An antenna for a weather balloon from old antennas
In this post I will build a dipole antenna for the purpose of decoding radio signals around the 403MHz mark or just under the 70cm band.
This antenna was made from an old magnetic loop trombone style antenna.
One where you slide a U bended piece of copper inside two foil coted pvc pipes to change capacitance for the loop.
The parts that I could summaries need for the build is the following :
- Approx 300mm of 15mm diameter copper tubing
- 1000mm of 20mm outside diameter PVC pipe
- Approx 750mm of RG58U / coax of your choice
- Female BNC 50Ω bulkhead
- Blowtorch for pipe soldering
- Soldering iron for BNC bulkhead
- Flush cutters
- Pipe Cutter
Take the piece of coax and strip about 3cm of the end, taking care not to damage the outer metal jacket.
Expose the center core of the coax and wind up the outer jacket into a wire like strand.
Take the soldering iron and tin both of these adequately.
The pipes where cut to about 137mm each.
They where then placed on top of our trusty gas hob.
Taking the blowtorch we then heat up the ends of the first element(pipe) for about 6-7 seconds with the tip of the blue jet stream of plasma.
Once the pipe starts smoking we firstly switch off the blowtorch, we then can proceed to carefully push our solder strand into the insides of the pipe, it will, if hot enough start pooling immediately and bonding to the copper.
When there is about a 1cm circular blob of liquid metal on the inside of the pipe start the torch back up, reheating for about 3 seconds and switching off the torch again, promptly carefully take the center part of the coax and submerge it in the pool of metal.
Take a deep breath, and start blowing...
You should see lots of flux smoke exiting the pipe which is a good sign, after blowing at it for about 3 seconds it will solidify and bond permanently.
Then patiently wait for the pipe to cool down.
After the pipe is cool enough to handle, repeat the above process for the outer metal jacket of the coax.
You should be presented with something like this.
We have successfully constructor the radiators of our antenna.
We then carefully take our coax and run it through the copper pipe connected to the outer jacket like so.