight of the coax and to make maintenance/disassembly easier.

<pic of baulin>

<Coax, lengths, stripping, crimping, sources etc>

Loop Antenna Option - A piece of sponge cake in one ear and a lump of jelly in the other....

We did some experimenting with a loop antenna. This type of antenna makes more of the magnetic part of the incoming electromagnetic radiation. This tends to make it less susceptible to RFI.

Unfortunately, the antenna turned out to be a trifle deaf. Nowhere near sensitive enough for RA. Not totally deaf... just mostly deaf. We might revisit the design to look at amplifiers, impedance matching etc.

Here's some picture of my Proof Of Concept antenna. The loop itself is made from a core of the wiring flex electricians typically use to wire a house. The ground plane is a 600mm square piece of MDF covered in chicken wire. 300mm pieces of wooden dowel for the stand offs and that's about it. I already had the MDF so the whole antenna only cost me about A$20.

The white plastic angle holds both ends of the loop to the standoff.