The simple approach to attach an Octagon LNB to a circular waveguide is to saw off the ridged scalar horn and push the cut end into a split 22mm pipe, but I decided to do it the long way. I made up a conical plug from brass to fit against the cone of the horn, then bored a tapered hole inside the plug to match the 17.5mm ID of the Octagon to the 20.2mm ID of 22mm water pipe waveguide. Probably totally unnecessary, but I had fun doing it.
Two friends have asked me to assemble dual-band 2.4GHz/10.4GHz patch/horn antenna feeds, known aa a POTY (for Patch of the Year).
The first one I made was from 1.5mm brass sheet, and I made it on the lathe and milling machine, but for these, a group buy was arranged on the UKMicrowaves group to get them cut on a CNC laser. The cuts are very slightly oversize, by about 0.2mm, but the edges are very clean and only needed minimal fettling to remove the inevitable pip at the start/finish of each cut. I had to open up the centre holes using a deburring tool to make the fit perfect. I also had to mark up and drill/ream the hole for the socket. I made a custom collet for the previous one, it supports the reflector at exactly 90 degrees. It is relieved near the solder joint.
It was a lot of effort and actually it would have taken about the same amount of time to machine them from scratch, but for folks without a machine shop, they are excellent.
The Radiall N socket my friend supplied is not the usual small-format N with 4.1mm PTFE sleeve over a 1.27mm gold-plated brass pin. It has a solder spill. I had to cut it back a little and solder in a suitable extension pin. Again, a lot of work for no benefit.
In addition the N socket and the TNC supplier by the other friend for hit POTY both have a 1.5mm raised boss behind the socket.
The N connectors I use are like this:
On the 0.9mm laser-cut material, that means the boss would intrude into the space between the element and reflector. I made up a precision washer to ensure the boss was flush with the reflector surface.
I soldered the reflector to the tube at the right spacing from the machined top edge of the tube (6mm+0.9mm+3mm) using 183C tin-lead solder paste, which makes a neat joint,
I used a 3mm U-shaped spacer to fix the driven element in place and clamped it, then soldered it. to the tube. Then I stuffed paper towel into the tube and soaked it with water to prevent the existing joints from melting, then soldered the N connector and spacer to the rear of the reflector.
After a good cleaning with isopropyl alcohol then hot water and detergent, I fitted the 10GHz dielectric lens and connected the 2,3GHz port to an HP directional coupler, signal generator and spectrum analyser
So far, I have not been able to get the correct double-dip response. I can get a very deep single dip to -33dB (!) but even doing asymmetric bends or dielectric slips, I can’t split the dips. At max RL, the axial ratio is pretty much that of a linear feed, and with it much flatter, the axial ratio is still around 6dB. I am making another of the original patter with 1.5mm brass parts and machining them on the lather and mill, with a good small-format N socket with 4.1mm PTFE over 1.27mm brass pin so see if I can find what is going on. Then I need to make another with these TNCs…
For the Goole Radio and Electronics Society entry in the RSGB SSB Field Day contest, we needed to fit some 3m extension scaffold poles on top of a pair of SCAM12s to support our doublet. I made up some 40mm spigots which are a push fit into the poles, and fitted a locking bolt, and made the SCAM spigot as a sliding fit at -0.15mm as usual. Made from aluminium extruded bar stock.
I made up some M8 threaded lockbolts by turning down some M10 bolts to 8mm diameter and cutting an M8 thread on the lathe.