small logo SETI League Technical Manual -- Antennas

What About Antenna Rotors?

A SETI League member writes to ask:

I am wondering if there is any detailed description of how to mount a parabolic dish properly. Is it sufficient with one motor- / rotor-mechanism or are two motors required? Is computer guidance of the dish necessary, and if so, what is / are the proper program(s)?

Actually, no motors at all are required! The beauty of operating a radio telescope in drift-scan mode is that it is in meridian transit mount, and requires no computer control, or even any active steering. You merely mount the dish in the fixed position of azimuth (on a north-south line) and assigned elevation angle, and let the earth be your antenna rotor. So no right ascension adjustment is required at all. Although an elevation rotor would allow you to readily change declination settings, it is by no means a necessity. If you tell us (via your Project Argus Participant Survey form) what are your aiming constraints, we will assign you an elevation angle which is practical for your location and mechanical limitations.

Now, what if you in fact want to cover the whole sky which can be seen from your location, not just an assigned strip of sky? You can do so with just one rotor, this to control elevation, while the earth's rotation gives you azimuth control. The physical rotor mechanism used is not likely to be an "elevation" rotor at all, but rather an "azimuth" one. Here's how that works:

Consider how the typical satellite TV antenna is mounted and rotated, using an equatorial (also called polar) mount. A fixed elevation angle is set, typically by adjusting a turnbuckle. This elevation angle represents the elevation to a Clarke orbit (geosynchronous) satellite at zenith, or on a north-south line from your location. The actual angle above the horizon is found as 90 degrees minus your station latitude (that is, 90 degrees on the equator, or 0 degrees if you're at one of the poles), with a minor correction for the fact that the satellites are relatively close to the earth, not out in deep space. Once this correct elevation (or declination) setting is established, it is locked down and not moved.

The single rotor associated with tracking of Clarke satellites is one which moves the antenna in right ascension, or hour angle. This is essentially an azimuth rotor, with its axis tilted so as to more or less parallel that of the earth. This rotor might be implemented by an electrically driven jack screw which pivots the dish on its mount, or a more complex and stable "horizon-to-horizon" arrangement might use a chain drive, gears and pulleys. It is this latter mount which we have found most useful for SETI in particular, or radio telescopes in general. But it is not mounted or used in the usual way.

The scheme we describe here has been used by several SETI League members, as well as on Project Argus Station #1 at SETI League headquarters. It is depicted in the accompanying photos. We used a commercial Paraclipse 3.7 meter dish on a horizon-to-horizon mount. The "elevation" turnbuckle (which is seen just to the right of The Big Switch in Figure 1) was first set to its shortest length, which pointed the dish straight up (the correct setting for geosynchronous satellite reception by a station on the Equator). Next, the mount was rotated on the mast, ninety degrees from its normal position, so the chain drive wheel is oriented north-south, not east-west. This turns what is normally the horizon-to-horizon declination rotor into a very robust elevation rotor, as seen in Figure 2. Finally, the stops on the motor control mechanism must be adjusted so the dish swings in a 180 degree arc from the northern to the southern horizon. In the case of the Paraclipse rotor we used, pointing is accomplished by applying a 24 volt DC power supply to the chain-drive motor. Reversing polarity reverses direction. An inclinometer or protractor with plumb-bob can be mounted on the dish for accurate elevation angle readout. The declination to which the antenna is aimed is a function of latitude and elevation angle, as follows:

Dec = Lat + Elev - 90 where
Dec = declination in degrees (north is positive, south is negative)
Lat = latitude in degrees (north is positive, south is negative)
Elev = elevation in degrees above the southern horizon (range is 0 to +180, +90 is straight up).

In meridian transit mount, Right Ascension is simply local sidereal time. This involves first computing Julian date. From this you can get Greenwich Sidereal Time (GST), and if you know your longitude, you can get Local Sidereal Time (LST). For the applicable formulas, link to Daniel Fox's Useful Formulas for Amateur SETI. Among the many computer programs which will give you real-time readout of LST is Atomic Clock from The Learning Company.

Click to email the Webmaster
| Home | General | Memb Svcs | Publications | Press | Technical | Internet | Index |
entire website copyright © The SETI League, Inc.
this page last updated 4 January 2003
Click for top of page
Top of Page