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Raysat Antenna

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Sep 23rd, 2008 at 2:29am  
Is Raysat COTM phased array antenna wide or pencil beam ?

regards,
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Reply #1 - Sep 23rd, 2008 at 4:34am  
one way or two way?

//greg//
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Reply #2 - Sep 23rd, 2008 at 10:54am  
Read more here:
http://www.raasys.com/webdata/SupportDocuments/61/StealthRay%202000%20Specs.pdf
http://www.raasys.com/webdata/SupportDocuments/59/StealthRay%20White%20ver2.doc

If the transmit gain is +27 dBi then this is equivalent to an 18cm diameter antenna, which will have a beamwidth of about 7.5 deg at the -3 dB points, so you will cause interference to adjacent satellites.  Consider the receive gain also.  If the receive beam listens to the adjacent satellites in a similar fashion your incoming interference from the adjacent satellites will make reception difficult.

If the sidelobe gain is -12 dB this means nothing to me.  The sidelobe gain pattern must be quoted in dBi and the off-axis angles quoted.  e.g. Sidelobe gain = 29 - 25 log(theta) dBi, where theta is the off-axis angle. This kind of formula generall applied beyond the main beam and out to -10dBi which applies all around the back.  For a special antenna like this one you need to get the measured sidelobe patterns over the full hemisphere.

Operation of these antennas is possible if you use a satellite with a large orbit gap on either side or spread spectrum techniques.  You will still need extra high power and high price from the satellite since the effective receive earth station size is small.  On the transmit side you need to use extra bandwidth (spread spectrum) and higher price so as to get the interference spectral density to others down.

Note that decreasing receive dish size from 96cm to 28cm will increase the satellite transmit power and price by about 10 times.

Note that spread spectrum will increase the satellite bandwidth used and price by 16, 32, 64, 128 times etc.
wxw
Talk to RaySat and get their advice as to how to comply with interference limitations. They will know on what satellites they have so far managed to get permission to operate. Good luck.

Best regards, Eric.
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« Last Edit: Sep 23rd, 2008 at 4:17pm by Admin1 »  
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Reply #3 - Sep 25th, 2008 at 2:36am  
Wow, talk with iDirect.  They have one on their COTM truck right now.
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Reply #4 - Oct 9th, 2008 at 11:21am  
Hi Eric,

>>If the transmit gain is +27 dBi then this is equivalent >>to an 18cm diameter antenna, which will have a >>beamwidth of about 7.5 deg at the -3 dB poi
How do you calculate this?

>>For a special antenna like this one you need to get >>the measured sidelobe patterns over the full >>hemisphere
Can you explain more on this? Cos' i want to measure the side lode Level.

Best Regards,
wow
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Eric Johnston
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Reply #5 - Oct 9th, 2008 at 1:28pm  
See antenna gain and beamwidth.  Just input various numbers till you get the answer you want.

Regarding measuring the antenna sidelobe pattern you need an antenna test range with a  turntable to support the antenna and a distant mast with 14 GHz receiver, plus instrumentation.  All this should be organised by the antenna manufacturer to get their antenna approved for use by Intelsat, Newskies, Asiasat etc.

If your equivalent antenna size is too small it will send unacceptable interference into nearby satellites.  I would consult the antenna manufacturer and ask which satellite operators have approved the antenna.  Then check with the satellite operators that this is true and discover any detailed limitations which may apply - like just one particular satellite and one spot frequency, spread spectrum, ultra low power density for example.

The preferred mobile satellite systems are Inmarsat and operate in L band.  They have very few satellites and they are spaced far apart around the orbit so small earth stations don't cause interference to nearby adjacent satellites, since there are none nearby.

Best regards, Eric.
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Reply #6 - Oct 16th, 2008 at 7:53am  
Hi Eric,

Raysat is a phased Array Antenna. So using this calculator for parabloic is valid ?

http://www.satsig.net/pointing/antenna-beamwidth-calculator.htm

Best Regards,
w0w
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Eric Johnston
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Reply #7 - Oct 16th, 2008 at 9:13am  
A phased array antenna comprises a number of physical parts, each radiating.  The phase fonts of all the radiations all combine up in flat sheet, normal to the particular direction where the beam goes.  The size, shape and amplitude distribution across this flat sheet gives the total gain and sidelobe pattern, exactly the same as the flat circular disk of phase front in front of a parabolic reflector.

My approximation is correct.  But because of the way this Raysat phased array works you need actual patterns.  The main beam gain will not the same in all pointing directions and the sidelobes will certainly vary a great deal in their detail.  It is important that the main beam is narrow enough not to radiate into the adjacent satellites and that the further out sidelobes are not a problem either.  

Measured antenna patterns are better then calculated estimates but calculated estimates are easier to do and avoid you building antennas that are certain to fail acceptance testing.

Best regards, Eric.
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Reply #8 - Dec 9th, 2008 at 11:37pm  
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« Last Edit: Nov 25th, 2011 at 10:13pm by N/A »  
 
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