I’ve just completed a batch of 24 of these W2IMU feedhorns for the 122 GHz band. Thread as usual is M8 x 0.5 mm. Horn ID is 4.03 mm, 27.1 degree internal flare, 2.00 mm reamed waveguide core, rear duplexer cavity for VK3CV boards, four M2 threaded holes, 4.00 mm reamed barrel, 3.98 mm spigot on the waveguide. This is the version 2 with a flat step as the end stop. Rather than relying on the 7.5 mm tapping drill to make the bottom of the threaded section, I now machine that using a centre-cutting M7 end mill. Part number for this version is DBN-122-IMU-0.7-02 and price to bona-fide 122 GHz experimenters is £14
I needed a couple of transitions, so decided to try to make a very simple narrowband design, optimised for 10368 MHz with low loss and a good match over a few hundred MHz. I ran up a design with rounded corners to the cavity to make it simple to machine using an 8 mm slot drill. I chose aluminium for the body as it performs well, although without anodising, it is going to need protection from the elements. I used some good quality Radiall SMA four-hole flange-mount sockets.
Although this looks a simple part, the instructions I make for myself show the level of detail.
I’ll publish the measured performance soon. So far, I can get around -23dB across ±100MHz. Once optimised, I will have some of these for sale to bona-fide experimenters. Email email@example.com for details
The original flanged coupler has a solid core, so as it is adjusted, the flange rotates. This is fine if it is used with an axisymmetric flange-mounted horn or a dish or lens which can rotate relative to the transceiver board. Where an experimenter wishes to connect the flange to something which cannot rotate, or where adjustment of the polarisation is needed, a new approach is required.
I separated the flange and central waveguide from the threaded barrel, and reamed a hole through the barrel. That allowed the flange to float and rotate, but it needed a clamp. I decided that a split nut which would fit over a raised ring on the shaft might work OK.
More pics to follow…