I' like to try for 24.5 West Intelsat 905 but it seems it is circularly polarized. One reference I read seemed to imply that if I rotate my existing linear lnb through 45 deg in the appropriate direction that it would then pick up circular. Is this right, or do I need a specific circular lnb?
C-Band circular polarization
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you need to insert a dialetic plate into the LNB to get circular polarity
Most of the C-Band LNB's I have allow this to happen...there's ittle slots on the sides to insert one
Most of the C-Band LNB's I have allow this to happen...there's ittle slots on the sides to insert one
Many C-Band LNBFs have a Dielectric Insert that is placed in the throat of the feedhorn to allow the reception of circular polarized signals. It is possible to receive circular polarized signals with a linear polarized LNBF, but with a loss of approx 3dB.
Edit: Looks like great minds post at the same time ..... LOL!!!
Edit: Looks like great minds post at the same time ..... LOL!!!
Excellent, guys, thanks. When I opened the lnb box I recall that a curious plate was there, but I did not have any idea what it was for. This plate is about a quarter inch thick, and about one inch wide by two inches long. I take it this is the dielectric plate? I'll get the lnb down and see where it fits.
Found and inserted the plate no problem, but could not get my channel at all. Rotated the lnb, rotated the whole polar mount to ease the lean on the dish at that azimuth, but no go. I am trying for BBC World, 3749 R 13.333. It seems that this bird is very sparsely populated with non-feed channels for my area, so not a lot to peak a dish on if they are not 24/7. Is anybody tuned into this satellite, recommendations on a time of day to renew my efforts? It could also be that I need to use something bigger than my six footer, later in the year I will be rolling out my 10 footers and will try again.
Thanks for suggestion, Ice, however the G13 is behind a leafy tree from where my 6 footer is currently located. I may just decide to up the pole and replant the whole dish about 100 feet west of where it currently is to make the far W satellites more convenient. I can replant the pole anytime I like, zero concrete option.
You really want the Corotor II+ wideband feedhorn. I use this, and it gets both circular and linear quite well, without having to go out to the dish to remove/install a dielectric plate every time you want to switch between the polarizations.
Ok there here is a pic of the guts of my CoRotor it had the plate fitted as standard. I thought they were all like that? There should be slots in the waveguide as well to locate it.
There is several ways to go on circ polarisation road. You can use a plate with a voltage switchable H and V feed but naturaly enough there is quite a loss when you go to receive normal H&V transponders with the plate in place. A plate in a normal H&V feed should improve signal strength of circ pol. signals by about 2dB.
With a plate in position, turning the feed horn around in the rings (support) will not make any difference. With the signals being circular polarised it is not dependant on skew. The main thing is the plate in 45 degs. to the antenna probes. Idealy halfway between the h&v probe This way you usually select H for RHCP and V for LHCP.
Alternativley, if your really keen on Cband circ. pol. siganls you can purchase or make a circ. feed horn. It's not that hard to make one you can do the old wave guide squeeze trick by getting a standard h&v switchable job and giving it a sqeeze in the vice about halfway down the wave guide 45degs to the probes. About 1/2 an inch will do it but it wont work quite as well as a proper of the shelf cast job. Where it properley steped inside the guide. Which will give about 3dB. (double the singal strength) better over receiving it with a H&V waveguide.
Edit: If you like I will post you a pic of my dedicated circular waveguide and a few other ideas for making a your own but will take me a day or two. Let me know.
Steveo.
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This is the wideband feed, which has probes a lot different than the normal Corotor. Looks just like mine.
if your feed doesnt have slots for the dielectric plate, insert plate at 45 degrees or in other words, looking through the feed, youll see the rectangular opening, place the dielectric diaganolly from corner to corner.
with circular it doesnt matter what angle you mount your feed horn.
if your using a dual feed, then your have RHCP on one and LHCP on the other. if your using a servo motor, then is a bit more tricky, youll need to adjust polarity for best quality signal, crosstalk could be a problem.
I use only dual feedhorns on my antennas down here so not a problem
with circular it doesnt matter what angle you mount your feed horn.
if your using a dual feed, then your have RHCP on one and LHCP on the other. if your using a servo motor, then is a bit more tricky, youll need to adjust polarity for best quality signal, crosstalk could be a problem.
I use only dual feedhorns on my antennas down here so not a problem
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