rule of thumb for making a waveguide to coaxial connector? (for NF probes)

[CopperCone]

Let's say I want to make big ass directional near field probe using waveguide. Like for 400MHz. It comes out like 15x7 inches. I can make this out of sheet metal.

It comes down to making a box that is open on one side with a rod going down in it. How do you do the calculations to design this?

I have seen a design that ignores the tuning screws for a 2.4ghz waveguide magnetron to coaxial connector (thought it is published by a neo nazi publication and I am not sure if I should link to the Zine here because its rather distasteful).

Is there a simple relationship that describes
-distance of injection probe from reflection wall
-length and height of waveguide
-minimum length of waveguide to be functional before open end

It's not great, but a bit easier then making a giant horn. Just curious really.

I was thinking about doing this for WR1500 waveguide. Perhaps making a set since its easy enough to make boxes out of thinner sheet metal.

Honestly, it would be easier to make a bunch of boxes then to design butterworth band pass filters for all this shit, you can make wr3200, wr1800, wr1500, wr1150, wr975 and nest them inside of each other for storage purposes then just use wideband amplifiers. It seems like less work and its smaller then a giant heavy ass dual ridge horn thats like 3x2x3 feet and weighs 25 to 40lbs (depending on materials thickness) compared to some dryer ducts and reinforcement wood, plus it seems reasonably directional.

A horn for the lower frequencies is ridiculous, it comes out to many hours of welding, cutting, and I don't even wanna think about fabricating the ridges. You need like 40 square feet of aluminum or more, and it cant be wobbly. Do I want to make one? yes. but this is really interesting due to the directional, cheapness and maneuverability from home depot so long its not mm wave dimensions.

I intend to make a higher frequency one first, for like WR975 dimensions, if I can figure out how to make the transition.

[AF6LJ]

Why not build a corner reflector antenna?
They use less metal and are very directional.

[CopperCone]

That kind of antenna will be a dipole with a corner reflector right? Meaning it will be much narrower bandwidth.

Can you combine a bicone and a corner reflector to make a wideband directional antenna on the cheap?

How does a corner reflector compare to a waveguide in terms of directionality? I am looking at the radiation pattern from corner reflectors on google images, and they seem pretty respectable, but the antenna element for the reflector seems like the hard part. Another beneift of the waveguide is that you can put it on something with a crap/no preslector, lacks yig filter, etc.. and it will probably do a very good job filtering out stuff that would require a more expensive system (i.e. compared to cheap SDR). The different waveguide shapes come in bandwidth chunks that are very easy to digest.


still interested in making one no matter what the alternatives are though.

[AF6LJ]

That kind of antenna will be a dipole with a corner reflector right? Meaning it will be much narrower bandwidth.

Can you combine a bicone and a corner reflector to make a wideband directional antenna on the cheap?

How does a corner reflector compare to a waveguide in terms of directionality? I am looking at the radiation pattern from corner reflectors on google images, and they seem pretty respectable, but the antenna element for the reflector seems like the hard part. Another beneift of the waveguide is that you can put it on something with a crap/no preslector, lacks yig filter, etc.. and it will probably do a very good job filtering out stuff that would require a more expensive system (i.e. compared to cheap SDR). The different waveguide shapes come in bandwidth chunks that are very easy to digest.


still interested in making one no matter what the alternatives are though.

Bandwidth for these antennas is typically eight to ten percent of the center frequency.
 
Using a Bicone with a corner reflector maybe problematic due to the corner reflector's focal length, and the (educated guess) the size of the bicone may interfere with the reflective properties of the corner reflector.