-
Circular versus linear polarisation
Hey guys, quick question -- why was it the case most of the time that they like/prefer to use circular polarisation for a lot of your C-band signals but prefer linear polarisation for Ku-band?
I want to take this question a little further though, if they do break into the Ka-band more into the future, which one do you think would be the preferred polarisation method?
I think they were using circular on some of the DirecTV Ka-band stuff in Americas at some point IIRC, but could be mistaken. Anik F2 at 111°W uses left-circular for signals at 19/20 GHz --
Last edited by irritant; 08-04-19 at 10:13 PM.
-
I have often pondered this question and come to my own conclusions based on history and engineering texts
c band was looking for coverage and compromise on size on the sats , the best available was circular and lefthand /righthand polarisation gave more bandwidth whereas with k band at a smaller size ie three hundred over the freq in mhz gives wave length, means feed horns can be manufactured easily and switching of polarities is easily obtained and isolation from one to the other by way of a short between the elements , so when you look into a k band lnb you can see the short and the polarity of the elements , it also allows for the distance from the sat to earth to be accurately accrued so when the signal arrives at the element it is at max ie skew of the lnb is easily managed and is the same for both hoz and vertical . whereas circular de emphasizes this concern with c band ., making skew not so important . ka band will remain as it is but we may see more of it down under its noticeable that the receive ifs are the same ie 950 -2100 mhz so there is no harmonic relationship between both bands cheers don
The Following 2 Users Say Thank You to hinekadon For This Useful Post:
irritant (09-04-19),Mr 672A (09-04-19)
Just to update, looking at this it seems the band they are using is technically the K-band, as the strict definition of Ka-band seems to be 26.5--40 GHz. Found that a bit weird, that these operators should say they are operating in the Ka-band.
This can get quite interesting because if one could employ circular polarisation at these higher frequencies, it may facilitate a whole new different design of new antennae perhaps. There are rumours some companies are looking into Non-Geosynchronous Orbit (NGSO) constellations of satellite for future communications, at even higher frequencies than Ka-band even.
Water vapour/moisture (and rain fade) is obviously much more of a problem at these frequencies, one effect being depolarisation of the signal. Would be interesting to see the effect linear v. circular.
hi there are heaps of readings on the effects of water etc on millimeter wave frequencies . As far as bands go for the life of me I cant understand the need for bands at all its much easier to understand a range of freqs , ie 5-6ghz ,20-40ghz than the a, b, c, d ------x bands and then try to remember the freqs and the sub bands but that always causes an argument lol
The Following User Says Thank You to hinekadon For This Useful Post:
Here is some information passed onto me by a friend who is a RF enginer who has a lot of experience in this field. This may help to understand the reasoning behind the use of the various polaraties.
Yes they use circular as you can use phased array antennas. All the high speed Ku and Ka always use circular. Only Australia opted for Ku linear..
It’s possible to use linear, but the phasing is way more expensive and not economical or cost effective.
That’s why all Ku small flat plane phased antennas are circular.
On the transmit paths, there’s no reason in either isolation on linear vs circular. There’s a heck of lot more circular transmit feed over linear.
If I was commissioning a point to point link, I’d opt for linear simply feed costs for the two TX sides.
The C Band was all originally circular with the old Intelsat’s (2,3, and 4) as globally it was an already adopted system for all the satellite cable services throughout the Pacific, Atlantic and Indian Ocean regions. Every island still to this day has working C Band circular 9m earth stations for cable restoration. Was part of the Inmarsat network as well.
Last edited by cmoore; 13-04-19 at 10:40 AM.
The Following 2 Users Say Thank You to cmoore For This Useful Post:
irritant (13-04-19),Mr 672A (13-04-19)
I would suggest you brush up on you knowledge of transmission and the levels concerned . Firstly polarity is NOT frequency dependant , the doppler effect is not mentioned .
What do you consider a phased array ???? and what has that to do with verticle /horizon or circular polarisation , a phased array is putting the active elements in the correct field the more elements in the right place the more signal is radiated or recieved this can also be placed by using other means according to the wave-lengths involved which is used by radio telescopes , IE they use multiple parabolic antennas set apart by a number of wave lengths to get the minute signals in below the noise levels from distant stations , to try to explain in simple terms two signals received at the same time contain only the same transmitted signal so by removing everything that is not duplicated gives you the signal of origin by the skew ok and the antennas being the same .
Shit I hope you understand this Its a bit heavy for this type of forum .OK don
Last edited by hinekadon; 13-04-19 at 11:37 AM.
Reason: more
Sorry as explained I am only passing on the info I was given. I am not the source of it. It was just a basic explanation. If you think it is miss information I am happy to delete it.
No dont delete it its better to have the thoughts corrected than to hide them you may be helping someone else as well by leaving them there I have no intention of insulting you or causing you any harm cheers don
Posting Permissions
- You may not post new threads
- You may not post replies
- You may not post attachments
- You may not edit your posts
-
Forum Rules
Bookmarks