RadioBanter - View Single Post - Analyzing Stub Matching with Reflection Coefficients

Richard Clark · April 15th 07, 02:39 AM posted to rec.radio.amateur.antenna

On Sat, 14 Apr 2007 16:36:11 -0500, (Richard
Harrison) wrote:

Richard Clark wrote:
"This conforms to my experience with many plumbing designs on the
microwave bench."

Good. Then, Richard Clark must also be familiar with the grooved
circular flange used in conjunction with a smooth flange to join
waveguide segments. This groove isn`t just used to hold a neoprene
gasket. It is also used as an electrical choke to keep the microwaves
within the pipe. It is approximately a 1/4-wave choke and its high
impedance across its open-circuit helps foil the wave escape. If virtual
open-circuits didn`t work, the "choke-flange wouldn`t work either.

Hi Richard,

You got the dimensions wrong for the wrong reason. It is a halfwave
shorted cavity (or transmission line sub-section). The cut channel is
quarterwave, but the distance from the middle of the longest side of
the waveguide is another quarterwave. The virtual short bridges the
joined, but non-contacting faces of the two waveguide sections. There
is no high Z action involved.

Further, there are two grooves, the outer one is for the neoprene
gasket.

For others, when two sections of waveguide are joined, there is always
the possibility that the two faces of the ends will not see a machined
match (like an engine head to the engine block - which even there has
problems of warp); hence the possibility of a break in continuity with
an open circuit. Rather than burn money as a solution, engineers
simply forced the problem by having the two faces separated and never
in contact!

What they did to offset this deliberate open is they machined a
circular groove around the flange of one section that was a
quarterwave deep, and a quarterwave away from the voltage node for the
waveguide's TE10 mode of transmission. The geometry of the groove and
its distance from the node was very simple to control in comparison to
guaranteeing fully flush mating faces (especially under the torsion of
ship's movement, or simple heat expansion). This is a hallmark of
engineering where the problem becomes the solution.

As always, a short is vastly more preferred than an open when casting
back into the transmission line and the design engineers went to the
additional length to see it incorporated into the solution.

Dare I call the choke joint a virtual gasket?

73's
Richard Clark, KB7QHC