Domenico, You are correct, the crossed dipoles fed in quadrature when on axis exhibit LH and RH circular patterns, but 90 degrees from axis they are linearly polarized. This gives poor performance at low angles as well as requiring both RH and LH rotations for a full pass reception.
Obviously the QFH antenna to be effective should point down at the earth, with the sides pointing to the horizon and the backside towards outer space. The one wave one turn can be optimized,(Length to Diameter ratio)to provide best radiation at the horizon. This will give good performance when the satellite is near zenith as well as provide improved performance at low elevations. Circular polarization does help to eliminate multipath and provide a steady copy, even while the antenna is mechanically rotating with the satellite for stabilization and temperature stability. Art, KC6UQH -----Original Message----- From: i8cvs [mailto:domenico.i8...@tin.it] Sent: Sunday, August 05, 2012 3:27 PM To: AMSAT-BB; Bob Bruninga ; kc6...@cox.net; Thomas Doyle Subject: Re: [amsat-bb] Re: Antenna Polarization Technical Question Hi Art, KC6UQH It is correct that a QFH is circularly polarized over the whole envelope of the antenna.If it is left wound the polarization is RHCP and if it is right wound the resulting polarization is LHCP. By the way the point is the satellite antenna. If the satellite antenna is made using two crossed dipoles mounted in the same mechanical plane and are supplied with 90° out of phase than the radiated polarization is RHCP along one axial direction and LHCP along the other axial direction. Since the satellite is thumbling orbiting in the space than the polarization coming from the satellite to earth or coming from the ground station to the satellite is continuing changing from RHCP to LHCP to linear passing through elliptical. The bad point is that a QFH can only radiate RHCP or LHCP depending on it's winding direction so that using only one QFH the QSB generated by the satellite thumbling cannot be completely eliminated and two switchable QFH's one RHCP and the other one LHCP would be necessary. 73" de i8CVS Domenico ----- Original Message ----- From: "Art McBride" <kc6...@cox.net> To: "'Bob Bruninga '" <bruni...@usna.edu>; "'Thomas Doyle'" <tomdoyle1...@gmail.com> Cc: <amsat-bb@amsat.org>; "'andrew abken'" <kn...@hotmail.com> Sent: Sunday, August 05, 2012 10:44 PM Subject: [amsat-bb] Re: Antenna Polarization Technical Question > Bob, > Just a reminder, a QFH antenna is circularly polarized over the whole > envelope of the antenna. A sharp null exists on the back side. A one > wavelength, one turn has gain at low angle side radiation and a 4 dB loss > overhead, where the distance to the ground station is the smallest. > Certainly this is a good fit for satellites. > > Turnstile antennas and patch antennas are linear polarized at the sides and > of course are the easiest to implement on a satellite. > > My point is all circular antennas are not equal and having an antenna with > gain on the sides opposed to the center of the antenna is very desirable. > > Art, > KC6UQH > > -----Original Message----- > From: amsat-bb-boun...@amsat.org [mailto:amsat-bb-boun...@amsat.org] On > Behalf Of Bob Bruninga > Sent: Saturday, August 04, 2012 3:25 PM > To: Thomas Doyle > Cc: amsat-bb@amsat.org; andrew abken > Subject: [amsat-bb] Re: Antenna Polarization Technical Question > > > Not sure why anyone would want to maintain the > > orientation of the satellite in such a way that > > would cause the direction of circular > > polarization to change during the path. > > Lets try this approach... As I said before, By the laws of physics, what > comes out one side of a circular polarized low gain antenna as RHCP comes > out the opposite side as LHCP. > > Now given that, and the fact that someone in Maryland is in the center of > the RHCP beam, then by the laws of physics, the guy in California must see > mostly LHCP. No matter how much one of those persons demands that he > deserves the RHCP beam, by definition, someone else somewhere will get the > LHCP one, and the geometry changes at least every 10 minutes or so and every > time the spacecraft rotates a bit. > > So one might say, "point it down" then only the person in Kansas will see > the main beam and those in CA or MD will be completely off the sides almost > 70 degrees from the main beam. Mot people do not realize how LOW these > satellites are. The only solution is to put satellites so high, that "down" > is about the same to everyone (geostationary altitude). But then that takes > 100 times more altitude, and that takes 10,000 times more power. > > Better to just live with the laws of physics... I guess. > > Bob, WB4aPR > > > > >On Sat, Aug 4, 2012 at 2:47 PM, Bob Bruninga <bruni...@usna.edu> wrote: > >>> I believe that is true but that does not explain > >>> why the optimum polarity setting on the receive > >>> end would change during a pass. > >> > >> That's easy. The circularity on a pair of crossed dipoles (about all > you can get on a spacecraft) May be designed for Right hand circularity when > viewed from the prime direction. But by definition, that save waveform will > be LHC when viewed from the opposite direction. > >> > >> And since the geometry to any one observer is constantly changing by > almost 180 degrees during an overhead pass, that is why it is very easy to > see, complete change in circularity. > >> > >> Bob, WB4APR > >> > > > > > > > >-- > > > >Sent from my computer. > > > >tom ... > _______________________________________________ > Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. > Not an AMSAT-NA member? Join now to support the amateur satellite program! > Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb > > _______________________________________________ > Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. > Not an AMSAT-NA member? Join now to support the amateur satellite program! > Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb _______________________________________________ Sent via AMSAT-BB@amsat.org. 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