Im trying to make my own Ku/C Band Feedhorn but I am failing

[John Popper]

Yes, well you did say you are getting pretty good Ku on that dish

The first picture shows a lnbf with a conical scalar ring for an offset dish. By using the Ku with a flat scalar ring for a PF you should pick up a point or two.

It looks like you should be using a six degree offset for you dish motor to move to the Ku lnbf. Are you doing that?

it's not a conical scalar ring, it's just a bracket to hold it, like this one:

and yep I have moved the dish but still get no signal when both lnbs are on

John - I initially wasn't paying attention, but your dish looks almost identical to one of mine, which I believe to be a 3.2m (10.5') Radyx. Do you have a depth measurement for it? My f/D calculates as 0.303, which is on the low side for a BUD. If your dish comes out the same, you will not have an easy time with Ku-band.

Pendra, I have no idea how to measure that

Is that so bad? With 1m offset dish one can catch just about everything NA wise in Ku-band at his location. Do I miss something in that comparison? Sounds strange JP, you're getting nothing at all.

when I use the bracket and the KU LNB only (as shown in the first pic) I do get almost every channel @ 83, 85, 93, 95, 97, 101, 105, 107, 113, 117 and 127.
I did blind scan the remaining satellites but got nothing and it makes sense since according to the lyngsat beam maps, Central Mexico is not covered at all by 87, 89, 91, 99, 103, so in short, that offset ku lnb in the first pic is working reasonably well, just not when I tried to put it next to the chaparral feedhorn.

it may match a 1m offset dish if the BUD was precisely aimed. from a real world tester, aiming ku on a BUD just doesnt seem worth the effort. it is so sensitive that a breathe seems to drop signal significantly. id deifnately opt for a 1m offset unless its absolutely necessary to use a BUD for ku. it is fun to tinker tho. haha.

crackt out,.

since my 10 footer is motorized and able to move from 137 to 83, it does make sense to wanna have both ku and c band on the same dish

 

[SatelliteAV]

The other guys are correct. If the KU Band LNBF works when centered, it will continue to work when offset in this small amount. Something is not aligning correctly.

Try this..... Pick a satellite that is closest to your true south that has both C-Band and KU-Band channels that you have successfully received and scanned. First aim your dish to receive the C-Band channels. Now select the KU channel that had the highest Signal Quality and switch to the offset KU LNBF. Slowly bump the dish to the west observing the Signal Quality. The KU LNBF may not be mounted exactly parallel to the C_Band LNBF and might need to be shifted up or down to place the KU feedhorn in the sweet spot.

A LNBF designed for a prime focus dish will increase signal reception, but your existing offset type LNBF should be providing acceptable reception.

 

[Hermitman]

The design of a prime focus dish is to reflect and concentrate the signals from the satellite to a single, small area called the focal cloud (focal point). This focal cloud is about the size of a golf ball and the idea is to position the feedhorn so the focal cloud is just inside the feedhorn's throat. That will give you maximum power transfer and your best reception. If your lnb feedhorn is outside the focal cloud, you may still recieve some signals but at greatly reduced levels. That's why, generally, there is only one feedhorn on a prime focus dish and its located right in the center of the dish. Good luck.

 

[pendragon]

Its interesting, what's the physics behind mesh type dish reflecting sat signals? One would think they should go through the mesh... And why its more difficult to aim a C-dish in Ku-spectrum, why its more sensitive to aiming errors?

If the wavelength of the signal is much larger than the hole size in a mesh, almost all of the signal will be reflected. A microwave oven uses this principle to protect you from the microwaves while letting you watch your food sizzle. A safe rule-of-thumb is to get the hole size down to 1/10 wavelength or less. On C-band the wavelength is around 7.5 cm and for Ku-band about 2.5 cm. For the mesh dish that John and I appear to have, the holes are larger than 2.5 mm, meaning there may be some reflection loss.

A larger reflector should ideally have a larger gain, which is a benefit of a narrower beam. You can visualize this partly because the narrower beam is collecting less noise. But it makes the dish aiming more critical. This is all theory. The practical problem with BUDs on Ku is the surface imperfections play a bigger role than for C-band. If the surface departures from an ideal paraboloid are (arbitrarily) less than 1/10 wavelength, the focus will be likely very usable. But holding a 3m dish within 2.5 mm is not particularly easy. This results in a spread out focal cloud with an effective gain reduction because the feed is not able to collect all the reflected energy.

Pendra, I have no idea how to measure that

Get someone to hold a tape measure that is perpendicular to the dish that extends to the center plate. Now sight from the edge of the dish, aligning your eye so the opposite edge is coincident. Read off this point on the tape measure. You can also use a string but the sighting method should be accurate enough.

 

[zamar23]

The design of a prime focus dish is to reflect and concentrate the signals from the satellite to a single, small area called the focal cloud (focal point).

Isn't it the design goal of any parabolic dish, offset or prime focus? But we see multifeed setups quite often, and one needs to compensate for the LNB shift by turning the dish a bit more (or less) to catch the same sat compare to prime focus position. Are you saying that offset parabolic dishes are more favorable for multifeed? Because they're offset, not mesh based, smaller in size and have bigger cloud, or why?

 

[1ADAM12]

JP- Yes it is a conical scalar. you can't see it but it is built into the Ku LNBF in your first picture.

What do you mean by "both LNBs are on"?

Maybe these pics. will help you. first pic. has Ku lnbf shown with plastic removed. This is like your Ku lnbf (conical scalar). Second pic. PF flat scalar on KU lnbf. Third Pic. you may be moving your dish in the wrong direction. for a 6 degree offset lnbf there is a 12 degree total offset.

 

[John Popper]

JP- Yes it is a conical scalar. you can't see it but it is built into the Ku LNBF in your first picture.

What do you mean by "both LNBs are on"?

Maybe these pics. will help you. first pic. has Ku lnbf shown with plastic removed. This is like your Ku lnbf (conical scalar). Second pic. PF flat scalar on KU lnbf. Third Pic. you may be moving your dish in the wrong direction. for a 6 degree offset lnbf there is a 12 degree total offset.

c band lnb and ku band lnb like on this pic

as it is in that pic right there I'm only able to get C band, no Ku Band, but like was suggested earlier I might test to offset the C band side and place the ku one dead center, will keep everyone posted

 

[John Popper]

The other guys are correct. If the KU Band LNBF works when centered, it will continue to work when offset in this small amount. Something is not aligning correctly.

Try this..... Pick a satellite that is closest to your true south that has both C-Band and KU-Band channels that you have successfully received and scanned. First aim your dish to receive the C-Band channels. Now select the KU channel that had the highest Signal Quality and switch to the offset KU LNBF. Slowly bump the dish to the west observing the Signal Quality. The KU LNBF may not be mounted exactly parallel to the C_Band LNBF and might need to be shifted up or down to place the KU feedhorn in the sweet spot.

A LNBF designed for a prime focus dish will increase signal reception, but your existing offset type LNBF should be providing acceptable reception.

thanks! I will try to do that tomorrow afternoon before I go to my local blacksmith and instruct him on how to make me some pieces for further testing!!!

 

[zamar23]

I found turning a shifted LNB 2-3 degrees below or above the dish center while turning it horizontally a bit to point to the dish vertical center line may help catching a bigger signal "cloud".

 

[Hermitman]

Isn't it the design goal of any parabolic dish, offset or prime focus? But we see multifeed setups quite often, and one needs to compensate for the LNB shift by turning the dish a bit more (or less) to catch the same sat compare to prime focus position. Are you saying that offset parabolic dishes are more favorable for multifeed? Because they're offset, not mesh based, smaller in size and have bigger cloud, or why?

It has to do with geometry and the satellite signal reflectivity angle. The offset dish is not really a parabolic dish. If it was, it would have the focal point in the center like the prime focus dish. An offset dish is a combination ellipse-parabola-hollow circle sector design to move the focal point below the dish face.

On a prime focus dish, when satellite signals from other than the targeted satellite are received, they are reflected at an angle that takes them well away from focal cloud area. This results in a narrow beamwidth for the dish and more received signals from the targeted satellite with less interference from adjacent satellites.

On an offset dish, the beamwidth is much wider because of its design. This allows signals from adjacent satellites to reflect at an angle much closer to the focal cloud of the targeted satellite. And if you position a second (or more) lnb near the center lnb, you will pick up the signals of adjacent satellites from these reflected signals. That's why offset dishes are used for multi-satellite reception without repositioning the dish.

Even though there is less surface area on offset dishes to reflect satellite signals, this is overcome by the increased power output on Ku and Ka satellites. The adjacent satellite lnbs are moved slightly higher or lower in relation to the center lnb (or the dish is slightly rotated) to compensate for the arc of the satellite positions (Clark Arc). If all the satellites were lined up horizontally, you won't have to worry about this. Hope this helps. Good luck.

 

[pendragon]

It has to do with geometry and the satellite signal reflectivity angle. The offset dish is not really a parabolic dish. If it was, it would have the focal point in the center like the prime focus dish. An offset dish is a combination ellipse-parabola-hollow circle sector design to move the focal point below the dish face.

Offset dishes are almost always just a section of a paraboloid. If you were to take a prime focus dish, cut off the lower half or a bit more, and fashion a rough oval from the original center to the top of the dish, you would have an offset dish. The focus is still where it was, but the dish is completely above it.

On a prime focus dish, when satellite signals from other than the targeted satellite are received, they are reflected at an angle that takes them well away from focal cloud area. This results in a narrow beamwidth for the dish and more received signals from the targeted satellite with less interference from adjacent satellites.

On an offset dish, the beamwidth is much wider because of its design. This allows signals from adjacent satellites to reflect at an angle much closer to the focal cloud of the targeted satellite. And if you position a second (or more) lnb near the center lnb, you will pick up the signals of adjacent satellites from these reflected signals. That's why offset dishes are used for multi-satellite reception without repositioning the dish.

A prime-focus dish will have a beamwidth that is roughly the same at any offset angle. An offset dish's beamwidth is generally dependent on its geometry. If it is cut 'wide' it will have a tighter beamwidth in-track to the Clarke belt (if properly skewed) than cross-track. Of course the converse applies. Actually unless an offset is cut very wide, it will have a larger off-axis loss than an equivalent prime-focus dish. If you want multisatellite reception from a single dish significantly off-axis, there are better choices than sections of a paraboloid, such as elliptical or toroidal geometries.

Even though there is less surface area on offset dishes to reflect satellite signals, this is overcome by the increased power output on Ku and Ka satellites. The adjacent satellite lnbs are moved slightly higher or lower in relation to the center lnb (or the dish is slightly rotated) to compensate for the arc of the satellite positions (Clark Arc). If all the satellites were lined up horizontally, you won't have to worry about this.

This is not really correct. The gain of a dish antenna is related to the square root of its projected surface area divided by the wavelength of the signal it is receiving. So a dish for Ku-band needs only about 1/9 the surface area of a C-band dish to have the same gain, because the wavelengths are 3X smaller.

 

[John Popper]

Update: I've made some progress by placing the Ku LNB below the Chaparral instead of to the side.


this is how I was testing it and getting nothing:

and this is how Im testing it now:

I checked to see if I would be able to get anything if I placed the Ku LNB below and Im now getting some channels when i move the dish to offset the difference, like the Azteca mux @ 93, CCTV mux @ 95 and others, still however not all the channels I was getting with just the bracket but at least I now know that I am able to get both c and ku with the same feedhorn, now this is not the final product because as it is right now im not even able to place the c band lnb so I need to make more modifications like cut a piece of the scalar ring so it ends up looking something like this (please dont laugh at my photoshop skills lol):

off to the blacksmith I go!!

 

[zamar23]

The mesh looks so big, I wonder if all Ku-signals run right through it...

On an optimistic note, may be you can make an extension rod to slide your Ku-LNB far from the dish while keeping it below the C-LNB to try catching a bigger "cloud". Just attach the slider on the rod at the bottom to the C-band Feedhorn at 2 distant points with small brackets. Or, you can attach the slider rod in 2 points to the mounting ring and main arm, using an extra supporting bracket, welded to the arm closer to dish center.

 

[John Popper]

The mesh looks so big, I wonder if all Ku-signals run right through it...

On an optimistic note, may be you can make an extension rod to slide your Ku-LNB far from the dish while keeping it below the C-LNB to try catching a bigger "cloud". Just attach the slider on the rod at the bottom to the C-band Feedhorn at 2 distant points with small brackets. Or, you can attach the slider rod in 2 points to the mounting ring and main arm, using an extra supporting bracket, welded to the arm closer to dish center.

Like I posted earlier all ku signals are not gonna run right regardless of what I do because my beam down here is not that good on half of the ku sats. I also need to get a better ku lnb, something for prime focus, this one im using right now is for offset so I know im not getting the best signal. Many folks have suggested the geosat but I don't know if it will mount right (because of its shape) with this frankestein I created, Im thinking the servo motor of the chaparral will not allow me a lot of movement with the geosat so I just ordered this prime focus ku lnb from dms which has a straight shape, we'll see how that works!

 

[Larry1]

The Ku LNBF in the lower position is quite a bit further back than when you had it beside the C feedhorn. If you have a Ku LNBF to maybe scrap, I wounder what would happen if you cut the outer rings off the Ku scaler then you could mount it a bit closer to the centre.... maybe even stick a 45 deg pipe elbow on the end to get it as close as possible if needed and grind the one side of the elbow to be very thin. (I have not tried this myself, just thinking out loud)

 

[1ADAM12]

BSC321SP lnbf claims to work well for offset and PF, Don't believe it.

By moving the Ku lnbf lower is the same as changing the dish elevation.

C band is more forgiving then Ku.

To get best performance the two lnbf should be side to side horizontally.

Then you would need to adjust your dish elevation,(FOCAL LENGTH) and azimuth to peak KU.

I'll second Larry1, the Ku lnbf is now further back.

You should have measured the face of the Ku LNBF in your very first picture where it was working. That is the critical focal length for the Ku.

You have mechanical issues that prevented you from getting the correct F/D in the side by side.

 

[1ADAM12]

OR you could just put the Ku back like you had it and side mount C Band LNBF.

The SatelliteAV LNBF would be best choice.

On the Mini Buds alot of folks have done this -

 

[John Popper]

BSC321SP lnbf claims to work well for offset and PF, Don't believe it.

By moving the Ku lnbf lower is the same as changing the dish elevation.

C band is more forgiving then Ku.

To get best performance the two lnbf should be side to side horizontally..

it never worked for me, I tested it several times, that's why I decided to try placing the ku below and Im getting some channels now

Then you would need to adjust your dish elevation,(FOCAL LENGTH) and azimuth to peak KU.

I'll second Larry1, the Ku lnbf is now further back.

You should have measured the face of the Ku LNBF in your very first picture where it was working. That is the critical focal length for the Ku.

You have mechanical issues that prevented you from getting the correct F/D in the side by side.

to be totally honest what I am most interested in is C band, my dish as is adjusted right now allows me a very solid c band experience, so I do not want to tinker with that, but I did want to get some ku band channels without having to buy another dish, change lnbs in the middle of the night or spend close to $200 for a corotor, so I decided to try this and for instance with this setup I am getting the cctv mux, the azteca mux, rtv, biography, history, the sports feeds @ 105 which is pretty much all I wanted to get anyway!

 

[John Popper]

OR you could just put the Ku back like you had it and side mount C Band LNBF.

The SatelliteAV LNBF would be best choice.

On the Mini Buds alot of folks have done this -

I will try to do that for a 7 footer I am trying to set up!

 

[John Popper]

well after many hours and several tries I have finished this little project and I can say I'm satisfied with the results (at least for the time being and specially since I have zero background on anything tech/science related, I studied International Relations and Economics lol). My C band reception is almost exactly the same as was before when I was using simply the c band lnb and feedhorn, however, I did lose 2 or 3 weak channels (like the nunavut channel @ A1 lol), but I can live with that.

As for the Ku band side, I get about 90% of what I was getting with the bracket and Ku LNB but the one important drawback is that I lost pretty much every channel @ 97, for some reason that bird is showing up real weak for me right now, however I never watched any of those channels for more than 2 seconds but I am still getting the channels I wanted, like CCTV, Azteca, RTV, Bio, History, the Satmex channels and the sports feeds @ 105.

Once I get the DMS "Prime Focus" Ku LNB i'll see if I do get a stronger signal than I am getting with the offset LNB or if the "prime focus" aspect was just bullocks. Im also still gonna buy the geosat many of y'all recommended!

Thanks to everyone for their suggestions!

here are some pics of the Chaparral Feedhorn with C band LNB & Servo Motor/Universal Ku Frankenstein