Experimenting with waveguides using the LiteVNA

[coppercone2]

One of the old HP amplifiers has a detent wafer switch on it that has a wedge shaped piece of aluminum attached to it. When you turn it, the detent goes into the correct position for the metal flap to press down on a micro switch. The mechanism is super fragile because its not made with the right interface.

For the rotary resitive card attenuator it would be possible to use that switch to make a stepped attenuator. I mean the one that operates like a guilotine not the one that I have.

I thought thats exactly how you made the attenuator there.

I don't know if those types of attenuators are used outside of physics class diagrams though. I think this one
https://www.torontosurplus.com/hp-j375a-agilent-j375a-variable-flap-attenuator-0-20-db.html

I thought that you 3d printed one and used a old wafer switch to adjust the flap angle

[coppercone2]

And for the shelf for the rack the way I did it is for most of my shelves I have 4 post full size adjusting shelves, but I had some single instrument deep shelves.


So if you put the coupling nuts with the little metal holder behind the front of the rack, you can bolt a 1/2 size shelf onto the front of the rack and it holds pretty good with just two screws. But you need to trim the back lip of the shelf, which I did with a plasma cutter. That way you can adjust your shelf in rack spacing holes.

But you might still run into nuts on the front, in that case you need to make a offset modification, which I did not do yet, I just got lucky with the shelf being able to be positioned where I wanted it to be. The idea with that would be that the shelf could fit over any thing sticking on the front of the rack.

I also wanted some quick release mechanism, maybe with springs, so you don't need a screw driver to reposition it. Or just hooks.If you put it on hooks, which I thought to bend out of some very stainless steel I have that is very tough (after plasma cutting and heat bending), it would just need a offset 'heel' to rest it over any rack shelf bolts so you can put it on any free hole. Like ladder hooks. The shape is easy, it would be this, I just need to work out the distances so nothing lands on a possible bolt

And you can possibly just cut it from square/rectangular tubing slices if you have the right size, not sure if any standard form would fit easily.

[joeqsmith]

"I thought thats exactly how you made the attenuator there."

All of the WG parts were made by LRL.   

My shelves are just PCB that I cut and soldered.  The weight of the equipment keeps them in place.   I used to just slide sheets under the equipment but it flexes and parts were always rolling off.   Mad two with a lip which solved that.  Sill, it would never support the weight I expect you are putting on them.   

[coppercone2]

Ohhh that paint on the switch part looks very much like the silver conductive paint. I thought that was 3d printed

[joeqsmith]

Ohhh that paint on the switch part looks very much like the silver conductive paint. I thought that was 3d printed

And while you may not want to come out and say it, it certainly looks crude enough to be home made. 

I'm not sure what they plated these with.  Maybe nickle. 

[coppercone2]

Ahh I replaced that screw that fell out of the vise jaws like 5 years ago and gave them a nice ultrasonic clean and all the stuff, so I can bend metal without it slipping wrong. I will try to cut some stainless steel strips for the hooks and see if I can find some form to bend it on. Having that shelf removable is gonna be nice, I think I can make it so it hangs on the side of the rack when not in use, almost walked into the attenuator

[coppercone2]

I got the 1 inch long WR62 flex. I think this is the one that is a solid piece without the wraps.

Its not very flexible. If you don't know what to buy, get the long ones. The short ones like this is OK for another misaligned wave guide, but if you want connector strain relief, its gotta be pretty damn close to nominal.

If your setup is really bad, I would recommend 1 foot or longer flex guide. Even the 8 inch one does not flex that much, its very stiff stuff.

[joeqsmith]

Lapped the flanges of the short section.  Next step is to make a new shorting plate for it.  Thumbscrews are to make it easier to insert the samples.   

[coppercone2]

nice reminded me to buy some thumb screws for the pasta maker because I stopped using it since it draws in dough into the gears and I need to unscrew it every time

I wonder if I can make that on my proxxon mf70. I think I can, with telurium copper and a new atlas 3 fluke non ferrous end mill. How thick are they? I made stuff out of electrical copper even on it, but the problem is the cutter is so small, it really should have a fly cutter, it would be mad hard to get it actually flat. I would need a new XY table though, it runs stiff, last time I made something some what big I was cursing at that stupid knob. The casting or whatever it is was cracked and I fixed it with super glue lol. The surface finish looks super cool though, if made with tiny end mill.

[joeqsmith]

My shorting plate is about 6mm thick.   Most likely will use the same material.   

[coppercone2]

You gave me an idea, I find it very annoying to put the nuts on the wave guide one at a time.

I wonder how hard it would be to make some kind of like thing to put over the wave guide to position the nuts. I like the idea of a 3d printed bracket shape that has magnets in it, so you can load 4 nuts into holes, have them held in place magnetically, so you can get it started and there is no chance of spilling it etc, then just lift it off. Like a nut starter or whatever specific for the wave guide size.

or ideally two tools, one with 2 pins, and 2 nut holders, and the other with 2 opposite nut holders. That way you can keep it balanced on pins while you put the first bolts on, then remove, and put the other plate on once  its already kinda held together. No bullshit. And the pins need quick release slide collets on them too, with like a spring, like the thing you have on sports bags to keep the nylon cord tightened. That I think would be like really good. I need a extrusion printer though because I know the resin parts will be too weak for this. It would be actually aligned on pins by the time you tighten the first two nuts, perfect, not some edge clamp that you guess with (according to the paper its a very small effect, but still, its like a purpose built spanner or something, nice to have even if you can do it with pliers). 

[joeqsmith]

Paperclips may be the fastest.   The threaded flange may be an option. 

[coppercone2]

What do calibration constants look for the plates you made?


I guess if its accurately machined, its standard numbers that are available?

Obviously I don't know what to do without the data disk.

[joeqsmith]

What do calibration constants look for the plates you made?

I guess if its accurately machined, its standard numbers that are available?

Obviously I don't know what to do without the data disk.

They are based on the Agilent standards.  Which includes the new shim. 

https://www.eevblog.com/forum/rf-microwave/experimenting-with-waveguides-using-the-litevna/msg4801919/#msg4801919
https://www.eevblog.com/forum/rf-microwave/experimenting-with-waveguides-using-the-litevna/msg4786121/#msg4786121
https://www.eevblog.com/forum/rf-microwave/experimenting-with-waveguides-using-the-litevna/msg4797443/#msg4797443

[joeqsmith]

Playing around with the frequency meter from LRL.   It's basically a circular cavity with a micrometer.  I shorted one side and plotted S11, uncalibrated  (yellow). Then using that old HP adjustable short to make another cavity, I was attempting to use copper foil tape to make an iris for it (red).   Lots of attempts.  Q seems a bit better.   

[coppercone2]

hmm speaking of metal iris, I never had strong feeling for a optical iris at having good continuity between the sections, but this one might (I saw it on adam savages channel for makers)
https://us.makerscabinet.com/products/iris

Fascinating idea, optimally I think it would be made of berylium copper sheet. I imagine the safe way to do it would be to make a stamping die for cutting the segments.

Or it might just be possible to get a standard manufacturer to assemble one without anodizing, or just send you the raw parts for alodine coating and assembly. I think it would need a extra lip for copper fingers to be really good, maybe.

If you stack a bunch of thick iris with offset cams I imagine you can approximate a conical taper with a very big mechanism to make a universal adapter for circular waveguide tubes

[coppercone2]

When you did your multi antenna tests, did you experiment with near and far field?

https://www.emc-directory.com/calculators/far-field-distance-calculator-for-horn-antennas

But its a bit bizarre because you get negative numbers for the low end.

need to be precise, about the frequency, it figures out cutoff I guess for the wave guide.

But for 12.5Ghz, with a 45cm2 aperture area, it says 16.5 meters far field. For 18Ghz on the same antenna, I get 1.1 meters.  And it goes down the smaller you make the aperture. So I am thinking that short horns might be useful for something too. Need to see what the parametric program does to flare angle. I think its adjusted for length automatically by an equation. But I think it might be interesting to print some shorter ones, if you are close to the VNA and stuff. I think you mentioned it in a video but if anyone is trying this...

θ=2*tan^−1(d/2*L)
              l = axial length

              d = aperture dimension

              θ = flare angle

that 16 meter figure is pretty huge, I was not expecting it, you really need to design antennas for experiments based on your work area.... very incorrect assumption. I guess it makes sense, its like a big air capacitor. But still I think I had the PCB near field probe thing in my head... so just plugging in those antennas with elbows infront of the VNA would be very near field if I did it.

[joeqsmith]

Say the horn is 80mm X 80mm, with a freq of 10G.   Wavelength would be 0.0288782 meters.   2D^2/wavelength is 0.426962 meters.   

Using the calculator you linked, with an aperture of 80^2 or 6400mm  gives us  385.04186074  meters.    Using an aperture of 80mm it gives us 0.06016279 meters. 

If you like on-line calculators, if I try:
https://www.everythingrf.com/rf-calculators/antenna-near-field-distance-calculator

and use the same settings, it show a far field of 0.42696198 meters.   

Could be a user problem.  Double check my math but it looks like the calculator you chose may have a problem. 

[joeqsmith]

When you did your multi antenna tests, did you experiment with near and far field?

Outside of showing that wire polarizer and swapping the E&M planes,  no.   To compare the radiation patterns, I was only interested in the far field and used the same setup each time.  The only thing changed was the antenna.  Well until some idiot decided not to use a wrist strap because after all, the horns are grounded....       Even so, repairing the amplifier appeared to have very little effect on the measurements.   

The problem as I mentioned is that to test the antennas from the papers I provided, they test them at much further distances.  I went over that in that video and talked about not being able to run them at the normal distance.   But because I was only interested in a relative comparison, I wasn't too concerned.   

[coppercone2]

hmm how did you damage the amp from ESD? The horn is connected to the chassis ground, the center pin should be floating deep in the wave guide?

Did you accidentally touch the center pin some how? I am reluctant to use a wrist strap too because it seems... well grounded and protected.

I need to attach a ground point on the rack I think so I can plug in right infront of where I am working. But being chained sucks

[joeqsmith]

Not the center pin directly.   Cable makes a decent capacitor.  ESD is pretty high frequency.  Bingo bango.   

Was just being stupid.   I tend to slack off with these low cost VNAs.  Did you check their math?  Sure seems wrong.

[coppercone2]

Hmm I never thought of the high frequency coupling thing from coaxial cable. Maybe I am lucky because its a concrete damp basement.

The protector for high frequencies are ridiculously expenisve.

I need to check the math still but not soon because the weather is nice today

[coppercone2]

I did think of a sanity check before digging out equations from god knows where:

for a parabolic horn fed antenna, the reflector is positioned in the far field region right? that might bound the problem, I never saw a antenna with a 50 foot feed

I am gonna guess before I do something else that they mean D = largest dimension that all the web pages say and the area picture on the site is a mistake and its not a mistake that they wrote cm instead of cm^2


so just confusing legend, I think, because I spent 15 minutes and I don't see anything obvious about using the area of the aperture as the calculator implies . so its probobly a... reduced equation. Wonder what the original looked like. I wish to see it. Need text book I guess.  I guess the formula  has like a scaling coefficent that is say 1/lesservalue^2 as a multiplier that results in 0.1% error or whatever, so they just got rid of it because its basically useless?


here we go, water cool your brain

[joeqsmith]

No need to chase down formulas or simplify anything as the link you provided for the on-line calculator you used provided the equations.  Just some basic math was all.  As for the area vs a sidewall, I had tried that and posted the results in my example.

Nothing on that site, or any of the testing I did uses a parabolic reflector.   Using no math and just thinking of a common sat dish, 2.4GHz, dish about 3' dia, horn about 1' away.   Total shoot from the hip.   At the 10+GHz you are talking about, I have no idea where that 15' is coming from.    Even the old sat dishes that were 8' or so dia, horn's focal point was only a few feet away.   

[coppercone2]

the how to web pages with formulas but not calculators say that the major diameter or longest diameter is used for the equation, not the area. But the calc I found shows area. maybe tonight I can try spread sheet comparison