Re: [amsat-bb] BIGGER dish

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

Bob,

I'll take a stab at this.  I have not actually had a primestar in my hands
so making some assumptions.  I will assume they are both the same oval
shape so if I calculate areas as rectangles the ratio between them will be
the same and that will give the gain ratio which is what you asked for.
{i.e. gain is proportional to dish area}

existing dish: 26x36 = 936
1.2m dish: 48x52 = 2496
ratio of area = 2495/936 = 2.67
gain = 10Log(2.67) = 4.26 dB

Another assumption:  Primestar uses the same LNBF in commercial use for
both dishes so the dishes will have the same shape-factor and f/d.  If
true, you can use the same helix feed.  Of course the focal distance will
be farther from the surface for the bigger dish (hope you have that info).

Whether your G5400 will hold up is one for the crystal gazers (haven't a
clue), but its probably on the high-end side of what the rotator can
handle.  You can actually calculate the dish area and maybe have the wind
area specs for the two yagis (from the mfr).  If so, add them up and
compare with what Yaesu says in the G5400 wind load spec.

As far as predicting squint performance, I suggest you use the AO40rcv
software (should be references on one of the amsat-NA/DL/UK web sites).  It
takes into consideration your NF, gain, dish gain, satellite range, squint,
uplink power, uplink gain, etc. to give you signal level and SNR.  You plug
in your numbers and the software gives you answers!

GL,
Ed, AL7EB

At 08:15 PM 7/25/2002 -0700, Bob V Johnson wrote:
>Hello
>I am currently using a primestar dish (26"X36"), a 3731AA downconverter
>with 5.5 turn helix feed.  I have a 1.2M primestar dish (48"X52"), I am
>thinking of using it, the following questions come to mind.
>1.  how much more gain would I realize?
>2. what size helix for proper illumination?
>3. will my G5400 rotor take it, in addition to a 2M 8el and a 70cm 18 el?
>4. how much  more squint angle can I get?
>Any other information any of you might feel is pertinent.
>73...Bob...W7LRD
>Seattle, Wa.
>"everything is a learning curve"
>
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