432 MHz FEEDHORN CONSTRUCTION

The design of the feedhorn was driven by my desire to fabricate a suitable 432 MHz feed system for my dish. The feedhorn requirements are that it must be able to provide both horizontal and vertical polarization signals simultaneously and it must be able to properly illuminate both a 0.43 f/D dish and a 0.3 f/D dish, as my current 0.43 f/D dish will become 0.3 f/D when the extension to 40' (12.2 m) diameter is completed. A dual-port, linear-polarized, cylindrical waveguide with an external scalar ring will satisfy both of those requirements adequately.

The horn will be used at 432 MHz and 436 MHz for EME and pulsar radio astronomy, respectively. The diameter of the cylindrical waveguide section was chosen to be 19-5/8" (50 cm), yielding a TE(11) mode cutoff frequency of 352 MHz and a TM(01) mode cutoff frequency of 460 MHz.

The guide wavelength for the mean frequency of 434 MHz is

Lg = 11802.85/(sqrt(434^2 - 352^2)) = 46.7" (118.6 cm).

The length of the waveguide section was arbitrarily chosen to be 72" (183 cm) which permits one and half guided wavelengths before the wave encounters the discontinuity at the end of the horn. Further, the long cylindrical waveguide length may result in a particularly quiet feedhorn on receive as the pickup probes are relatively far from the waveguide entrance.

The cylindrical waveguide section was formed by rolling 0.040"(1mm) thick aluminum sheets into tubes and riveting the overlapping edges together.

The scalar ring will be used to adjust the illumination angle of the feedhorn for the dish used. The size of the scalar ring was chosen to be 0.4 wavelength wide by 0.25 wavelength deep. At the selected mean operating frequency of 434 MHz (air wavelength of 69.1 cm) 0.4 wavelength corresponds to 10.8" (27.6 cm) wide and 0.25 wavelength corresponds to 6.79" (17.4 cm) deep.

The backplates for the waveguide and for the scalar ring were cut from a 48" (122 cm) x 48" (122 cm) x 1/8" (3 mm) thick aluminum plate as concentric circles.

The following photos illustrate how the horn and scalar ring were constructed.

Photo above: Cutting the scalar ring backplate from square sheet. An electric sabre saw was used to cut the plate by hand. (April 14, 2010)

Photo above: The cylindrical waveguide backplate was cut from the center of the scalar ring. (April 14, 2010)

Photo above: The aluminum sheet was hand rolled to form a cylinder. The scalar ring backplate was used to assist the process, as shown. A twisted rope was used to help pull the sheet into the cylindrical shape. (April 15, 2010)

Photo above: Hose clamps were used to temporarily hold the sheet into the desired cylindrical shape. (April 15, 2010)

Photo above: A pneumatic riveter was used to rivet the overlapping cylinder edges together. 1/8" (3 mm) diameter x 1/4" (6 mm) long aluminum pop rivets were used. (April 15, 2010)

Photo above: Rivet holes were drilled using an electric hand drill. (April 15, 2010)

Photo above: Fabrication of the first section of cylindrical waveguide is complete. A second section 36" (92 cm) long will be fabricated and inserted 12" (30 cm) into this section to form the complete waveguide. (April 15, 2010)

 

Photo above: Riveting the mounting sleeve onto the scalar backplate. (April 21, 2010)

 

Photo above: Fabrication of the two-section cylindrical waveguide and scalar ring is completed. Adjustable monopole mounting to be added next so that the feedhorn may be precisely tuned for resonance with optimum return loss on each port. (May 17, 2010)

Photo above: Monopole probes were fabricated using type-N coaxial connectors, half-inch (12.7 mm) diameter copper tubes, and 7/32" (5.6 mm) diameter silvered brass tubes. Nominal length of the monopoles are 5-5/8" (14.3 cm) long. (May 20, 2010)

Photo above: A 2" x 5-1/2" (5cm x 14cm) rectangular slot was cut into the approximate monopole location to accept a moveable monopole mounting. (May 18, 2010)

Photo above: Monopole mounting plate, interior, showing backing plate, mounting bolts, and monopole in position. Monopole tubing length is approximately 6-1/8" (15.6cm), diameter is 7/32" (5.6 mm). (May 18, 2010)

Photo above: Interior view of feedhorn showing vertical and horizontal monopole probes. (May 20, 2010)

Photo above: Monopole mounting, exterior. (May 20, 2010)