----- Original Message ----- From: "Phil" [email protected] To: [email protected] Sent: Thursday, April 15, 2010 1:19 PM Subject: [amsat-bb] Circular polarisation question
Thank you for reading this.
Ten years ago I built a pair of circular polarised aerials primarily for satellite use. The UHF aerial has 12 elements in each plane while the VHF aerial has 6 elements in each plane. My UHF received signal strength often seems to be below that of other stations and so I'm rethinking how I have set up these aerials. The received signal strength via VO-52 is S7 at the best, without a pre-amplifier, and so I'm happy with that. The UHF aerial is where the problem is even thought they are set up in the same way.
Both aerials are mounted in an X orientation. One set of elements is a quarter wave ahead of the other set. Because of the X mounting I cannot tell if the vertical set is ahead of the horizontal set or not. Each aerial is fed via a power splitter via equal length cables.
My question is; does this sound like a correctly set up right hand polarised aerial? Originally I thought it was now I'm not sure.
-- Regards, Phil
Hi Phil
Since one set of elements is a quarter wave ahead of the other and each aerial is fed via a power splitter via equal lenght of coax cable then your antenna is circularly polarized.
Depending to which side of each dipole you have connected the inner conductor and the braid of the coax cable the polarization can be RHCP or LHCP
If you remember how the inner conductors and the braid of coax cables are connected the the rear dipole and to the front dipole it is possible to determine if the polarization is RHCP or LHCP
By the way for general information if you add a 1/2 electrical waveleght of coax cable to the fed line suppliyng the front dipole than the sense of polarization reverses from RHCP to LHCP or vice versa.
In a separate email I have sent to you a drawing showing how both linear polarized fields components of your antennas adds to generate a resultant circularly polarized wave.
Since the drawing is made using two crossed driving elements one vertical and the other horizontal you only must ideally rotate the drawing right hend or left hend by 45 degrees and nothing changes in your reasoning.
By the way your antenna is circularly polarized for sure but without to make the above analisys it is impossible to say if RHCP or LHCP
Have fun
73" de
i8CVS Domenico