I've been half following this thread and can't make out the reason for a less than hemispheric antenna pattern. GPS needs several birds to lock up, and if you look at a single bird, Dopplar will make the signal useless as a frequency reference.
Best, -John ============= > Hi Jim: > > I've got a spare Ku band satellite dish and would like to use it for GPS. > In an ideal application the GPS antenna would be mounted in the normal > manner and above it would be a sub-reflector aimed at the Ku dish. > That way the antenna might pickup sats near the horizon directly and > from a narrow part of the sky by means of the dish. > The dish might be aimed at a WAAS GPS sat. > I've heard that you can just use the TV dish with a normal GPS antenna, > and it gas gain even though the polarization is reversed. > > Have Fun, > > Brooke Clarke > http://www.PRC68.com > > > jimlux wrote: >> Bill Janssen wrote: >>> Magnus Danielson wrote: >>>> On 10/08/2010 03:35 AM, jmfranke wrote: >>>>> When I said the feed would work, I was meaning it would work if LHC. >>>>> The illustrations and text imply you could just place a normal GPS >>>>> receiver at the feed location, but the polarization would be wrong. >>>> >>>> Which was what I reacted on... >>>> >>>> I am by no means a practical antenna expert, and the EM-theory is a >>>> bit fuzzy on the edges, but I do distinctly recall that signal is >>>> RHC and reflections becomes LHC so an antenna with RHC orientation >>>> will provide some first-degree damping of the LHC reflections. For >>>> this antenna setup the intended RHC signal is reflected and should >>>> become LHC... just as the interference... so it relies on the >>>> antenna gain of the dish to out-perform the other reflections for >>>> the half-space receiver that a normal GPS antenna is. The choke ring >>>> for a dish head has a distinct different pattern (forming an inner >>>> cone rather than flat space). >>>> >>>> So, a normal antenna would kind of work since the antenna gain would >>>> overcome the poor LHC supression of a simple RHC antenna... yay. >>>> >>>> If an LHC antenna was used instead... now we are talking. >>>> >>>> Cheers, >>>> Magnus >>> So a dish reflector and a sub reflector and the GPS receiver at the >>> dish would work? What is that >>> configuration called? I can't remember at this early hour. >>> >> >> Depends on the relative curvatures and focal points: >> >> Cassegrain if the subreflector convex. >> Gregorian if the subreflector is concave parabolic. >> Dragonian if the subreflector is concave hyperbolic >> >> IEEE Ant and Prop Magazine a few years back had a series of articles >> on designing them all. >> >> All of them can be done offset or coaxial >> >> Any would conceivably work.. It's all about what your pattern looks >> like, what sort of efficiency you need, any mechanical constraints, etc. >> >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > -- > Have Fun, > > Brooke Clarke > http://www.PRC68.com > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.