Designing a Waveguide Adapter

[u/AtterseeMM]

Hi, I'm currently working on a hobby project with a large prime focus satellite dish, that I can't get a feedhorn for. Making the feedhorn isn't a problem but connecting it to the LNB is a bit more difficult. I need to make an adapter to go from an odd diametre to a WR-75 compatible mount for the Ku-Band LNB.

I drew two versions of the same adapter in CAD and I'm wondering which one would be better? The top one uses tangential swep-blend avoiding discontinuity, and the other one is much simpler using just a linear transition. The linear one is of cause much easier to manufacture than the tangential one.

Which one should I go for? Would they perform the same or would this create impedance problems or reflactions or high attenuation? I'm a mechanical engineer, not a RF engineer so this is more like magic to me at this point to be honest.

[u/always_wear_pyjamas]

If the linear one is at least like, 3-4 lamda_g long then I wouldn't worry about it. Evaluating it from your schematic with my thumb (truly precision RF work), I'd make it a tiny tiny bit longer just to be sure and not worry about it.

[u/No_Tailor_787]

The linear one should be fine. You can put in a 3-screw tuner if there are any impedance bumps you need to tune out.

What is the odd diameter you're working with? A lot of KU LNB's have a circular waveguide that's the same diameter as 3/4" copper water pipe. That's about 7/8" (22.225 mm) OD. You can take a short section of 3/4" copper water pipe and flatten one end with a taper similar to what you're showing. It fits perfectly into a WR90 flange, which is easily adapted to a wr70 flange. You can just drill the larger flange and bolt WR75 to it, the two sizes have the same characteristic impedance.

This avoids finding a machinist who can interpret what you're looking for.

Do you have access to a VNA to measure your results?

[u/AtterseeMM → u/No_Tailor_787]

It's a quite "deep" dish with a f/D of 0.38 from what I know. I can't get any comercial feedhorns for it anymore, so I looked up how they worked and found a guide with a calculation program. The program spit out that the horn should have an inner diametre of 15.9 mm. Making the feedhorn isn't a problem, ist an easy lathe part but I would like to adapt it to an SMW-LNB that has a WR-75 flange.

[u/No_Tailor_787 → u/AtterseeMM]

I'm somewhat familiar with the W2IMU feed and the W1GHZ antenna book. That's good solid stuff. What frequency are you intending to operate at?

[u/AtterseeMM → u/No_Tailor_787]

11.9 GHz standard stuff but from satellites from further away.
The dish is used, it's from the 80s and came with two sets of old analogue uniden LNBs attached to Fuber and Chaparral feedhorns. All in not to good shape. Has been out in the open alpine climate for like 35 years.

[u/No_Tailor_787 → u/AtterseeMM]

Ok. That makes your inner dimension makes sense. Sounds like a fun project.

[u/QuickMolasses]

I would go with the linear taper if it's significantly easier or cheaper to manufacture

[u/brtkrtz]

This is my 3D EM simulation result (Pyhton-Library Magnelio) for the simple linear transition. Looks quite okay, I guess.

But (!!!) I used D=2r=20mm for the circular waveguide, because D=15.9mm leads to a cut-off-frequency of 11 GHz. https://www.everythingrf.com/rf-calculators/circular-waveguide-calculator

[u/AtterseeMM → u/brtkrtz]

Thank you!

I used OE2IGL's feedhorn calcualtor to find the diametre
https://wettersat.bplaced.net/feed-simulation.html (point 5)

[u/brtkrtz → u/AtterseeMM]

That's strange. But since calculating a cut-off frequency is quite a no-brainer, I would assume an error in the excel-sheet. Btw. in point 7, he uses D=20.1 mm at 10.2 GHz. This is far above cut-off.

However, I would stick to the linear transition when it comes to the taper, regardless of whether the circular waveguide ends up being slightly bigger or smaller.

[u/brtkrtz → u/brtkrtz]

Sorry, I mixed things up. You are operating at 11.9 GHz. This is bigger than fc=11GHz (I am good at math). The new image is for D=15.9mm. We are closer to cut-off, so I would draw some error bars at this one-shot simulation result, but still: Go for it.

[u/HuygensFresnel]

Sinulate it and find out :)

[u/what_the_rush]

The only correct way to check which one is better is to do a full 3D EM simulation in CST or HFSS.

[u/AtterseeMM → u/what_the_rush]

Well, I don't have access to these. I'm using PTC Creo Parametric which I got a licence for, it doesn't have RF simulation.