Rotators
| Rotators | |
|---|---|
| Also called | Rotor, antenna rotator |
| What it does | Turns a directional antenna to point at the other station |
| Main parts | Motor and gear unit, brake, position sensor, control box, control cable |
| Rotation | Usually 360°; some models 450° with overlap |
| Ratings and mounting | |
| Wind load rating | Effective antenna area, in square feet or square meters |
| Other ratings | Turning torque, braking torque, vertical load |
| Mounting | Inside a tower with a thrust bearing, or on top of a mast |
| Variant | Azimuth-elevation (az-el) rotators for satellites and moonbounce |
A rotator, often called a rotor, is a motorized unit that turns a directional antenna so it can be pointed at the station being worked. A Yagi or other beam concentrates its signal in one direction and rejects signals from others, which is only useful if the antenna can be aimed. A control box in the shack shows which way the antenna points and turns it. For DXing and contesting with a beam, it is an essential part of the station.
Classes of rotator
The right rotator depends on the size, weight and wind area of the antenna it must turn.
| Class | Typical antennas | Well-known examples |
|---|---|---|
| TV-style, light duty | Small VHF and UHF beams; TV antennas | Consumer TV antenna rotators |
| Light-duty amateur | VHF/UHF arrays, small HF and 6-meter beams | Hy-Gain CD-45, Yaesu G-450 series |
| Medium-duty | Full-size HF tribanders | Hy-Gain Ham IV, Yaesu G-800 series |
| Heavy-duty | Large HF Yagis and stacked arrays | Hy-Gain Tailtwister and HDR-300, Yaesu G-1000 and G-2800 series |
| Azimuth-elevation | Satellite and moonbounce antennas | Yaesu G-5500 series |
TV rotators are inexpensive and adequate for a small VHF beam, but they turn slowly, often lack a brake and aren't built for the weight and wind loads of HF antennas. Amateur rotators add stronger gears and housings and, in the medium and heavy classes, a mechanical brake that locks the antenna in place. Without a brake, wind can turn a large beam back and forth, wearing the gears. At the top end, some large stations use industrial drives or surplus aircraft propeller-pitch motors.
An az-el rotator turns the antenna in both direction (azimuth) and elevation, so it can follow satellites across the sky or track the moon for moonbounce.
Ratings and wind load
The main rating is the effective antenna area, in square feet or square meters, that the rotator can handle. Antenna makers publish a wind area for each antenna, and the total for everything on the mast should be comfortably below the rotator's rating. Makers calculate these figures in different ways, so leave a generous margin, especially in windy or icy locations.
Area isn't the whole story. A long-boom antenna has high inertia, and starting and stopping it strains the gears even when its wind area is modest. Braking torque is how hard the rotator resists the wind trying to turn the antenna. Vertical load is the weight it can carry, which matters most when the rotator supports the mast by itself. Ratings usually assume mounting inside a tower with a thrust bearing; a rotator carrying the load on top of a mast should be derated.
Mounting
On a tower, the rotator bolts to a plate a few feet below the top, and the mast runs from the rotator up through a thrust bearing at the top of the tower. The bearing carries the weight of the mast and antennas and the sideways force of the wind, leaving the rotator only the job of turning. Purpose-made antenna masts are much stronger than ordinary water pipe.
On a mast or roof tripod, the rotator mounts on top of the support and the antenna mounts directly above it. The rotator then carries everything, so keep the antenna close to it and the load light.
Leave a rotation loop in the coax at the rotator, long enough for full travel and secured so it can't snag on antenna elements, guy wires or the tower. Use the control cable the maker specifies, with heavier wire for the motor on long runs to limit voltage drop. Route the control cable through the station's surge protection at the entry point, as described under Grounding and lightning protection. Tower work is dangerous; use a proper safety harness and never climb alone.
Control boxes and computer control
A basic control box has a meter showing the bearing and buttons to turn the antenna clockwise or counterclockwise. With a braked rotator, the antenna should be allowed to stop completely before the brake engages or the direction reverses; many controllers add the delay automatically. Digital controllers let the operator enter a bearing and turn the antenna to it.
Most modern controllers, and add-on interfaces for older ones, connect to a computer through USB or a serial port. Many use a command set based on Yaesu's, which is widely supported, so logging, contest and satellite-tracking programs can turn the antenna toward a station with a click.
Bearings from maps and logging software are true bearings, measured from true north, which differs from magnetic north by several degrees or more across most of the US. Calibrate the rotator to true north, and put its end stop in the direction you need least.