KU Polarizer/Adapter for C Band feedhorn.

[zodiak]

What the heck is this? Originally found here. Sorry for the ALL CAPS-- that was the original description.

NEW, CREATED FOR INSTALLATION ON CHAPARREL STYLE C-BAND FEEDHORNS BACK IN THE DAY, ONCE MOUNTED ON THE EXISTING FEEDHORN WITH THE SUPPLIED STAINLESS STEEL CLAMP THIS ADAPTER IS TRANSPARENT TO C BAND SIGNAL YET TRAPS THE REFLECTED KU SIGNAL ALLOWING C AND KU OPERATION ON THE SAME DISH. YOU WILL STILL NEED A KU LNB. HOBBYIST SEEM TO CONTINUE TO FIND USES FOR THESE AND I WILL CONTINUE TO SELL THEM TILL GONE, THE TRICK WAS IN THE BLACK PLASTIC FEED HORN IT UTILIZED A BUILT IN REFLECTION DEVICE TO MAKE THE TRICK IDEA WORK.

 

[Alec]

zodiak, great find. It places the Ku lnb in front of the C band scaler with the Ku lnb facing back towards the C band feed horn, Interesting concept.

 

[Stefan]

I actually purchased one of those from that same seller. I think he sells those pretty regular on ebay. he seems to be a pretty good guy. I purchased one, but he sent me 2 by mistake. When I contacted him he said to just keep the second one. I've never actully tried using it yet but I'm sure they probably work. How well they work and whether they degrade the signal or not however is perhaps the question. Anyway, if you note they have a polar servo to control polarity and accept a WR-75 flange LNB. So, if you use one of these, in it's original configuration you'll need to purchase a WR-75 Ku LNB and you'll need some way to control the polar servo. Some FTA recievers, like the Pansat 2500, will control a polar servo and have connections to hook it up to the reciever, but many FTA recievers do not. So, this could be an issue for some. There is however a workaround. If you notice the polar servo attaches to the device at what appears to be a C120 flange. Therefore you could simply remove the polar servo and attach a C120 LNB that supports 13/18v polarity switching.

 

[Alec]

Thanks Cascade, but the cone issue, is 2nd now, due to the mysterious black Ku feed that Stefan has. Id greatly like to understand the contents and construction of the "black cone" !!

 

[Stefan]

As far as I can tell it's simply got styrofoam in it. Whether or not there's something under the styrofoam I don't know. Anyway, if your really interested in one try emailing the seller. I'll bet he's got plenty more and he'll probably sell you one directly. I seem to recall him saying that the company that made them went out of business and he bought out their remaining inventory. So, he's likely got loads of them.

As I said earlier, I haven't tried them out yet. So, I really can't say how well they perform or if they degrade the signal significantly.

 

[Alec]

I havent used Ebay in 2 yrs, old registration data expired. Just took me an hour to re reg. Ebay says I have to wait 3 days before I can contact seller. In 3 days I'll be out of town for a week........and all I want to do, is tear it apart.

 

[caddata]

Alec:

While laying out the geometry for the cone type scalar in Autocad, using Rayydio's dimensions, I've discovered that the scalar needs to be elliptical and not round to fully illuminate the dish. Same is true when the flat scalar is used. Have you or anyone else noticed an elliptical shape to any of the existing scalars? The round geometry fails to illuminate the last 1" to 1.5" of the dish periphery in one plane when full illumination is achieved in the other plane (horiz. and vert. planes). This is opening up a whole new can of worms (complexity) in the design.

Thanks,
Harold

 

[zodiak]

I havent used Ebay in 2 yrs, old registration data expired. Just took me an hour to re reg. Ebay says I have to wait 3 days before I can contact seller. In 3 days I'll be out of town for a week........and all I want to do, is tear it apart.

Ahem... It's probably not my business but just as a suggestion. Why don't you ask Stefan to sell you an extra one he had because of the seller's mistake. He's not using either anyway.


caddata,
Are you taking into account the fact that the dish is actually elliptical itself? I don't know about yours but all my "round" dishes are about 10-15% taller compared to their width.

Besides, even if 1.5" of peripheral relfection is lost (which I doubt is lost entirely) that would represent about 0.17% of the 36" dish surface, which is a very little reason to complicate the design and manufacturing.

Finally, my understanding is that scaller's function is more to attenuate (shield) the reflection from the dish periphery so that background noise from behind the dish won't spill around edges and won't contaminate the pure signal, which is actually much weaker in absolute terms.

 

[caddata]

Zodiac:

Yes my 90CM Fortec is elliptical (37" vert. X 33.5" horiz.). At the offset angle of the normal feed, the feedhorn sees the dish as being round. I'm concerned in the under illumination because the 36" dish turns into a 33". This configuration uses the C Band feed positioned 12 degrees east of the normal focal point. The dual feed might not be practical. Both feeds located at the normal focal point would be the best. Locating the C Band feed at the normal focal point is more practical. Antenna gain for the C Band is marginal with the 36" dish while the Ku Band is slightly more that really needed.

 

[Alec]

Harold, nice input, Ive been doing some reading myself. Appearently if the dish is oval, large top to bottom, round cone is fine. If dish is toroidal, wider to the sides, than an oval feed horn design is needed. Id notice on a site last night an obvious cuvred oval feed horn. My interests at this time, is that dishes seem to have a singel best focal point, side by side lnb configs less efficent, thus, my interest in the reverse feed concept utilized by " black cone " http://www.satcure.co.uk/tech/index.html "We do not supply feed horns because the feed horn is part of the dish that it matches. You can't "mix 'n' match" feed horns."

 

[Alec]

I searched for some time, trying again to locate photo of oval feed horn. 2nd link is interesting Java script. 3rd is some wave guide info. Thanks guys http://community.webshots.com/album/378655130OEYTWY http://www.falstad.com/embox/guide.html http://www.fnrf.science.cmu.ac.th/theory/waveguide/

 

[zodiak]

Dish gain is proportional to its surface area. The difference difference between 36" and 33" is about one sixth of a percent. Is it worth it to be concerned about?

Speaking about dual feed, oh yeah, it will be very practical. I'm also contemplating my next C/Ku project (I've got a spare CM 1.2 laying in the basement for about 6-7 months now) and unless Sadune finally releases C/Ku LNB for some reasonable price, multifeed setup the way to go.

Yes, you cannot place both LNBs in the absolute focus point. However, there is not such a thing. It is not a point but rather a "cloud" about the diameter of the wavelength. That means if you are off it center by less than 1/2 of the wavelength you won't notice any difference. You can try that by tuning to a satellite, unscrewing the LNB and try to move it a little watching signal meter.

That is why it actually may make sense to put the C-band off focus because it's much more tolerant to the absolute displacement. Specifically about 4 times as tollerant, e.g. if you put a Ku LNB 1.5" off focus, which more than whole Ku-band wavelength, you're most likely to loose the signal completely, while for C-band it would be less than half so you wouldn't even see the change on the meter!

Note that we are not even taking into account that we can change the dish aim. If we cannot put an LNB in the focus (which is obviously impossible for every LNB in a multifeed) but can adjust the dish aim we can get significantly better results. By adjusting dish aim we can actually put the LNB mouth back in the center of the focus cloud. The only difference now is that the cloud has irregular shape, which is actually pretty complex and assymetrical and grows very fast once you more than 10 degrees off primary dish axis.

In our case if we put two LNB's direcly side by side it would be 5 degrees at most, which wouldn't make the cloud bigger than normal significantly (I'd guess a quarter of an inch at most). Even for Ku there will be almost no difference, the C wouldn't be able to notice a thing even theoretically!

The only pitfall I can see here is ability to control such a multifeed with the receiver since we won't be aiming anymore at the true satellite. I'm pretty sure it can be done with a PC but honestly I don't know if it's possible to specify such an adjustment in the receiver control.

 

[caddata]

Check your calculations, the loss of area between 36" vs. 33" is almost 16% if you calculate them as flat discs (Pi x r x r = area). That's substantial on a small dish like this.

I have experimented with the C feed positioned 15 degrees East of the Ku feed. It works. The geometry required that the C feed be located .500" higher than the Ku focal point on a radius equal to the focal length (the satellites are going down the arc to the West so the C feed had to be the inverse). The o.d. of the scalar prevents spacing closer than about 10 degrees.

I programmed the receiver (Fortec Ultra) with individual C & Ku locations (two different entries in the data base) and just told the receiver the C was 15 degrees away from it's true position and the receiver didn't know the difference. The feed angles involved with the configuration shaded a small portion of the Ku from seeing 100% dish.

 

[zodiak]

Yes, caddata you are right my calculations were wrong-- the area is NOT negligible. However, I doubt you should treat treat it as totally lost reflection area. Its contribution may be reduced thanks to suboptimal scalar but hardly by 100%-- this is unfortunatelly only my speculation.

 

[Stefan]

Thanks Cascade, but the cone issue, is 2nd now, due to the mysterious black Ku feed that Stefan has. Id greatly like to understand the contents and construction of the "black cone" !!

I took it apart today. there's really no magic to it. It's simply a black plastic cone or funnel filled with styrofoam. The Ku "feedhorn" is just a round waveguid (pipe) with a flange on it that attaches to the polar servo. I believe the way it works is the Ku signals are simply reflected back out of the mouth of C band feedhorn and the "plastic funnel" and styrofoam simply serve to hold the Ku waveguide centered in front of the C band feedhorn and aimed into it's throat such that it catches the reflected Ku signals. I think you could achieve the same thing by simply taking an LNBf, sawing off the end of the feedhorn (scaler ring looking part) , and positioning it directly in front of the throat of the C band feedhorn centered and looking right down it's throat. youd want to leave some space between it and the mouth of the C band feedhorn so as to not obstruct the signal reflected from the dish into the C band feedhorn (probably about 3 to 5 inches) . Anyway, here are some pics of it taken apart.

 

[zodiak]

caddata,
Sorry, I was so pissed off to find my error I didn't notice your success report. Congratulations!

When you design a multifeed it actually makes a lot of sense to play with the off-center LNB placement once you think you've put it in theoretically perfect spot. You can learn a lot (mostly about how irregular is the focus cloud shape) and often improve your initial results. So don't make it permanent until you moved it in all 3 dimentions.

In any case, placing any lnb 10 degrees of dish axis WILL degrade the signal. Not much but pretty noticeable. I went as far as 18-19 degrees with barely acceptable results but 10 degrees is when it really starts to drop. So for the final setup (or even the next step) I'd actually put both LNB off axis and effectively reduce the displacement angle for both.

If you want to maximize your C-band performance you may safely assume you can put if up to an inch off center (C-band wave is about 3" long), which is, I'd guess, about 2-3 degrees in your case. The Ku LNB would then have to go at about 3" (7-8 degrees) off center.

Unfortunatelly, I have positively no time to do my dish projects until August. I'm actually thinking of putting both LNBs in the same "caddy" that you could slide into different positions and find the setup that works best for the intended application.

 

[Alec]

Stefan, thanks for taking that thing apart !! I was wondering if there was any reflective suface inside the cone, appearently just the waveguide. I had been wondering about placing Ku lnb in reverse in front of the C band scaler, just to check it out an see what happens. Will be away for 1 wk, looking forward to coming back home and tweaking the dish some more. Thanks !

 

[Alec]

If any persons may try this " inline " concept, rather than cut off the cone on your lnb ( subjecting it to damaging tooling vibrations ) , find a piece of 8" - 10" copper or aluminum tubing that will fit snuggly into one of the smaller rings inside.

 

[zodiak]

... or get a spare old-style scaler-free LNB-- you know, the one with the starndard flange tha is meant to be screwed onto the feedhorn.

Personally though, I don't believe in this concept. It may or may not work for prime-focus BUDs, where this contraption is located in the shade of the feedhorn/LNB assembly. However, in a smaller offset dish this will directly interfere with the concentrated wavefront. Even assuming 100% of the Ku signal is reflected from the C scaler (which is of course impossible) the distortion from the waveguide, the LNB, the cables, etc will be much greater than the one you get if you place LBN's multifeed style.

It won't hurt to try it but I'd never saw or irreversibly modify any equipment in the process. Just my $.02...

 

[caddata]

Did anyone notice this interesting adaptation? Gets everything out of the dish's focal point. Maybe you could cut a hole in the center of the plate the diameter of a Ku wave length to allow the Ku feed to see the dish while reflecting most of the C Band energy to the C Band feed positioned below. However, the loss might be substantial because flat reflectors cause an out of phase condition with the signal. Cassegrain feeds like this use a hyberbola as the secondary reflector which keeps the microwaves in phase with each other. You can think of an out phase wave as being negative in nature, which will cancel a like wave that is in phase, compounding the loss. The reflectors in the photos are most likely hyperbolic in shape. A hyperbola is a convex parabola which also has a focal point.