Satgen290 Satellite Problem No3 Antenna Polarisation by GM4IHJ 15 Oct94 BID of this msg is SGEN290 PLease use this BID if you retransmit this msg A low earth orbit satellite approaching your station can have a fixed antenna pointing vertically down towards the earths centre, ie not your vertical. Or it can have an antenna at right angles to the line to the earths centre in its "horizontal" plane but rarely your station horizontal plane. Worse still as the signal comes down to your station through the ionosphere , it plane of polarisation is rotated, many times at say 10m , less often at 2m and hardly at all at 70cms. It should be no surprise therefore that most satellite designers avoid fixed polarisation antennas. The design solution is circular polarisation , whereby the signal plane of polarisation is rotated at the transmission frequency in Hz. Eg a 145.987 MHz signal is rotated 145987000 times per second. Which should provide a useful downlink provided the ground station antennas is also designed for the correct direction of circular polarisation. Herein lies the problem for many radio amateurs. At L 1269 MHz or S 2401 MHz the answer is relatively easy - use a helical antenna which is strong neat and compact at these frequencies , and easily made from say central heating tubing. At 435 MHz J band the helix is getting a bit large , and it needs to be strongly built to stand up to the wind and weather . Good helical antennas have been available from a limited number of suppliers, but not many radio amateurs have bought them. Meanwhile at 145 MHz the problems are far worse , the helix is very bulky and difficult to mount effectively. So most amateurs use another solurtion at 435 and 145 MHz to get circular polarisation. This entails using X and Y yagi beam antennas mounted staggered on the same shaft. In theory this sounds fine. All you need is say a yagi with vertical elements , with exactly one quarter wave in air ahead of its driven element on the centre support, the driven element of a horizontal yagi. Then if you look from the back of the antenna you connect your matched feeders coax centre of one cable to the upper dipole driven element of the vertical , and coax centre of the other feeder to the right hand dipole of the horizontal driven element, for RHC right hand clockwise circular polarisation. For LHC left hand circular simply collect the coax centre to the left hand instead of the right hand end of the horizontal driven element. BUT, BUT manufacturers rarely put their production XY yagis exactly a quarter wave apart at the satellite frequencies of interest. So the real problem starts when the coax feeder lengths have to be adjusted to allow for this. So if you can , buy an XY yagi which has about the correct phase spacing. If you cannot , remember to trim your coax feeder lengths to allow for the less than optimum spacing and, do not forget to allow for the velocity factor of the coax. ie most coax can duplicate a natural quarter wave in air by using a length of about 0.7 or 0.6 of the wavelength dimension in air. The exact figure depends on the coax velocity factor. If in doubt grid dip the coax test lengths until you get exact quarter waves and then duplicate that measurement in your coax connnector suitably modified to allow for driven elements not a quarter wave apart. Sorry this is so complex but it will perhaps serve to show that if you have not done this before , you should consult someone who has. 73 de GM4IHJ @ GB7SAN