Re: Coax Probes into Waveguide


Hello Andy and other readers.

An interesting project.....but I want to make sure that you're not presuming that a probe forming a waveguide-to-coaxial transition needs to be a quarter wavelength long - it's usually better not to be so.

Apart from off-the-shelf transitions made by the likes of Hewlett-Packard (e.g. X281A), the best-matched 'probes' that I have seen are not just straight 'rods' but they look more like inverted cones with the broad end in the middle of the waveguide.  And they're not a quarter wavelength long either.  I have done some trial-and-error experimentation and have confirmed that probes like these have worked very well in various projects.  I remember one case making a probe of increasing diameter using several pieces of brass tube of incrementally increasing diameter.  (Sorry, you can't work in waveguide and avoid some metalwork!)

And being a little shorter than a quarter wavelength, it is intuitively allowable for an adjustment screw to be introduced through the opposite wall, adding a little capacitance there.  But adjustment screws may need to be better placed at another location and the old trick of leading a small ball bearing around inside by way of a bar magnet held on the outside, is a great way to find where the 'sweet spot' actually is.

Approaching from another angle, a very good way of bringing any trial-end-error waveguide transition into submission is to have a close-fitting back short, the position of which is adjustable by sliding in and out a little.  Known good directional couplers (waveguide and/or coaxial) are your friends here!

Just a few comments to start.....

Cheers and 73,

--Doug Friend, VK4OE.

On 3/08/2016 20:12, Andy Talbot andy.g4jnt@... [ukmicrowaves] wrote:
Background first:
I'm taking a new look at the old G3JVL iris coupled filters in waveguide  as used in his transverter.  I have all the original design software, and a copy of MYJ where the design theory originally came from.   But I want to do it with a difference; in that the original used WG in and out, but I want SMA in/out

So, for a three cavity design, originally we had four irises;  two outer ones with large holes setting the Q or input/output transformation and two inner small irises defining the coupling, or K values.    I want to replace the outer irises by coax probes, so only two iris plates have to be installed, and the rest is drilling and tapping (about the only sort of metalwork I'm happy doing !)

So, how to design the probes for adjustable coupling.   Coax to WG transitions are a complete field in their own right, and there is no one correct answer; there are more variables than there are needed to do a complex match, so all sorts of responses can be produced.

However, for a filter, particularly a narrowband one, a  reduced coupling is going to be needed over that for a transition, so a shorter probe nearer to the backplane.   Which where it gets complicated if I don't want to make false starts in cutting metal and have to throw away bits of prepared and soldered guide.

IF I insert an SMA probe into the broad face of WG, I get a simple transition.  Now, the $24M question:  If I place a tuning screw on the opposite face of the WG so it would meet the probe, does anyone know how much it changes the coupling, or, indeed, in which direction.  Ie could I use a very short probe at a decent spacing from the backplane (say 20% of a wavelength) and rely on adjustment screws to get it right

Is there a better place for a coupling adjustment screw ?   

Or if I use an offset probe, not on the centreline of the broad face that will have reduced coupling, where then is the best place for a tuning screw ?

Anyone with experience in this area?

The  G3JVL transverter RF input filter, designed for 60MHz bandwidth, was originally targeted with a 28MHz IF in mind.  It would be nice to be able to do a 10GHz system that could feed directly into a high performance SDR radio without having to go via 144/43/1296 etc.   Even a 50MHz IF would suit many modern SDRs.

With just two iris plates to make and install instead of four, the JVL filter becomes an awful lot easier to build for those of us who aren't all that happy with metalwork

Andy  G4JNT

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