Waveguide flange

The connection between a pair of flanges is usually made with four or more bolts, though alternative mechanisms, such as a threaded collar, may be used where there is a need for rapid assembly and disassembly.

The atmosphere within waveguide assemblies is often pressurized, either to prevent the ingress of moisture, or to raise the breakdown voltage in the guide and hence increase the power that it can carry.

[citation needed] It is also possible to form air-tight seal between a pair of otherwise unpressurizable flanges using a flat gasket made out of a special electrically conductive elastomer.

[citation needed] Electric current flows on the inside surface of the waveguides, and must cross the join between them if microwave power is to pass through the connection without reflection or loss.

This side branch is designed to present a low input impedance where it meets the broad walls of the waveguide,[3] so that the surface currents there are not obstructed by the gap, but instead flow onto and off of the separated faces of the flanges.

This is somewhat longer than a quarter of the free-space wavelength, since the electric field also varies in going around the ditch, having two changes of polarity, or one complete wave in the circumference.

The ditch thus constitutes a quarter-wave resonant short-circuit stub, and has a high (ideally infinite) input impedance at its mouth.

[citation needed] Because the working of a choke connection depends on the wavelength, its impedance can be zero at at most one frequency within the operating band of the waveguide.

However, by making the gap extremely narrow,[1][3] and the choke ditch relatively wide,[1] the input impedance can be kept small over a broad frequency band.

The choke ditch in these flanges is some 8 times wider (around 20% of the waveguide height), although the proportions vary considerably, as the width-to-height ratio of the standard mid-size guides deviates from 2:1.

Claimants to the invention of the choke connection include Norman Ramsey[10][11] with the assistance of Shep Roberts while the two were working at the MIT Radiation Lab during World War II.

Winfield Salisbury also claims to have made the invention while leader of the Radio Frequency Group at the MIT Radiation Lab between 1941 and 1942.

The two pieces are soldered or brazed together to ensure an uninterrupted conducting path between the inside surface of the waveguide tube and the mouth of the flange.

[5][6][7][8][9] MIL-DTL-3922 is a United States Military Standard giving detailed descriptions of choke, gasket/cover and cover flanges for rectangular waveguide.

IEC flanges are identified by an alphanumeric code consisting of; the letter U, P or C for unpressurizable[2] (plain cover), pressurizable[2] (with a gasket groove) and choke[2] (with both choke gasket grooves); a second letter, indicating the shape and other details of the flange and finally the IEC identifier for the waveguide.

The Electronic Industries Alliance (EIA) is the body that defined the WR designations for standard rectangular waveguides.

Figure 1. A UBR320 flange on R320 (WG22, WR28) guide. This type of flange has no choke or gasket grooves. The through-mounted assembly is made evident by the distinct colours of the copper waveguide-tube and brass flange.
Figure 2. A UG-1666/U (MIL-standard) choke flange (left), and matching gasket/cover flange (right). These flanges are aluminium and are socket-mounted onto aluminium WG18 (WR62) waveguide.
Figure 3. E-plane cross-section of connected choke and gasket/cover waveguide flanges from figure 2.
  1. waveguide tubing socket-mounted into...
  2. choke flange and...
  3. gasket/cover flange
  4. gap between flange faces ( not to scale )
  5. point of contact of flange faces
  6. short at bottom of choke ditch
  7. O-ring gaskets to allow pressurization
The gap between the flange faces has been exaggerated by a factor of four to make it clearly visible. The choke flange can also be mated with a plain cover flange and still form a pressure seal.
Figure 4. Plastic caps over disconnected flanges prevent dirt [ 4 ] and moisture entering the waveguide, in addition to protecting the face of the flange from damage.
Figure 5. RCSC 5985-99-083-0003 choke flange through-mounted on WG16 (WR90) waveguide. Machining down the end of the waveguide tube has left a clear pattern across the recessed face and the end of the tube. The flats on either side of the flange are to allow a threaded collar to be manoeuvred over it, [ 1 ] while the notches at the top and bottom are for alignment. The O-ring for pressurization is in place.
Figure 6. Non-standard quick-disconnect (threaded collar) flanges on WR102 guide