Building a prime focus dish for downlinking GOES full disk images

[Cyntrifical]

Greetings,

 I just recently acquired a 10ft (3.048m) Prime Focus Mesh Satellite Dish. It's not in too bad of shape for it's age (probably late 70's or early 80's?) and I still have yet to test the actuator arm or motor. Those aren't too expensive to replace. It currently has a Dual motorized C/Ku Band LNB feedhorn on it, which is in the range of 3.4-4.2 GHz, from my understanding, is almost entirely TV satellites. I'm looking to downlink GOES Satellites to receive full disk images of earth, and it seems I need some type of LNB in the L-band range of 1600-1700MHz (can't remember exact frequency at the moment).

 I've searched and searched online and it seems that there's not a lot of L band LNB products for prime focus dishes. Most are made with an Off Axis dish, however I've heard of other's using a prime focus dish for this exact purpose. Does anyone have any links to a product page for a receiver/lnb that will work? Or is there a DIY method for making my own? Would greatly appreciate any help! We are planning on mounting it onto a two wheeled vehicle trailer, and repairing the actuator arm and using a positioner system to control the direction of the dish. Thanks!

[Alpollo]

As long as you have the downconverter you need, you should just be able to locate it at prime focus it'll should work.

Assuming your dish was design for a higher frequency, means the mesh is tight enough to use for low frequencies, so no problem there. The diameter should be great too, I don't know how long a 1600mHz wavelength is off the top of my head, but your dish should be more then a 1/4 of it.

It might take a little trial and error to get the best focus, but just pop it on there however you see fit.

The focal plane is right at the entrance to the wave guide on the existing LNB , (the mouth of the round tube part) That is where you want to locate the entrance of your new downconverter. You might find slightly better focus in or out a bit, you could try it and see, but you wont be far off. 

If you don't need the current LNB any more, maybe you can carve out the center area to mount the new LNB which I assume will be smaller. That way you could reuse the support trusses as they are, by keeping the the outer ring section of the old LNB with the screw holes. You could even reuse the nose cone then.

 

It sounds like an interesting project, keep us posted if you don't mind.

[SilverAstro]

On 05/03/2017 at 17:07, Cyntrifical said:

,

 not a lot of L band LNB products for prime focus dishes. Most are made with an Off Axis dish, however I've heard of other's using a prime focus dish for this exact purpose. Does anyone have any links to a product page for a receiver/lnb that will work? Or is there a DIY method for making my own?

 

As Apollo says, an off axis feed horn will work if placed at on-axis focus and oriented right, (but may not be optimum for other weak sig applications )

If your  LNB can be dismounted from its off-axis horn then it is an easy job to DIY a new feed horn ( an LNB/down-converter can be DIYed but needs specialist electronics and would probably not be economic or time value these days ) If you can access the ground plane and output ( in old style LNBs it will be easy via co-ax ) then mount the ground plane to a copper cylinder/tube of approx 3/4wave dia. closed with copper disc at one end, mount a 1/4wave monopole in the cylinder connected to the o/p, this is your transition from the coax to the cylindrical waveguide.

The length of the cylinder will govern the angle of the feed pattern and how it illuminates the dish, too long and it will not efficiently exploit the full dia. of the dish, too short and it will spill the dish wasting gain (and in weak sig condx will pick up thermal from the background to the dish). Formulae are available online (mine are packed away in boxes in the attic ! )

Variation : (1) silver plate your copper for extreem efficiency  or (2) use a bean-can instead of copper tube for strong sig condx

 

[Dave Simons]

Guys I'm embarking on a similar project here in South Australia, so your comments are gold!  What do you intend to do to record and analyse your readings?

Good luck and keep us posted.