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

Originally Posted by Hermitman

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.

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!!

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.

Originally Posted by 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.

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!

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)

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.

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 -

Originally Posted by 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..

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

Originally Posted by 1ADAM12

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!

Originally Posted by 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 -

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