Dish Depth

[kelleyga]

I was measuring my 10 feet mesh dish Depth, and need someone to clarify something for me, please.

To measure it, I ran a cord from one side of the dish to another. Then I used a measure tape and measure from the inside center of the dish to the cable going across. Exactly from the middle of the dish (which I think is called the hub??) it measure 18.5 inches. When I was doing the measuring, I noticed that the solid circle in the middle of the dish (in the inside) is NOT completely flat. This solid circle in the middle has another small circle about 3 inches in diameter that is a little on top of (or closer to the cord when measuring) of the whole solid circle.

If I measure from the middle of the dish (small circle) I get 18.5, but If I move just a little away from the center of the inside of the dish and measure from the other flat area of the solid circle in the middle I get 19.5

This 2 measure will give me different f/D ratios:

18.5 = f/D = 0.40
19.5 = f/D = 0.38

1) Which one should I used? I was thinking it has to be from the EXACT middle of the dish (inside), but the middle is closer just because of that small little circle I was talking about. I am confused now.

2) I also read somewhere else I have to measure from the middle of the dish but from the HUB (the outside of the dish). If that is correct then I was doing it wrong from the begining....???

As always thanks for your great help.

 

[Channel Hopper]

Ignore the step that is in the centre of the dish, try to imagine the depth at the centre would be an extension of the natural curvature of the parabola if there was no step and make the measurement with this in mind (you might be better off drilling a small hole right in the centre, measuring from the string to the back, and subtracting the thickness of the reflector material from the measurement.

Whilst f/d ratios to set up the perfect focus are great for the initial setup of the feed, there is (usually) far more to be gained (sic) by ensuring
1) the dish surface is not distorted, dented or warped in any way, and
2) the feedhorn is set right in the centre of the dish by varying the individual feed arms.

 

[Channel Hopper]

By the way the calculation of the focal distance of your reflector, if the dimension of 18.8" is used (assuming about 0.3" is safe to use for the additional depth behind the disc) - the f/d at 0.398 - at 47.87 inches. So I would hedge my bets that the dish reflector is a 0.4 design.

What make is it ?

 

[kelleyga]

I do not know what make it is. I got it for free. I will check tomorrow in the morning, but I think 18.8 as you said should be ok.

Now, I have some dents but they are not super dents. I tried to fix them but if I put my hand on one side the dent will go to the other. So I just left it going out toward the back of the dish but they are small. I also have a little part very little where the mesh is tear but very small. With my hands I aligned it and decided to put some electrical tape in the back, a small piece, IS that bad? there is not a hole thru.

Thanks for your help, I appreciate it.

 

[PaulR]

Well, as this is for C band reception then the dents aren't as important as if were for Ku band. Same for the rips in the mesh, don't lose too much sleep over them (unless they're as big as a chicken that is).

PaulR

 

[johnjr]

hi, one way to set up the feedhorn/lnb is to aim the dish at the sun and adjust for max sun noise....then look at where the shadow of the lnb/feedhorn is lying on the dish surface. Adjust the feedhorn/lnb until its in the centre of the dish. This ensures that you will not get a lobsided plane when aiming the dish at a satellite.
ie. Instead of the dish pointing directly at the satellite for max signal, you point slightly away from the satellite.

Once this is done, you can then adjust the lnb/ feedpoint focal distance for max signal.....remember if you over illuminate or under illuminate the dish, then you loose gain and signal strength.

cheers

 

[Channel Hopper]

The sun moves approximately 15 degrees every hour, so adjustment on the feed and arms would have to be rather quick. Additionally the weather conditions will cause peaks and troughs in signal so a clear day would really be the only option for fine tuning.