Rem%IMSSS@sri-unix.UUCP (09/07/84)
It would seem to me that horizontal polarization is ambiguous, is it N-S or E-W, while vertical polarization is bad design, because the Earth is nearly exactly under the satellite (point on equator is exactly under satellite, while anywhere else on Earth such as North American ground station is at most 4000 miles deviation from exactly-under at a distance of 24,000 miles) and electromagnetic signals propagate only at right angles to their electric field change (and at right angles to their magnetic field change; the three vectors form an orthogonal set in 3-space), thus using cosine rule conversion from antenna beam field and transmission direction you'd get very little signal at Earth. I suspect by "horizontal" they mean "horizontal at ground station that is due North of beam point" which would mean east-west, while "vertical" would mean north-south. This still leaves the question of whether they are talking about the electric or the magnetic field vector. Can I assume they are talking about the electric field vector?? Or are they really using some strange kind of circular polarization that appears horizontal from all points on Earth (except directly under satellite where it can't be received at all)? -------
karn@mouton.UUCP (09/12/84)
The direction of polarization of a radio wave always refers to the electric field. (It's easy to remember, a vertical whip antenna produces vertical polarization.) The "vertical" and "horizontal" polarizations of a geostationary communications satellite are referenced to the spacecraft as you would see it if you were lying down directly under it (on the equator) with your head pointed north. For the spin-stabilized birds (HS-376, SBS, Westar, etc) this is also the position in which you usually see the spacecraft in ground tests, and when it is mounted on the launcher. Of course, you have to do a coordinate rotation when adjusting your TVRO feed horn, but it is easy when you visualize what is going on. Phil