Bent dish blues... Any suggestions on fixing the dish?

[dish_in_the_sky]

Still having difficulty getting a READABLE signal. I think I got AMC 3? at ~136 deg georaphic [lat 47.8N lon 122.2 W], and the skew acts normally, showing I do have a linearly polarized signal. Peaked signal, could not get any quality, scanned all transponders on all visible satellites (LO 10600/22 kHz ON,) nothing detected.

The Tech SAT LNBf seems to be working, but the dish (parabola surface) was noticeably, but not severely bent in shipping several years ago. The amount it was bent seems to be significant as demonstrated by the weak signals I can see (~ 1-3 dB above the noise floor)

Any ideas to get it to the right shape? My wife will mutiny if I order a new 36" dish!

Let's see... 12 GHz radio waves are EM radiation, same as light is. Therefore the same optical laws apply.

Lambda at 12 GHz is ~25mm. Usual shape and surface quality on telescope (parabolic) mirrors--a dish is the same but at RF wavelength-- is lambda/10 for a good image, lambda/4 for a poor image.

So, a non-offset (I think this also applies to an offset) dish needs to match the pareabolic shape and surface finish combined to at least 6 mm, and better 2.5 mm for good signal and pattern.

Lambda/4 gives a 90 deg phase shift, if you get towards 180 deg, you get destructive interference which reduces signal strength.

 

[PSB]

Should be able to use strings across the front of the dish (up and down, also left to right) and if the two strings touch in the center then your dish is not warped. If it is warped the strings can be used as a guide when 'warping' it back into shape!

When peaked on the signal stand behind the dish and pull on both sides of the dish, this sometimes helps to collect more signal, the dish may need re-peaked after this process but I have seen this work many times!

 

[dish_in_the_sky]

Gotcha!

I took the dish section off the mount, and instead of stringing it, I put it on the (pretty flat) garage floor for measuring. I then gently bent it where the rim wasn't flat

Repeat until well done...

Reinstalled the dish, tightening the bolts just enough to stably secure te dish to its mount.

BINGO! Finding FTA satellites is almost as easy as the DSS birds now.

Currently on IA5 to keep the inlaws happy until I get the motor installed.

Later, once its all working, I'll gently tweak the dish to grab as much signal as possible. Probably I'll do that on SatMex 5, which is a fringe bird for me with about 39-41 dBw EIRP in my location (extrapolated map)

NO WONDER I was chasing my tail last weekend!

 

[mark_calgary]

Later, once its all working, I'll gently tweak the dish to grab as much signal as possible. Probably I'll do that on SatMex 5, which is a fringe bird for me with about 39-41 dBw EIRP in my location (extrapolated map)

NO WONDER I was chasing my tail last weekend!

I've received that sat using a 90cm/36in dish up here at 51.7N 114.1W so my EIRP estimates might be similar (or worse) than yours. I can't tell you what my quality readings are as I'm at work and it's not a satellite I often motor to but maybe I'll check tonight.

 

[PSB]

I took the dish section off the mount, and instead of stringing it, I put it on the (pretty flat) garage floor for measuring. I then gently bent it where the rim wasn't flat

Repeat until well done...

Reinstalled the dish, tightening the bolts just enough to stably secure te dish to its mount.

BINGO! Finding FTA satellites is almost as easy as the DSS birds now.

Currently on IA5 to keep the inlaws happy until I get the motor installed.

Later, once its all working, I'll gently tweak the dish to grab as much signal as possible. Probably I'll do that on SatMex 5, which is a fringe bird for me with about 39-41 dBw EIRP in my location (extrapolated map)

NO WONDER I was chasing my tail last weekend!

Glad thats its sorted out now!

 

[PSB]

Here is a good link that should help members do the same thing with a warped dish!


http://www.satsig.net/axiflat.htm