Difference between Ku and Ka reflectors

  • 2
  • Question
  • Updated 3 years ago
Archived and Closed

This conversation is no longer open for comments or replies and is no longer visible to community members.

Could someone from Exede, or some oddly intelligent genius, tell me the difference between Ku and Ka reflectors, parabolic reflectors, etc? I've been thinking about this, looking about at all of the old abandoned dish's on my family farm. From my experience in the telecom industry, and dealing with a lot of RF-based systems, I know that there are 2 important principles. 1) relative "permeability" of the reflective surface, and 2) the relationship between the LNB location and the reflector's center of focus. Apart from that, ruling out dish diameter due to the required gain, Ku and Ka dish assemblies should be relatively interchangeable, assuming the geometry is correct. The ONLY think I can think of is surface treatment, like maybe a Ka reflector has a special coating, designed to reflect 20-30 GHz RF. Otherwise, I don't see how there would be anything different. I'm not questioning the necessity to have a correct tria/LNB, correctly positioned...I'm simply asking why a 1.2M dish, made of some polymer, can't reflect Ka, when it does a great job of reflecting Ku. Anyone? I need some sleep, and this is keeping me up. Whether or not I need a different hobby is debatable.
Photo of Jason O'Brien

Jason O'Brien

  • 3 Posts
  • 1 Reply Like

Posted 3 years ago

  • 2
Photo of JEP

JEP

  • 986 Posts
  • 718 Reply Likes
While I don't meet either of your requirements, I do believe there is no difference between a Ku and Ka reflector per se.  Obviously, based on the original intended use by the vendor, the dish has been sized accordingly for the required gain, beam width and side lobe attenuation. I think as long as the reflector is a non-mesh, solid metal surface (typically an aluminum alloy or coated polymer) you are good to go.  The Ka band wavelengths are less than Ku so it would suffer deterioration in performance more rapidly due to dish dents or warping.  ( I used to install 36" round FTA dishes and would lay them down on a concrete floor and flex them until the rim touched evenly all the way around before installation. )  I used to use a 36" dish on my DirecTV to reduce rain fade and it worked very well.
Photo of Jason O'Brien

Jason O'Brien

  • 3 Posts
  • 1 Reply Like
Well, now, I'm not so sure you aren't at least *most * of one of those. That's not what I'd call common knowledge, by any means. I'll admit that your experience is what I'd decided, until I started pulling the specs on aftermarket "dish's", and they always have a band/range they're designed for, i.e. Ku or Ka. I get the whole C band deal...less power, needs more gain, needs a larger reflector, etc. I was actually wondering if I could swap my tx/rx Ka dish for a 1 meter unit, just to provide more headroom in rain and so forth, and all of the old dishes lying around are what lit the spark. In the interest of full disclosure, I've not done a single speck of math to determine my current headroom or gain, etc...just wondering. It could be that my current link has copious headroom, but I know Ka is much more susceptible to fade, if anything is, and I rely on consistent Internet access for my profession.
Photo of JEP

JEP

  • 986 Posts
  • 718 Reply Likes
Dunno...The Exede dish isn't round, but is roughly 26.5" x 28.5", so your 1 meter dish would give you ~ 3.1 dB more gain.  I don't know how that translates into resistance to rain fade.  I got about 6 dB with my DirecTV experiment and that was very noticeable.  It would also be a tad trickier to point the 1-meter dish, since the beam would be more narrow and you might also need a sturdier mount.  Are you Exede certified to align the dish?  I'm sure they frown on do-it-yourself alignments. ;-)
Photo of david

david, Champion

  • 522 Posts
  • 365 Reply Likes
I have a suspicion that unless you're on the edges of the sat, a bigger dish won't make any real difference if your dish is pointed correctly. Ours doesn't go out unless there is some serious rain between us and the sat., just your average rain doesn't affect it. We're also in the southern part of the middle of the country, though. A bigger dish probably wouldn't give more than a couple minutes extra internet on an outage. I would imagine the dish we have now is on the edge of diminishing returns size wise, at least for here.

As far as your original question, with a solid metal dish or one with a solid metal coating, I can't see where it would make a difference as long as the dish was big enough for enough gain. On the old mesh dishes, I think it did make a difference in the size of the mesh depending on the frequency.
(Edited)
Photo of Dave

Dave

  • 116 Posts
  • 20 Reply Likes
Here is the frequency chart that may help you somewhat...
Dave


(Edited)
Photo of Jason O'Brien

Jason O'Brien

  • 3 Posts
  • 1 Reply Like
So, first, thanks for taking the time to comment, thoughtfully. If you're like me, then time is is your greatest commodity, and I genuinely appreciate the fact that anyone would intentionally stop what they were doing, give this some serious consideration, and go through the motions of forming a rational reply. Truly, truly appreciated. Second, the rain fade is certainly worse in Ka bands, but I don't live in the tropics, so I'm not sure how much of an issue it is to begin with, but I've heard many stories about peoples bad experiences. Not necessarily with Exede, just Ka systems in general...especially on the uplink. I get it, too. The waves are so small that they absorb and "back-scatter." Some frequencies exacerbate this phenomenon, for the good and bad. An OTDR, for example, uses the specific frequencies that excite the atoms inside a fiber optic strand. The transmission wavelengths of 850,1300,1310 & 1350 nm are those that are most easily passed. In order to do it's job, and OTDR uses frequencies outside of this range, to intentionally "light up" the molecular components, and give it something to bounce light off of. I suppose what I'm saying is that atmospheric "dirt" works in much the same way, in that it provides the beam with the very thing that's counter to it's efficiency. Figures. Either way, I am most impressed with Exede and Viasat. The throughput they offer to the average consumer is a technological freaking miracle...rain fade or not. I clean up a lot of messes caused by last-mile DSL and half-a$$ broadband installs, and have been recommending Exede as an alternative, simply based on my own experiences. To my original point...how about an array? Scoop up a truckload of of old dishes, grab a sack of 2GHz combiners/splitters/diplexers, and a piece roll of RG6? Anyone? I'm sure there's an app for that ;-)
Photo of Exede Beau

Exede Beau

  • 380 Posts
  • 74 Reply Likes
Jason, judging from the thread it doesn't sound like you're experiencing any issues, but I just wanted to check in. If you are please shoot me an email at exedelistens@viasat.com with your account and contact information.

PS I think your choice of hobby is fantastic!

This conversation is no longer open for comments or replies.