cep4478@CS.ROCHESTER.EDU> (01/11/90)
>> 2.1) How are commercial (broadcast) TV signals polarized and >> how does this affect the choices? > >Most are horizontally polarized. If they are using circular polarization, >which can reduce ghosts, they make a big deal about it (It costs them more), >and special antennas are usually available locally. I have two questions: - to transmit omni-directional horizontally-polarized, are they using some kind of 'halo' design? - What kind of antenna would radiate a circularly-polarized yet omnidirectional pattern? I've never seen any circular designs other than twists and dishes. Chris N2JGW cep4478@ultb.isc.rit.edu
Steve Lamont <spl@MCNC.ORG> (01/11/90)
In article <1899@ultb.isc.rit.edu> cep4478@ultb.isc.rit.edu (C.E. Piggott) writes: > - to transmit omni-directional horizontally-polarized, > are they using some kind of 'halo' design? Most TV transmitting antennae are of the "turnstile" variety -- or at least they were a few years ago when I concerned myself with those sorts of thing. There are usually four elements, spaced around the feedline. They look more ore less like this from the side: ------|------ \ | / \----|----/ \ | / +--|--+ / | \ /----|----\ / | \ ------|------ I wouldn't say that they are completely omni-directional -- they have pretty lumpy patterns -- they look like a clover leaf, if I remember correctly. The horizontal and vertical polarization come from the current on the horizontal and more or less vertical conductors. > - What kind of antenna would radiate a circularly-polarized > yet omnidirectional pattern? I've never seen > any circular designs other than twists and dishes. There are some antenna systems which PURPORT to emit an omnidirectional pattern (the FCC, in fact, MANDATES that all FM and, if I recall, all TV to be omnidirectional), although numerical analysis as well as actual measurement indicate that no such beast exists. There are some strange animals which look like more or less like a scissors jack that are fed in all sorts of bizarre ways -- but again, they aren't anywhere close to omnidirectional. Often, an antenna has to be tuned by placing reflectors and other such garbage on the supporting structure. There are FCC rules and regulations which control exactly how much tweaking that can be done. spl (the p stands for personally, the only TV I know is a transvestite...) -- Steve Lamont, sciViGuy (919) 248-1120 EMail: spl@ncsc.org NCSC, Box 12732, Research Triangle Park, NC 27709 "That's People's Commissioner Tirebiter -- and NOBODY'S sweetheart!" - F. Scott Firesign