what are the differences between different sizes of dish? I know that 30" is the minimum dish size requirement for FTA, but whats with the bigger sizes?
FTA Dish
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Requirement is such a strong word. Let's change that to read, say something like: A dish size of at least 30" is a good general rule-of-thumb.
Lak7 is spot on. I'll add some other benefits:
Larger dishes are also useful with weak transponders, or perhaps receiving a signal at the outskirts of a beam footprint. There are more than a few of these in the sky here - where my 30" will not provide enough signal for the receiver to work, but the 1.2m (or 47 inches to spare you the conversion) works just fine.
thanks...what about the ellipticals with multi LNBs. How are they compared to the bigger diameter circulars?
The oval dishes have a focal pattern that is smeared out to pick up multiple birds. Superdishes and Direct world dishes can be modified to FTA use and are a bargain (free).
I'm curious where you've read this {ie about the smeared focal pattern}? I have assumed that elliptical dishes are just elliptical sections out of a regular paraboloid {which will naturally have a smeared pattern for any lnbf not at the focal point}, basically no different from a regular dish with respect to only one focal point. If, in fact they are not paraboloids as it sounds like you're saying, I'd be interested in reading more the theory involved.
Now, I have assumed that those torroidal things, which I know nothing about, ARE something similar to what you describe. I've assumed that they must be parabolic vertically, but not parabolic horizontally. As I said, I don't know anything about these, but I assume that they wouldn't work very well unless used with the double reflection thing, but I could be wrong.
In any event, regardless of design, it is my opinion that any time you put more than one lnbf on a dish, it is a compromise from the start, because there is only one focal point of the dish, and any lnbf that isn't at the focal point will have reduced performance. Seems to me that smearing out a focal pattern would make the dish a complete non-performer, as it wouldn't have ANY focal point.
It's also my assumption that on multi-lnb dishes, that since the lnbfs are designed for an F/D based on the height, that each lnbf will only illuminate a portion of the dish consistent with a circular dish with the dimensions of the height of the elliptical dish, and the focal pattern of any off center lnbf will be smeared, and and be less efficient than the center lnbf.
It's obvious, however, that despite the obvious reduction in performance, that many people are having good luck with these multi-installations, although I really wonder, just how good the interferrence rejection is, compared even to a circular single lnbf dish with a height equal to the height of the elliptical or torroidal dish.
Anyway, just my opinions, and I'd welcome opinions to the contrary, particularly about those torroidal things, which I've never understood very well.
If your looking at buying a setup, check out some the gold sponsors at the top of the page. I would recommend a winegard 2076 (76cm) dish if you are thinking of going with the 30in size. These perform very well. Otherwise if you can locate an old primestar dish these work very well also.
The Toroidal dishes are supposed to work by using a primary reflector (the large dish itself) to reflect the satellite signals equally onto the plane of the secondary reflector (at which the LNBFs are aimed) in a linear pattern. From what I've read, this works better in certain geographic locations than in others, but it basically increases the range of arc that the dish can reliably receive.
One of the oft-overlooked advantages of larger dishes is the tighter beamwidth. This can be very effective in rejecting interference from adjacent orbital locations.
Elliptical dishes have advantages over a traditional offset dish when it comes to multisatellite reception. Normal offset dishes are taller than their effective diameter, which is fine for a single satellite. However the gain falls off very quickly off-axis for an offset dish, compared to say a prime-focus dish. By making the dish wider, this effect is reduced. In addition the increased width yields a narrower beam pattern between satellite locations, which is more useful than reducing beamwidth perpendicular to the Clarke belt. Admittedly the tighter beamwidth is not important for DBS applications.
Toroids are a much different geometry in that they can achieve a focal "line" rather than a focal point. By being generally wider than taller and spreading the pattern, it's much easier to line up a string of LNBFs compared to other designs. A toroid will have some gain loss at the extremes of its range, but in my measurements (Wave Frontier T-90) was only down 3 dB around +/-20 degrees compared to a prime focus (Fortec 1.8m) at +/- 7 degrees and a normal offset (GeoSatPRO 1.2m) at +/- 3 degrees.
Elliptical dishes have advantages over a traditional offset dish when it comes to multisatellite reception. Normal offset dishes are taller than their effective diameter, which is fine for a single satellite. However the gain falls off very quickly off-axis for an offset dish, compared to say a prime-focus dish. By making the dish wider, this effect is reduced. In addition the increased width yields a narrower beam pattern between satellite locations, which is more useful than reducing beamwidth perpendicular to the Clarke belt. Admittedly the tighter beamwidth is not important for DBS applications.
Toroids are a much different geometry in that they can achieve a focal "line" rather than a focal point. By being generally wider than taller and spreading the pattern, it's much easier to line up a string of LNBFs compared to other designs. A toroid will have some gain loss at the extremes of its range, but in my measurements (Wave Frontier T-90) was only down 3 dB around +/-20 degrees compared to a prime focus (Fortec 1.8m) at +/- 7 degrees and a normal offset (GeoSatPRO 1.2m) at +/- 3 degrees.
Thanks.
In reading this though, I was trying to imagine how you took these measurements? Did you measure the signal while centered, then move the dish the indicated angles off center, then hunt for the position of the lnbf that gives you best signal? That's the only thing I could come up with, and that seems reasonable with the torroidal, or prime focus, but it seems like it would take quite a bit of searching for the best spot with an offset. Or am I missing the obvious (as usual
).Are you just getting started and have no equipment? If so I'd recommend finding a free dish network super dish, DirecPc dish or primestar dish. Place a wanted ad on craigslist or freecycle. You can find receivers on ebay for about $30. An install kit with all your wires and ground block is about $20 (ebay). For $50 you will be receiving fta ku band on one satellite. Enjoy it for a while.
Then start reading and read some more on this forum. Eventually you'll be craving a better receiver, a bigger dish, switches, c-band,....
Now that you are hooked and have a case of techno-lust come back and visit the sponsors. They've got all the fta-crack you can handle.
Then start reading and read some more on this forum. Eventually you'll be craving a better receiver, a bigger dish, switches, c-band,....
Now that you are hooked and have a case of techno-lust come back and visit the sponsors. They've got all the fta-crack you can handle.
Thanks.
In reading this though, I was trying to imagine how you took these measurements? Did you measure the signal while centered, then move the dish the indicated angles off center, then hunt for the position of the lnbf that gives you best signal? That's the only thing I could come up with, and that seems reasonable with the torroidal, or prime focus, but it seems like it would take quite a bit of searching for the best spot with an offset. Or am I missing the obvious (as usual
This was quite laborious, but as I was trying to decide which path to take I put up with the headaches.
The toroid required the most measurements, but it does have a scale that can be calibrated to a coarse offset position given a center orbital location. I used the same LNBF and satellite for all measurements, and tried to hurry so all measurement conditions were as close as possible. I calibrated my signal meter, but also took spot checks of a specific transponder with my HP spectrum analyzer. Basically I skewed the toroid with the LNBF at the center and peaked it (azimuth, elevation, LNBF skew, LNBF focal position and pointing angle). That was the reference point. I then slid the LNBF down the rail to a known angle offset and skewed the dish according to the new virtual center. I then repeaked everything as I did with the center position before reading the signal power for that offset. I did a practice run-through before doing it for the money and got quite good at peaking everything in a matter of a minute or so.
The results of the toroid measurements were more impressive than I expected, so I was a little more perfunctory with the offset and prime focus measurements. They were also easier because both dishes were already motorized and tracked the arc accurately. The center reference required pulling the existing feeds out, which I hated because I would have to repeak everything afterwards. Originally I employed a hair-brained scheme to avoid this. Later I came to my senses and decided to take advantage of pulling the feeds to make some modifications I came up with to partly rationalize why this was a good thing to do.
So with each dish steered to approximately the centered position, I peaked the mounted, reference LNBF and tweaked the position. I then replaced that LNBF with an identical type just to keep the support arms anchored. I then steered the dish to a known offset position and located the off-axis peak with the reference LNBF. I only did a few measurements as it became clear that theory and practice were reasonably close.
By the way, the angle offsets I state are by orbital position. The actual offsets as seen by the dish (as in azimuth and elevation) are somewhat larger for obvious reasons. I could have done the latter, but it was more meaningful to me to do the former. The one trouble with this is the relationship between the two varies depending on where one is centered in the arc, and doing it from the dish's reference might have been more useful to translate to other results. But I had one center in mind and avoiding unnecessary math was the benefit.
I think my spreadsheets have enough data to ultimately unwind this, but I don't have much interest at this point. I also will confess that the toroid measurements were done with dinner looming, so they were quite hurried by the threat of a prematurely terminated and thus invalidated experimental run. In a perfect world I would like to measure this again, along with some effort put into determining the effective beamwidth of the toroid and perhaps a rough cut of how cross-polarization varies by offset. But my toroid is set up and tweaked with a lot of LNBFs, so unless I decide to buy another this is unlikely to get done.
I recall reading about the Superdish design while working for DN as a technician, the shape is neither truley parabolic nor elliptical, but a mixture of the two in order to maximize the signal delivered to each lnb, while minimizing the adjacent satellite inferference. The focal pattern looks like sideways hourglasses. I assume some serious math boys did the calculations and that mass producing them cheaply was a main consideration.
? Perhaps since the Superdish is designed for specific separations, maybe it's sort of 3 separate dishes connected together, sort of with the shape gradually changing from one dish to another as a factor of the % difference between the aim points of the 3 lnbfs?
If I get some time, I may do the laser pointer/white paper ray tracing on a SD.
The main advantage of using them is zero cost and acceptable performance on most birds. I had interference trouble using one for 123W when PBS went live on 125W and ended up switching to 1.2M DTN dishes for 123/125.
They are handy for getting new people into FTA, as it reduces the cost of entry to ~$40-60 for a cheap reciever and linear lnb.
The main advantage of using them is zero cost and acceptable performance on most birds. I had interference trouble using one for 123W when PBS went live on 125W and ended up switching to 1.2M DTN dishes for 123/125.
They are handy for getting new people into FTA, as it reduces the cost of entry to ~$40-60 for a cheap reciever and linear lnb.
I am seriously thinking about setting up one. Just wondering what's out there to watch... I just don't want to waste a couple of hundreds for mediocre shows and transmissions. More so with watching low def shows on my 1080P 73" TV...if you know what I mean...
You two have a very interesting discussion here. You raise some very fine questions which make me desire to go back into my antenna theory classes from my college days. I now wish I had paid more attention to this subject matter. At the time, it was only a sideline of my main education experience. Now that I am getting involved in this hobby, it would have been valuable for me to research this more way back then.
The idea of a "smeared" out focal point is a bit unorthodox, but I understand the concept. If you think about it as two (or more) dishes blended together, they would actually have more than one focal point. This has to be correct and very well designed with mathematics and physics as we know that they are designing these antennas and they do function.
Obviously, DN, DirecTV, Bell Express and StarChoice are able to design a dish antenna which can get two, three or more satellites to be received on a single dish.
I feel that these satellites are probably using a higher power transmission to make this possible on such small dishes. Most often with the circular polarized signals as that would also benefit this in obvious manners. But, Star choice is doing it with two linear sats for their customers.
I would very much like to research this and develop my math and antenna design skills to understand this better.
Thanks to you two, I have a newfound interest in this subject. It will be a lot of fun simply to research this information.
Thank you guys for inticing me to do more research! A lot of great fun here!
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