[NLRS] [Mw] Boradwall Waveguide slots for antennas

[Dr. Gerald N. Johnson]

On 12/3/2015 8:07 PM, Paul Wade W1GHZ wrote:
> the calculations in my spreadsheet are for a round end.
>
> slots cut with a CNC machine, square with rounded corners,
> ended up slightly lower in frequency.  then length adjusted to
> compensate.
>
> cut them however, measure and adjust.  that's how the round
> end numbers were derived years ago.
>
> or get yourself access to a CNC machine.  I'm learning to use
> one at our local Makerspace.

Around 42 years ago I had access to a 2 axis CNC vertical mill. I 
developed software for making paper tapes for cutting arbitrary profiles 
from equations. Many were NACA wing profiles. Even though it had 1 mil 
steps by taking advantage of geometry varying what part of the 
circumference of the end mill cut the tangent of the desired surface it 
cut such airfoils within 1/2 thousandth. One airfoil was for a NASA 
research project and an ISU mach 3 or faster shock tube. NASA came to 
test the part and came with the opinion they were the only shop capable 
of meeting their dimension tolerances. After a machinest smoothed my 
carving they were surprised that it was as good as they could make. The 
shock tube was a tougher test, some of the 1 mill steps caused waves 
even after lots of smoothing.

It would not be suitable for cutting a gang of slots in a waveguide 
antenna since it didn't have power on the knee or the head for 
penetrating. And penetrating is part of the problem of cutting slots.

My concern with sawing is that the aperture on the interior of the guide 
probably sets the impedance as seen in the guide while the aperture on 
the outside face with sloped ends from saw cutting sets the radiation 
pattern and that taper may not have a great effect on the detected 
resonance. When the original literature MIT Rad Lab is based on 
neglecting the effect of guide wall thickness, that effect isn't clear.

It would be more fun cutting brass guide than copper just because brass 
machines so much nicer.

73, Jerry, K0CQ
>
> 73
> paul
>
> On Thu, Dec 3, 2015 at 4:55 PM, Dr. Gerald N. Johnson
> <geraldj at netins.net <mailto:geraldj at netins.net>> wrote:
>
>     I know there has been some mention in references I have about round
>     ends vs square ended slots, though most of the computation appear to
>     be based on square ends to the slots and no thickness to the
>     waveguide walls. Is there any literature showing a difference
>     between the resonances and radiation of round vs square ended slots?
>
>     Then is there any that allows for the slot in the inside of the
>     guide being a different length than on the outside, as it would
>     naturally be from cutting the slots with a metal circular saw blade?
>     Or does all the literature presume all slots will be squared up by
>     hand after the milling operations. With copper not a fun material to
>     cut slots in with an end mill because its so gummy and grabby, I'm
>     thinking cutting with a fine toothed saw blade would make much
>     smoother slot walls and destroy way fewer cutting tools. Filing
>     isn't much fun either with a file small enough to fit in a 10 GHz
>     slot. The stroke can't be very long and the file has to be thin and
>     weak. I have a large supply of 3" and 4" diameter metal saw blades
>     that I can use, if the slot resonance and radiation aren't messed up
>     by sloping ends on the slots. I also have collected a few 3/4" and
>     1" diameter but those won't get the ends perfectly perpendicular to
>     the surface and have very coarse teeth compared to the bigger ones.
>
>     73, Jerry, K0CQ
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