sidelobes - a way to eliminate them?

[skysurfer]

I'm trying to figure out a way to eliminate some sidelobes that I noticed after I repeaked a c-band/ku-band dish.

My dish is a 10' fiberglass dish, f/d of .30, focal length 35.5 inches, corotor ii feedhorn, and the dishg is not perfectly parabolic anymore due to aging - the bottom is slightly flatter, the sides and top are fairly parabolic curved shaped still.

I noticed my dish feedhorn wasn't centered so I re-centered it and repeaked the system after the fix because of the change in dish tracking. I used Ku-band for the peaking because of how intolerant it is of reception errors compared to c-band.

Now that I've finished, I can see some minor sidelobes on c and ku-band (i.e. when on G27 Ku, I can see a very weak but visible hump of digital signal from Horizons-1). When I look at G25 C-band, I can see some very weak visible humps from Galaxy 16.

The sidelobes aren't bad (they are not strong enough to be lockable and doesn't seem to affect digital locking of desired signals) but the noise floor isn't perfectly smooth between the digital/analog signals across the polarity.

I didn't experience sidelobes before, probably because my signal was 2-4 dB s/n weaker on digital signals than now so the sidelobes weren't noticeable with my dish not in great alignment.

Am I getting sidelobes due to my dish not being perfectly parabolic anymore or could it be because I have more stronger signals now so they become apparent? Is my dish type prone to sidelobes (with a .30 f/d) or...?

I"m trying to figure out what kinds of things I might want to try to do to eliminate sidelobes and have the clean noise floor between signals that I"m used to, even though the sidelobes don't seem to harm performance in the signals I've tested so far.

 

[tvropro]

If the dish isn't true parabolic you will see more sidelobes. Also make sure the feed F/D and focal distance is correct and perfectly centered. If the sidelobes isn't effecting the BER on your signals and causing breakup I wouldn't worry about it much.

 

[14karat]

Just thinking out loud here...
If the dish face is sagging at the bottom and drawing up at the top, couldn't a couple of well-positioned (perpendicular) tension cables (stainless aircraft cables) across the dish face bring it back into shape?

 

[Anole]

silly idea

Was it just gravity over 20+ years that warped the dish, or a collision with something?
If gravity, is it possible to remount the dish upside down?
Wouldn't work for offset dishes, as their shape on the top half is different from the bottom half.
But, a prime focus dish (center feed) is the same no matter how you mount it,
...and there isn't anything but the actual mount points that differ.
... if that.

This is not a serious proposal, so much as a question...

 

[tvropro]

Use a blowtorch to reshape it Just kidding

 

[ACRadio]

Was it just gravity over 20+ years that warped the dish, or a collision with something?
If gravity, is it possible to remount the dish upside down?
Wouldn't work for offset dishes, as their shape on the top half is different from the bottom half.
But, a prime focus dish (center feed) is the same no matter how you mount it,
...and there isn't anything but the actual mount points that differ.
... if that.

This is not a serious proposal, so much as a question...

If it's a one piece then turning it 180 degrees is the only option. If it's a 4 piece then disassemble the dish and swap 2 opposite panels and reassemble face down on a flat surface such as a parking lot. Tighten the rim bolts first and work around to the center. This helps pull it back into better shape.

 

[skysurfer]

I love the thinking of this group!

Yes, 20 years more or less of gravity has taken it out of parabolic.
I got it as a used dish in summer 1989 (geez, almost 20 years in the hobby!!!) from a buddy who installed c-band dishes for residential and commercial. I think it may be some type of Scientific Atlanta fiberglass dish since I can see part of the Scientific Atlanta logo bleeding through where some of the paint has worn off.

I like the idea of rotating it 180 degrees, but with its weight and a lack of a good flat place to be able to put it when rebolting the 4 panels and then carry it back onto the mount, I am favoring the guy wire idea. I can put a bolt through the lip with a eye to connect guywire at 4 points (90 deg apart) and use a turnbuckle more or less at the center to adjust things until the thing is back to being parabolic.

I know I don't want to use the feedhorn as attachment points -- even tho the dish is quad leg, I know the stresses may pull on the feedhorn and get that out of alignment from looking straight into the center of the dish properly.

I'm trying to think how I'll know when I"ll be at that point of being parabolic as I tighten down the turnbuckles, using a method other that eyeing it. Maybe the dish will be parabolic when the distance of N/S and E/W are equal?

I also have considered just doing N/S with guy wire (since E/W appear to be in good shape) and equal the distance of N/S to that of E/W. My drawback to N/S only is that the N/S tension may make the dish more oval shaped instead of parabolic by pulling in E/W sides a little bit but hopefully the equal distance would prove things are still parabolic rather than oval shaped.

 

[truckracer]

It will be very difficult to get the dish in perfect parabolic form. You may need to just get a new dish if you can spare the extra cash to get one. Or Find a free one (solid metal or mesh dish) from the neighbors who don't use it anymore.

 

[skysurfer]

Here's what I found out.

I peaked my dish and got it great for N/S (you can pull up or down on the lip on extreme sat and not get any stronger of signal).

I noticed when I pulled side to side for whatever reason that the signal strength got stronger when I pulled the side lip out. I figured out I must have "squished" in the sides when I re-centered the feedhorn so I drilled some extra adjustment holes so I could bring the ends out further. After I did that (and while on extreme sat), I am unable to get any stronger signal by pulling the lips in or out, meaning my dish is probably about as parabolic as it is goign to get on the N/S and E/W dish face crossings.

It seems to perform a little better now (still a llittle sidelobe on c-band, ku-band is now clean).

I am pondering as a project if the winter is mild or for spring if I want to take my 10 foot fixed mount fiberglass dish, remove the dish from its fixed mount, and remount it on my steerable polar mount. that 10 foot fiberglass is in 4 sections (not 8 like the dish I'm working with) and is perfectly parabolic. I don't have the grand or so for a good 10 foot dish delivered so I wlll just make do with what I have.