Hexbeam antenna solid transmission line

Made to order. Owner did not update me on results.

Context

A hexbeam antenna is a simple directional antenna, with stacked elements for multiple bands. Each element taps into the transmission line. The easy way to do this is to wire together multiple short coax patches with stripped ends.

According to the owner, hacking coax like this incurs significant losses. The solution is a solid transmission line made out of aluminium profiles (in this case) with specific dimensions. 

Mechanically, the problem is to keep two aluminium tubes inside one another and be able to acces the inner conductor without shorting it to the outer one.

I'm not sure the loss on the coax is large enough, nor that the tiny 1-3dbd worth of gain on the hexbeam warrants this much work, alas, I have never received the comparison between regular tapped coax and this solid transmission line. F/B ratio might be useful. 

BOM

• 20mm square tubing
• 8mm round pipe (or 8mm bar, should be sturdier)
• M3-M4 screws, 5-10mm (pilot)
• M3-M4 screws, 8-12mm (permanent)
• M3-M4 tap 
• M3-M4 washers

How to

1. Print one pair for each band. Print extra spacers if you want to distribute them evenly. The spacers are the cuboid parts with 3 holes.
2. Take the square tube, mark the tap holes appropriately for each band.
3. Drill a hole for the screws, on both sides. The exit hole will be tapped for the “shield” signal. 
4. Enlarge the entrance hole to whatever is comfortable. The “hot” signal is routed through this hole. Drill, slowly.
5. Deburr the inside. You can enlarge the entrance hole as much as needed, since the insulator part (the flat part with side lips and center tube) aligns one the profile with its sides. These parts were made specifically to hide my good craftsmanship.
6. Thread the exit holes.
7. Add spacers to the round pipe. Distribute them accordingly to the markings on the square tube.
8. Clamp the tube, mark, drill, tap. I used M3, but I suggest M4. I suggest aluminium bar over pipe, for better threads.
9. Add the pilot screws to spacers. The head should be flush with the spacer. If it stands proud, the spacers won't fit.
10. Insert the round bar into the square tube, with the spacers. Don't push too hard or you'll strip the threads.
11. The pilot screws should be aligned with the entrance holes.
12. Remove one pilot screw. Add one insulator. Add one wire for the hot side (antenna element), add a washer, screw down the assy with the permanent screws.
13. Use the threaded exit hole to screw down the other end of the wire. Use short screws, enough to make contact with the square tube and no deeper. The insulator should prevent you from shorting the two profiles, but you have much more leverage with a screw against a printed part.
14. Repeat for each spacer.

Notes

1. Originally, my insulators were shorter (like in the pics). 
2. Antenna connector - I'd like to say this is an exercise for the reader, since my solution was not ideal: create one more set of taps.
3. The antenna connector can even be designed at the end of the assy, through the hexbeam mount plate
4. Here is some theory on designing TLs. This is one online tool to design arbitrary TLs with a known impedance, too.