Equatorial Adapter for a Ham Satellite Antenna

There are many Ham satellites currently orbiting the Earth equipped with electronics that enable Amateur Radio license holders to communicate with each other. This Instructable shows how to build a poor man's tripod equatorial adapter that simplifies hand-aiming an antenna at a Ham satellite as it passes overhead. 

1-1/2" PVC MPT threaded plug

1-1/2" PVC female MPT thread to slip adapter

1-1/2" PVC slip plug

PVC glue

2-1/2" thumbscrew hose clamp

1/4-20 tap

8-32 tap

8-32 x 3/4" screws

Camera tripod

VHF/UHF antenna

The Radio Amateur Satellite Corporation, AMSAT, works with universities, industries and governments to promote building, launching and communicating through non-commercial Amateur Radio satellites. They also publish, in real time, each satellite's status as reported by Hams around the world.

Ham satellites are in low orbits, traveling about 17,000 MPH and circle the Earth about every 128 minutes. That means a Ham satellite is only accessible while it remains above the horizons, for fifteen minutes or less. This brief window requires Hams to optimize their communication equipment and procedures.

Newcomers to Ham satellites typically use affordable hand-held antennas, as shown above. It can be advantageous to support the antenna on a tripod to free-up a hand for adjusting the radio, holding the microphone or taking notes. An ordinary camera tripod can pan and tilt, but it can't easily follow an overhead arc. Adding an equatorial adapter enables you to follow a satellite by occasionally advancing an antenna along a single plane.

The equatorial adapter is made with three 1-1/2" PVC fittings. The lower plug is drilled to mate with the tripod. The upper plug is drilled to mate with the antenna boom. And the center fitting creates the "swivel" action as the upper plug rotates on its threads.

The bottom plug has one 1/4-20 tapped hole for the tripod's camera screw and four #16 drill holes to receive the tripod's anti-rotation pin. Four pin holes give you mounting options.

The top plug has two #19 drill holes to pass the 8-32 screws that hold it to the antenna boom.

Before gluing the lower plug to the female adapter and drilling into the antenna boom, rehearse how the equatorial adapter swivels the antenna on the tripod. Look for potential collisions between antenna elements and the tripod. Proceed with gluing and drilling after the final adapter height and boom mount location have been established.

Because the antenna elements dictate where the threaded plug can be screwed to the boom, the antenna may not be perfectly balanced on the tripod. To compensate for the imbalance, some friction should be added to the swivel. I accomplished that most reliably by making four saw cuts through the fitting's threads to make them springy. The right amount of friction to stabilize the antenna's position can be added by tightening or loosening a 2-1/2" thumbscrew hose clamp positioned over the threads (first photo). Rubbing a little bar soap on the threads helps smooth the swivel motion.

My first attempt to add swivel friction with a section of foam inside the PVC female adapter proved to be not as reliable (second photo).

There are a number of computer programs and smartphone apps that inform Hams when and where the satellites will be overhead. I use the iPhone app called SatSat. It provides three essential numbers needed for deploying the antenna-mounted tripod: AOS (acquistion of signal), LOS (loss of signal) and Maximum Elevation. AOS and LOS bracket how long the satellite will be overhead and where it will cross the horizons. The Maximum Elevation specifies how high to aim the antenna. The trajectory plot helps me align the tripod so the swivel axis aims the antenna at both horizon crossings when rotated fully left and fully right.

Watch the video to see how the tilted antenna rotates on the equatorial adapter.

I hope this Instructable has inspired you to learn more about Ham satellites and how to enjoy using them.