Coupling two 1.80m dishes as one 3.60m dish

[al_madhi]

Hi guys , wondering Is it possible to couple two 1.8m dishes to act as if it is one 3.60m dish by connecting the outputs of the two 1.8m dishes through a combiner switch ( two in & 0ne out ) providing the two 1.8m dishes are aimed at the same satellite which requires 3.6m dish in order to receive it .

Any thought you would have be greatly appreciated .


cheers

 

[n4bbq]

Two 1.8m dishes does not equal one 3.6m dish.

You are onlly doubling your surface area and if I remember correctly, that is only about 3dB of gain over the 1.8m alone.

Then do get another 3dB over that, you'd have to add two more 1.8m dishes.

Look at the surface area of a circle to give you an idea.

A=pr^2
A[1.8m] = (3.14)*(1.8m/2)^2 = 2.54 m^2

A[3.6m] = (3.14)*(3.6m/2)^2 = 10.17m^2

The 3.6m gives you a lot more surface area to snatch RF from the air. And with a parabola, you actually have more surface area involved than a circle.

 

[rkrenicki]

And the combiner will have a 3 or 4db insertion loss, so if anything there will be a net loss.

 

[Wholeshoe]

Plus you can't tell if they would be in phase with each other if they were not you could have either adding or subtracting of the carriers and a detriment to the system. You would also be adding in the noise of the second LNB hurting your CNR by 3 dB.

I don't think it would work very well but it might be fun for somone to get 2 18" dishes and play with it.

Wholeshoe

 

[Wholeshoe]

My boss just said that Paul Allen is investing money in something like this for SETI. They plan on using a bunch of 3.8M dishes to simulate a larger one.

It is also similar to matching antennas in a cable system to give better directivity to picking up the signal. I know when doin that you have to match your cable lengths before going into the two way combiner.

Wholeshoe

 

[Wholeshoe]

In giving this some more thought if what you are trying to do is build a c-band reception antenna out of smaller antennas it won't work. It would actually be worse than a single dish with a c-band lnb. This is because the size of the dish is what determines the selectivity of seeing a single satellite. With two smaller diahes you would have twice the amount of adjacent satellite interference than a single dish of the equilivalent size.

Wholeshoe

 

[al_madhi]

Two 1.8m dishes does not equal one 3.6m dish.

You are onlly doubling your surface area and if I remember correctly, that is only about 3dB of gain over the 1.8m alone.

Then do get another 3dB over that, you'd have to add two more 1.8m dishes.

Look at the surface area of a circle to give you an idea.

A=pr^2
A[1.8m] = (3.14)*(1.8m/2)^2 = 2.54 m^2

A[3.6m] = (3.14)*(3.6m/2)^2 = 10.17m^2

The 3.6m gives you a lot more surface area to snatch RF from the air. And with a parabola, you actually have more surface area involved than a circle.

To - n4bby thanks for reply,
I know I made a mistake I should say coupling 4 X 1.80m I just rememberd that after I hit ( Enter ) key . Any way the main thing here is about coupling

best regard

 

[rkrenicki]

4 1.8m dishes will be closer to the surface area of a 3.0m dish, but with the combining, your going to run into all sorts of trouble.

The doubling of your signal gain (or quadrupling in this case) will be negated by the insertion loss of the combiner. That and if your cables from the LNBFs to the combiner must be exactly the same length, otherwise you will have phase problems, especially with digital signals.

That, and you will increase your noise floor by about 3db per dish... so your carrier to noise is going to be in the gutter.

Adding more dishes, is just going to make it worse.

Now, how the SETI stuff works, is generally by shifting from dish to dish sequentially, or using seperate recievers on each dish and comparing the output.

 

[Wholeshoe]

Thanks for the SETI info ultatryon.