Dish mods from the "Dark Side"

[melgarga]

Being bored, I decided to travel a few villages of the "Dark Side" and came across something that caught my attention. It seems that if this could work, something would have shown up here courtesy of our resident Mad Scientist ( ya'll know who you are!) The guy claims 123, 125, and 129 on a 30" dish w/ this setup. Using 3/4" copper pipe for a waveguide is one thing I guess, but what about a scaler assy?
Wierd.

 

[Larobpra]

Does he switch the one wire to one of the other LNB's when he wants to watch a different sat? Looks bogus to me.

 

[melgarga]

I dunno, I was so distracted by the "plumbing" I didnt really think about it.......lol
I'm curious about the connection point between the lnb input and the copper pipe.

 

[pendragon]

I'm curious about the connection point between the lnb input and the copper pipe.

Looks like the copper pipe goes into a C120 flange on the left LNB. It would be around the correct diameter. Actually this might work; if you look at the feeds DN put on the Superdish, these pipes are only a little cruder.

 

[Smith P.]

Umm, then where is initial conus ?

 

[melgarga]

Looks like the copper pipe goes into a C120 flange on the left LNB. It would be around the correct diameter. Actually this might work; if you look at the feeds DN put on the Superdish, these pipes are only a little cruder.

I defer to your knowledge of these things, and after researching the OP's other posts, I see no reason to question his qualifications or claims, but the damn thing just looks really weird. I'm all for thinking 'outside the box' and this demonstration gets moved to the top of my list in that respect.
.........so if the focal point is at the end (or beginning?) of the "waveguide", the distance to the actual lnb input element is irrelevant?
Makes me start invisioning a homebrew multi port eliptical (or conical?) scaler assy..........

Merging ideas from two different Mad Scientist's research projects, if one used the same diameter copper pipe as the throat of a C band LNB(F) theoreticly could one yield the same result in moving the C band assy to a position that is more "adjacent LNB" mount friendly?
or
optimizing the mini bud C band feed assy and boring holes in the scaler for the adjacent Ku LNB(s) to be mounted in a more position adjistable mount.

Could this be a solution for adding one or more Ku LNBs to a prime focus dish to 'pick off' the correct focal point for Ku without compromising the C band portion?
Microwaves....gotta love 'em.....lol
Damn, I'm starting to sound like my grandkids. "What's this for?, How does this work?, What does that do?......"lol

 

[Smith P.]

From law of physics traveling distance is affect signal level and adding noise; as I questioning before, missing provision of entrance's conus and scalar ring will induce drop the signal, we should remember for the type of normal parabolic [offset] reflector exist only one optimal focal point; when you get adjusted sats you'll get less energy in second/third pseudo-focal points. Therefor you'll need to make bigger entry throat and avoid adding signal traveling distance to the probes inside of LNBF.

 

[pendragon]

I'm a little rusty on waveguide theory, but it is important to note that in a circular waveguide the diameter is critical to the range of frequencies that can be successfully transferred. One would need to calculate the modes to see whether standard pipe diameters would be practical for FTA applications. Eyeballing one of my C120 LNBs puts 3/4" copper in the ballpark, although it might be a little large. One does need to carefully manage transitions in dimensions and the primary/secondary probes cannot be arbitrarily placed. There are rough rules-of-thumb available in design, but one is pretty much stuck with either heavy computational modeling or cut-and-try methodologies.

Good waveguides can move signals at least a short distance. Minimizing surface imperfections/irregularities is important. I recall losses such as 0.2-0.5 dB/ft at Ku frequencies but that's just memory. This doesn't necessarily mean the CNR will be reduced by this attenuation, because the dish will 'amplify' a variety of noise sources, including terrestrial, solar and galactic. That may still put the noise floor at the LNB probe higher than the LNB's equivalent noise.

There's no need to be bashful about using waveguides in FTA. DN used about 6" to split 119 and 121 on the Superdish. My circular ortho C-band feed is about a foot long to accommodate its 1/4 wave plate. There is no scalar on the Superdish 119 feed and the 121 has only a slight flare. The use of scalars is often beneficial, but may be obviated by other aspects of feed design. My gut feeling is a longer feed will be more directional and thus reduce the benefit of a traditional scalar.

The optimal location in the feed for the focal 'point' pretty much needs to be determined by trial and error because the focal clouds of dishes can be quite different and complex. Enlarging the feed mouth for off-axis reception may win or lose depending on the energy density of the off-axis focus. I'm not sure I would characterize off-axis focus as 'pseudo' focal points, however. They are real, but as in any parabolic reflector, they will exhibit coma which will spread the received energy.

 

[Smith P.]

[Just short note from math: a parabola had only one focal point. If we talking about parabola reflector. ]

 

[pendragon]

We are arguing semantics. I tried to be careful to not characterize off-axis focus as a focal point(s). Calling out "second/third pseudo-focal points" has an air of specificity when in fact it is a continuum. Off-axis focus is also three-dimensional. Those were my only concerns. The scientific nomenclature is generally to refer to aberrations as the inability to bring the energy to a precise point.