Satgen521 First Catch Your Sat by GM4IHJ (BID SGEN521) 1999-03-20 Listening to an Iridium mobile phone satellite one finds that a typical signal on a near overhead pass dopplers from say 1625.892 MHz to 1625.832 MHz in just a few minutes. Most satellite watchers do not have the accurate narrow beam antennas and rotators for following these high speed az/el dashes . So some hand operated antenna tracking would seem necessary . But this becomes impossible when it has to be combined with receiver tuning, to follow the far from linear, high speed doppler shift. If you are a typical satellite chaser . You rarely know the Keplerian orbital elements or the identity of the satellite you are tracking. So auto tracking is out . Equally you certainly cannot afford an antenna farm covered in beams and dishes, for every frequency, for every satellite of interest. Omni directional antennas are fine , but lack gain, so you miss all the difficult ones. Is there another solution ? Yes there is. If you look at satellite orbits, depending on your station latitude, there are azimuths where orbits "bunch up" American LEO orbiters such as NOAAs go just west of the north pole on ascending orbits and come just east of the pole on descending orbits. By contrast many Russian launches such as Iridium amd Meteors, do the opposite. Eastward polar pass ascending, Westward polar pass descending . So polar pointing of a quite narrow beam will collect most of them if your station is within 45 degrees latitude of the pole. A similar situation applies in the southern hemisphere. For satellites in lower inclination orbits, ground station latitude is important. Eg Eastward going Mir and Globalstar pass across either north or south of your station ( depending on your latitude) on several consecutive orbits. So a useful type of antenna is the colinear, mounted , NOT IN ITS FAMILIAR VERTICAL POSITION,, but mounted horizontally aligned east/west, beaming north south at temperate and high latitude stations . Or mounted beaming east west at equatorial stations. For S band and higher frequencies , a change of antenna type is probably dictated, and it will usually be home brew. At IHJ stacked helices are working succesfully, and a pair of dipoles stacked in a corner reflector are undergoing trials. The helix provides wide frequency coverage , mounted with the twin helices horizontal its beam width is too sharp for broad azimuth reception but with the helices stacked vertically it works well, The stacked dipoles are much more frequency restricted, but are very easy to make and to match ( unlike the helices ). Indeed these antennas are often picking up several Iridium satellites at the same time . Which raises the question. How does one sort them out? See Satgen next week. Iridium channels heard so far are 24 in number :- At least Six channels each Megahertz 1622 to 1625 MHz inclusive Each megahertz set dopplers down through roughly (on 1622 for example) 1622.060 to 1622.000 : 1622.226 to 1622.166 : 1622.393 to 1622.333 1622.560 to 1622.500 : 1622.726 to 1622.666 : 1622.892 to 1622.832