# ow-loss coax (such as LMR-400 or CNT-400)
# 50mm of copper pipe (~10mm internal diameter)
# 61mm of flat brass bar (~12mm wide by ~0.5mm thick)
# 30.5mm of brass pipe (~4-4.5mm outer diameter)
# female n-connector
Start by cutting off a 50mm length of copper pipe, and cut some slots in one end, making the length of the slots as close as possible to 30.5mm.
Clean up the slots with a small needle file (for the car buffs, a points file works quite well too).
Clean up both ends of the pipe with a file, and use some sandpaper to clean up the external surface of the copper pipe. Also cleanup the inside of the copper pipe (the cut end, as you'll need to solder it, and the other end to ensure a good connection to the coax braid.
I clean up the pipe by holding it in the chuck of my drill (holding the drill on the workbench), and then using sandpaper and a file on the rotating pipe.
Cut off 30.5mm of the small brass tube to make the balun, and clean up the ends with a file.
Using a small drill bit, drill a hole near one end of the brass tube. This hole will make it easier to solder the coax core into brass tube.
For the dipole, we used some brass plate, approx 12mm wide by 61mm long. The length of the brass plate isn't too critical just yet, as long as it's at least 61mm long. It'll be trimmed to the correct length once the parts have been soldered together.
Mark the centre of the brass plate, as this is where you'll have to solder the small brass pipe.
Then hold the copper pipe against the brass plate (with the slotted end against the brass plate), and mark it's location.
hen hold the copper pipe against the brass plate, and mark it's location
Now cut the brass plate along the blue line, and clean up the cut ends with a file. I find a junior hacksaw works quite well for this.
Clean up the cut edges, and remove the tarnish with some sandpaper.
We've used both RG-213 and CNT-400 coax for these modifications, and they require slightly different approaches to the coax core.
Note that CNT-400 or LMR-400 is recommended, rather than RG-213, due to the lower impedance.
Strip approx 30mm of the black outer sheath off the coax.
Fold the braid back over the remaining outer sheath.
Strip off the central insulation, and if using coax with a stranded core (ie, RG-213), double each strand of the core over, and tighten up the bends with a pair of pliers.
Push the copper pipe over the folded-back braid on the coax, until the brass tube protrudes past the end of the copper pipe by at least a few millimetres.
Note that you may need to un-braid the coax braid, to as it is a pretty tight fit.
Tin the two pieces of the brass plate where they need to be soldered to the brass tube and the copper pipe.
Tin the end of the brass tube and the end of the copper pipe with some solder.
Solder the brass tube onto the previously marked centre point on the larger of the two brass plate halves.
Now slide the copper pipe down against the brass plate, and solder it to the brass plate, ensuring the two slots are aligned against the long sides of the brass plate.
Solder the other half of the brass plate to the copper pipe, ensuring there's an air gap of approximately 1mm between the two brass plate sections.
Measure the overall length of the brass plate, and trim the length to make it 61mm long. This is the dipole, and its length should be as close to 1/2 wavelength as possible.
Using the original dipole as a template, measure, mark and drill the the holes in each end of the dipole. These holes are used to locate and hold the dipole in the feedhorn.
If you've got access to a coax crimper, use it to crimp the copper pipe onto the coax braid, to ensure a very firm connection, and trim the excess braid which is still protruding past the end of the copper pipe.