Ancient Web: VK3NX Pointed a Backyard Dish at the Moon

Part of the Ancient Web Collection.

Most people look at the Moon. Some radio amateurs aim a dish at it, bounce a signal off its surface, and hope another operator hears the echo.

Visit VK3NX's dish and control page.

VK3NX, an Australian amateur-radio station, preserves a detailed account of the mechanics and control system behind an Earth-Moon-Earth, or EME, antenna. This is a narrow, specialist survival rather than evidence of a dated 1990s launch. Its older-Web character comes from the design of the reference itself: photographs, schematic links, paragraphs of hands-on specifications, and descriptions of modifications written for other builders.

It is unusually concrete. The dish is not just 'large.' It is a 3.7-metre Joysat mesh reflector attached to a 50-millimetre mast and a 100-millimetre pole set partly in concrete.

Moon tracking, by jackscrew

The technical notes describe azimuth steering with a 36-inch actuator and elevation with a 24-inch actuator. Reed switches in the jackscrews produce 32 closures per inch of travel. Those pulses become position information for a VK5DJ beam indicator system. The builder explains that two positions of the azimuth jackscrew cover different arcs of the Moon's apparent travel.

Instead of assuming the antenna is aimed correctly merely because its mechanical readout says so, VK3NX also uses a wideband noise detector to measure noise from the Sun and Moon. That offers a real signal against which the pointing can be checked and refined. The page discusses a 141 MHz detector centered on a 4 MHz bandwidth, with an AD8307 logarithmic detector and approximately 40 dB of gain ahead of it.

Another small marvel is the feed-changing system. Different microwave feeds are built as removable assemblies held by four wing nuts. The author says lowering the dish and swapping feeds takes around ten minutes. Power amplifiers live beneath the dish, in an enclosure nicknamed the 'moon buggy,' while other equipment sits in a tin shed.

The control section credits John Drew, VK5DJ, for the underlying controller and points to his documentation. It explains that the VK3NX system was later rebuilt to combine the position controller with motor controls. A link to a separate noise-meter article retains the possibility of following the design further.

This is the kind of engineering lore that rarely fits into a product brochure. The page records decisions, compromises, and one amateur's solutions to moving a giant outdoor antenna accurately enough to use the Moon as a reflector.

Read the dish-control notes and linked schematics.

Part of the Ancient Web Collection.