Separate names with a comma.
Discussion in 'DIRECTV General Discussion' started by Sixto, Jun 17, 2010.
So you don't think it is a stretch to extend all the way to 3GHz?
No, not at all;
All the way down into the Ku band perhaps is.
The feed horn tube is just a short run waveguide, which is like a transmission line for microwave energy. And like all transmission lines it has no sharp cut off above or below the bandwidth it was designed for, but gradually tappers off on either end. So 1 additional Ghz shouldn't be a problem.
And again, for all we know the Ka band feedhorn dimensions may have even been designed that way from the beginning with the future RDBS band in mind, to have a lower frequency cut-off somewhere well below 17.3 GHz.
The question isn't so much about the feedhorn, but the antenna(e) that are mounted near the end of that horn to receive the signals.
Then take a look into 99/101/103 LNBF combo's RF probes ... or if dish is more friendly for you, DP500+'s combo LNBF with 118/119 FSS/DBS RF probes.
Well to receive the present 18.3-20.2 GHz Ka band, the probes' bandwidth to center frequency ratio is already around 10%. Add the RDBS band makes it about 16%.
Is only a 6% increase unfeasible?
Probably not. Standard waveguide bands (including horns) cover much wider percentage bandwidths.
Do you have any pictures of the innards of a SL3 LNB that show the arrangement is between the antennae? I'm thinking the separation in orbital slots has an impact.
As I said previously, I'm considerably more concerned with the antennae that the waveguide feeds and how they are arranged given the closer relationship between the wavelengths.
Ah...but doesn't the fact that the satellite signal IS circular polarity mean that the only requirement is that the probes for LHCP and RHCP be 180 degrees apart? As I understand it, and I'm sure one of the RF engineers will correct me if I'm wrong, this means that even if the Ka probes can't pick up RDBS frequencies (something that is not a given), an LNB could easily have 2 pairs of probes, say with Ka at 0 and 180 and RDBS at 90 and 270. Theoretically, as long as the waveguide can focus the beam, you can have as many pairs of probes as needed. The only limit is really the size of the probes and how many can fit within the focal point (and the cost).
Whew .... :blink:
Well ... just about every horn antenna design tutorial and analysis I can locate online seems to descend into the deep depths of mathematical hell, but the best I can surmise is that the pickup probes used in a "Choke Feedhorn" antenna, which is the actual type of feedhorn largely used by home satellite dishes, based on it's width to height ratio and proper distance from the short circuit wall at the back of waveguide, generally has a bandwidth of around 20% of the center frequency of operation.
So again, the current Ka band probes should be sufficient for the nearby RDBS band as well.
As HoTat2 points out, different frequencies are optimal at different distances with respect to the short circuit wall.
If having optimal antenna element lengths and spacing weren't important, our OTA antennas would surely all be simple dipoles.
I suppose if you have your mind set on a certain opinion any post could be twisted to be considered supportive of your side of an argument ... but your assertion that RDBS cannot be received on an OTARD compliant dish is not supported.
Leave antenna and LNB design to the experts. I'm sure they will come up with something that works. Whether or not you or I or anyone else here understands why it works is not the issue. The experts will figure it out.
I'm not one, play one on TV, or stayed in a Holiday Inn Express last night, "but" I've worked with waveguide and know a bit about signals.
With the current configuration, expanding Ka-hi usage above 20.2 GHz looks to be where the RDBS will be, using the 2150- 3000(?) MHz output of the LNB.
I "suspect" DirecTV has already designed for this.
Or maybe better described as extending below the Ka-lo band to 17.3 GHz for reception and above the Ka-hi band range for LNB conversion, placing it somewhere between 2150-3000 MHz?
So do I ...
To extend Ka-lo would require being below 15.9 GHz, which isn't likely due to the cutoff of the waveguide. Waveguide doesn't "roll off" like coax, so extending Ka-hi is much more reasonable.
Sorry, you've lost me here VOS;
The downlink frequency band for RDBS (in ITU region 2, the Americas) is 17.3-17.7 GHz, and the current Ka d/l bands extend from 18.3-20.2 GHz. So won't the feedhorn have to cover only a 2.9 GHz span from 17.3-20.2 GHz which I see as totally feasible?
Why does extending 1 additional GHz below the Ka-lo band require going under 15.9 GHz?
And how does extending the Ka-hi receive band currently at 19.7-20.2 GHz, higher going to help receive the RDBS band below at 17.3-17.7 GHz?
Currently 17.3 - 17.8 GHz is Ka-lo, with 19.7-20.2 GHz Ka-hi.
Ka-lo is a low side conversion from the 18.05 GHz LO, which means 18.3-18.8 GHz would be the high side and fall into the same 250-750 MHz IF output, so both can't be used.
18.8-19.7 GHz interferes with the Ku output, so it can't be on the same [current] coax.
This leaves two ranges to have the IF above 2.150 MHz: above 20.2 GHz or below 15.9 GHz. [18.050 ± 2.150 GHz]
To use 17.3-17.7 GHz for RDBS, either the currently Ka-lo goes away, or a whole new LNB with another oscillator will be needed.
OK, I see what you're saying now;
But I can't imagine DIRECTV downlinking on any band other than what their RDBS license is authorized for at 17.3-17.7 GHz, nor would it be permitted.
Otherwise it wouldn't really be "Reverse DBS."
Anyway, as I wrote earlier in this thread, what I'm guessing is their may be another L.O./mixer/LNA circuit in the current Ka/Ku LNBs which has been dormant up until now such that for example using the same 200 MHz guard band of the current stack after LNB conversion;
Ka bands - 18.05 GHz L.O. = 2150 MHz to 1650 MHz and 750 to 250 MHz
RDBS 17.7 to 17.3 GHz - a 14.95 GHz L.O. = 2750 MHz to 2350 MHz
Otherwise, if such a circuit does not exist in the current base, then yes a new LNB would be required which has it of course.
Thanks for the "Reverse DBS", as I really didn't know what RDBS was, but now you've got me
Googling comes back with two links dating back to 2006, which seems to be about Ku SATs and their 17 GHz uplink.
Isn't DirecTV already downlinking at 17.3-17.8 GHz for Ka-lo?
I'm not sure we have gathered the whole picture of what's going on.
DIRECTV is downlinking between 18.3-18.8 GHz for the Ka-lo (or the Ka "B") band. This is then converted by a receiver circuit with a 18.050 GHz L.O. injection in the LNBF to output at 250-750 MHz.
The RDBS band would be coming down from the satellite between 600-1000 MHz below the lower edge of the Ka-lo band at 17.3-17.7 GHz and would need a different receive circuit and L.O. in the LNBF to translate this band somewhere above 2150 MHz.
17.3-17.7 GHz is the same band (less 100 MHz) used for years as the standard Ku band uplink frequencies between 17.3-17.8 GHz for the Broadcast Satellite Service (BSS).
Thus the expression "Reverse DBS" (or RDBS).