Hi Domenico, Henk & all,
Domenico, you are totally right, that is indeed what is happening. When looking boresight, it is true CP, but looking more to the side it progresses slowly to more and more elliptical to almost truely linear when looking 90 degrees from boresight. (I liked your comparison to a phonograph record!)
Henk, PA3GUO, your plots are more than just art, besides looking nice they are truely useful, and they support our observations that Delfi-C3 is slowly tumbling around one of the 2 short axis. Any tumble around the long axis would be hard to observe from polarization data, since the antenna of the s/c appears to be very nicely circular with good axial ratio figures. During the development, we have performed measurements on a 1:1 scale antenna model and these measurements showed the same. Delfi-C3's antenna is indeed, like Domenico said, comparable to two dipoles fed 90 degrees out of phase. Actually, they are 4 monopole antennas which are fed with 0, 90, 180, 270 degrees from a phasing network, which also connects the other transceiver to the same antenna system. (Side effect of this is that switching to the other transceiver reverses the circular polarity sense).
Oh, speaking about art, did you know that Delfi-C3 actually carries a work of art into space? See
http://www.delfic3.nl/index.php?option=com_content&task=view&id=78&a...
73
Wouter Jan Ubbels PE4WJ
On Mon, May 5, 2008 at 6:37 PM, i8cvs domenico.i8cvs@tin.it wrote:
----- Original Message ----- From: "Henk, PA3GUO" hamoen@iae.nl To: amsat-bb@amsat.org Cc: pa3guo@amsat.org Sent: Monday, May 05, 2008 6:42 AM Subject: [amsat-bb] Creating Artworks based on Delfi-C3... or ?
Dear all,
Previous days it was observed already, then in periods of maximum signal strengths on LHCP, there is a minimum on RCHP (and visa versa). Triggered by that I wanted to try something else: simulations reception of the two antennas of my 'X' antenna (2x6 elements DK7ZB), and plot the signal strengths over time.
Most likely this can be considered as a totally useless experiment, but I wanted to do this for once (could have taken another satellite as
well).
Both antennas are only 6 cm distance from eachother, which is
compensated
by a bit of extra coax cable. The two cables enter the shack.
One cable has been connected to an FT817. Other cable was connected to an TS2000. Using this set-up I did a simulatious reception of Delfi C3, and plotted the received signals above eachother: Result: Maximum on one antenna is minimum on the other, and visa versa.
Have a look yourself at the graph:
http://home.vianetworks.nl/users/hamoen/pa3guo/dc3_04may08_2000utc_pa3guo_po larization_145.930.jpg
Conclusion of PA3GUO: fun to do, but I have no clue what this tells us
:-)
Conclusion of PE1ITR: it's beautiful, but one does not know what one can
do with it... it's ART ! :-)
... still:
- if Delfi C3 has circular polarization, shouldn't both signals have
been the same ?
- how does this relate to the LCHP/RCHP opposite maximums (see text
above) ?
Looking forward for your thoughts !
73, Henk, PA3GUO http://www.qsl.net/pa3guo
Hi Henk, PA3GUO
Delfi-C3 is transmitting circularly polarized no matter if RHCP or LHCP because it depend on which side the satellite look at your antennas and more on this later. As you stated you are receiving with an "X" antenna made of two linearly polarized antennas one is oriented at 45° "/" and the other one at 135° ""
The point here is that the transmitting antenna of Delfi-C3 is not looking straight boreside at any time to your receiving antennas because the satellite is slowly tumbling.
If we assume that at a certain time the transmitting and receiving antennas have been randomly looking straight at each other (coaxial) then a circularly polarized wave will appear circularly polarized with the same amplitude into your "X" antennas.
If the antennas are not looking directly at each other because there is a continuously variable squint angle with the satellite tumbling then a circularly polarized wave will appear elliptical to your linearly receiving "X" antennas.
This effect is easily visualized if you look at a phonograph record ; looking directly onto the playing surface , it appears round .As the record (the satellite CP antenna) is rotated toward an edgewiseview ,it appears more and more elliptical until, when you are looking directly at the edge of the record, it appears as a straight line.
This effect can be applied here and when your two crossed linearly polarized "X" antennas are looking directly into a circularly polarized antenna, the X components are equal:As one moves around to the edge of the antennas (satellite tumbling) one of the X linear components increases while the other X component decreases or approaches to zero.
If the linear component that appears as a straight line is oriented exactly at 45° "/" with respect to your "X" antennas then the signal received by the 45° "/" antenna is maximum while the signal received by the 135° "" antenna is minimum or zero.
Your question 1 : if Delfi C3 has circular polarization, shouldn't both signals have been the same ?
My answer 1 : They are the same strenght only when Delfi C3 and your "X" antennas are looking straight at each other.
Your question 2 : how does this relate to the LCHP/RCHP opposite maximums (see text above ) ?
My answer 2 : If you are receiving a LHCP or RHCP wave with an "X" linearly polarized antenna the signals is the same but if you receive with a CP antenna the effect changes. The antenna of Delfi C3 is made with 2 crossed elements located in the same plane. To generate circular polarization one element must be feed with a 90° phase difference. Been the crossed elements in the same plane without any reflector the antenna radiates RHCP in one axial direction and LHCP in the other direction. If you receive with a circularly polarized antenna it happens that depending on wich face the satellite is looking to your "X" antennas you can receive a RHCP or a LHCP wave and this is why a polarization switcher is necessary.
I have extracted this basic concept from " The Advantages of Circular Polarization for Amateur Satellite Ground Stations" by John J. Nagle, K4KJ publisched into the "1975 ARRL Technical Symposium" page 80-87 of text with 25 pages for drawings and pictures.
73" de
i8CVS Domenico
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