Some big dish questions ?

[Vipersan]

Hi friends ..
back again to pick the brains of the resident big dish experts..

Some 20 years ago I purchased a 1.8mtre dish and polar mount 2nd hand ...and I have used it quite extensively in the past..
..mostly during the analogue years.

I have no idea who manufactured it but identifying it would be a start.
It is a six petal solid metal dish
The centre of which has a black discwhere the petals terminate.
Its cream in colour and has 3 support legs mounted from the outer edges of 3 of the petals which terminate at the lnb mount plate.
These 3 support legs appear to be made of some kind of carbon fibre.

Origionally there was a mechanical motorised polariser ..a large blue block mounted to the inside (dish side) of the lnb mount plate ...but there was never what you could describe as a feed horn or scalar plate ..simply a waveguide tube mounted at the focal point.

I no longer have this assembly as it fell to bits some years back.
Instead I improvised a feed assembly in order to mount a c120 universal lnb.
The approximate distance from the plate at the botom of the dish ..to the assumed focal point is roghly 31 > 32 inches.

Id very much like to put this old dish back ito service and play around with both KU and C band if its suitable .

My questions are ..
Does anyone recognise the dish from the description ?
Does anyone know the correct distance to the focal point ?

what is the correct type of feed horn/plate for this type of dish ...and why would there have been nothing more elaborate than a waveguide tube origionally ?

Im absolutely convinced the dish is not correctly illuminated by the feedhorns I have tried in the past ..as I would compare signal strength or gain (ku band) with a 1 metre offset dish..
..its as though most of the dish surface area providing wasted signal.


Any info or help regarding the dish ..and making it useful again would be excellent.
..and yes ..after reading another thread on c band reception (unknown territory to me) ...I ordered one of those cheap C band lnbs on ebay.

rgds 2 all
VS

 

[Robbo]

Might be an idea to post a photo? Would certainly be easier for someone to recognise I would have thought.


The correct distance to the LNB can be calculated based on the diameter and depth of the dish:-

_http://www.satlex.de/en/fdratio.html


The scalar rings on the feedhorn are there to narrow/focus the LNB onto the dish. Some feedhorns are adjustable like the Invacom adjustable feedhorn. With these, with the rings fully forward the beam is narrowest and when it is moved to the back it is widest.

I noticed on an old prime focus dish (1.8m) that I played with for a while, that like your feedhorn, there were no scalar rings, but what just looked like an open ended waveguide. Though I can confirm that it appeared to work well, and I was not able to improve on it using the adjustable feedhorn instead.



What feedhord are you using at the moment? As a standard one designed for an offset dish will work very badly, with performance similar to a 1m dish or thereabouts. This would be because the F/d ratio would be too large (narrow) for the dish.

 

[Vipersan]

Thanks for replying Robbo..
I'll take a few pics on sunday ..weather permitting.

..I'll include a picture of my homebrew feed assembly.
Im using a prime focus feed with scalar rings ...but as you say ...they appear to do very little to iprove the signal.
Im convinced Im not getting the most out of this beast ...as my TD110 outperforms it totally.
I mounted the mouth of my scalar feed as close to where the mouth of the open ended waveguide tube origionally was....but the feed was designed for a smaller ..1mtre prime focus I suspect.
rgds
VS

 

[Vipersan]

..Still 'persisting down ' here atm ...so pictures of the dish are delayed.
I'll try and snap it later today.

In the meantime I got the dish measurements to calculate focal distance.

Diameter = 182cm
Depth = 25.5 (assuming the piece of wood I stretched accross was true)

calculator returned :-
Focal point (dist to lnb) = 81.19cm

F/D ratio 0.45

..a quick measurement showed me I have it almost correct ...insofaras ..the lnb feeds waveguide opening is at 80cm

would 1cm differance be enough to account for the dishes poor performance ?
I have my doubts as the origional measurements could be innacurate by a few mm.

I 'll get those photos up asap.
rgds

 

[Robbo]

1cm out would not make the dish perform that badly. It won't be optimised though of course. I suspect that the problem is the feed horn not being matched to the dish, which is where the adjustable one can come in handy. As I said, I managed to get similar results with it as I did with the matched feed, but didn't use the adjustable in the end as, well I had the matched feed so there was no need.

Never touched a petalized dish, the one I played with was solid, but that could be a problem too, if you have not all the petals aligned properly.



Just as a side note, I stuck a Sky minidish LNB on it as well, the dish then did actually perform better than a TD110.

 

[Vipersan]

..hope I did this right ..

 

[Robbo]

This is better:-

 

[Vipersan]

Thanks Robbo ..
I could do with an idiots guide as to how you did that ..lol
cheers m8

No actuator arm on the dish atm ..as its well rusted.
I intend to replace it ..if I can get the dish to perform better.
..also ignore the exposed f connectors and wiring..
Thats just temporary whilst I connect a sat meter.

..you can see what I mean about the feed assembly being small ..and that it may not correctly illuminate the dish.
Chances are I'm only using the centre area of the returned dish signal.
rgds

 

[Robbo]

Click on the manage attachment button under additional options:-

 

[Vipersan]

For the sake of clarity ..and to help with the dish identification..

I've modified the feed image to show what it would have origionally looked like..

 

[paranut]

Hi

Did you get any feedback on this dish?

I have the same one I am just putting together so it may save me some grieve if you've already got some answers.

The FD of the dish being .45 I guess will rule out a lot of LNBs. I have one that came with the dish so I'm hoping it will be suitable but it is old. A website I found suggested it was a 2.3dB. It is labeled a MEGA800 with a Swedish Microwave noise reducer. It has a separate voltage input on the side, but I am unsure if it required AC or DC and the voltage, so for now I'm going to try 12V DC and see how that goes, unless anyone here can shed some light before I dive in? Or maybe I should just go for a new LNB. Any suggestions?

The intended use for the dish is to receive Sky broadcasts. I live in Slovenia so I'm on the outside of the footprint, needing a 1.8m dish theoretically. All set to go, apart from plugging the LNB in and aiming the dish.

Regards
Brett

 

[Vipersan]

Hi Paranut...
Yes I did find out who made the Dish ..
Apparently a firm called Alcoa
and it was a very nice dish in its day ..
It was equipped to receive ku when I bought it second hand ...but with a motorised mechanical polariser feed ...which as you saw in the photo ..mounted to the inside of the mount boss ..
This puts the focal point a good few inches inside the boss ..
I calculated the focalpoint to be 81 > 82 cm from the dish centre to the lnb feed assembly ...and this proved to be accurate under real test conditions.
The dish wasn't really designed to use a scalar feed horn ...but simply a tube wave guide whose 'throat' was located at the focal point...though I have had some success in the past with scalar feeds set slightly back from the focal point ..
For this reason you would be advised to buy a new lnb with matching adjustable feed and scalar plate ...such as the Invacom C120 LNB & Feedhorn (see ebay)
By using this assembly you will be able to move the scalar rings and optimise the signal gain at the focus.
You will however have to do some engineering to get the feed/lnb a little closer to the dish ...as it doesn't naturally fall there if you mount the new lnb on the boss (remember the mechanical polariser occupied a few inches of space)..
You might achieve this by shortening the carbon fibre tripod supports ...or by engineering the mount boss closer to the dish face by a few inches ..
How are your DIY skills ;)
Its a superb old dish ...and I currently use mine for C band ..with great results ..and its nice to see another one back in useful employ..
Let me know how you get on ?
Some pics would be nice
rgds
VS

 

[Vipersan]

This might help you Paranut

The picture shows my modified ku feed /lnb & boss mount

The green line is approx 9.3 cm
The Yellow line approx 10.8 cm
The Red line shows the original mechanical polariser stripped down and re used with the actual feed section of the original assmebly removed and replaced with a modern ..though not matched scalar feed (Blue Line)..
This 'Blue Line' section is what you will replace with the invacom adjustable feed..
If you have the mechanical polariser ...you could also strip it down to use as wave guide extension (the back section 'Red Line')
Best of luck m8

 

[paranut]

Hey, thanks for all the information. Very helpful!!!

I spent hours researching it on Google yesterday too and so I'm a lot more clued up than on my original post and came up with the following info.

As far as I could tell, the manufacturer was called Patriot. They made a dish that matches the dimensions of this dish and the build type. I can't find any info on Alcoa, maybe they were a distributor? See link below, where I got some info;

_http://www.bigdish.info/mainten/lfocal.html

It gives an Focal length of 81cm and an FD of 0.45. It was used by large commercial operations so was expensive and of good build quality, despite it being a 6 petal. After assembling mine and measuring it it is without distortion and damage. Yippee. That said, I can't find any pics of a Patriot, so I can't be 100% that this is the manufacturer, but it all fits..

The problem is, as you say, getting the LNB/feed sorted out. The Invacom feedhorn only goes to 0.43, so if that is used it will show about 9% background noise, beyond the rim of the dish. That's a fair bit. I haven't been able to track down a feedhorn that will allow for 0.45, but I am sure they exist.

My current plan is to use the Invacom feedhorn, but to take the adjustment negative. Hopefully that will give enough shielding of the outer dish. I'll combine it with the Inverto Twin C120 as I have a 40 meter cable run so the Invacom LNB may struggle with that cable length. Other than that, just a lot of fiddling to do to get it all in the right spot. My DIY's ok, so hopefully I'll get right in the end. I'll keep you posted!

I'll forward some pics later...

Brett

 

[paranut]

I found this website useful for pointing the dish fairly precisely. Just wait for the sun to move around at the allotted time and you have a direction for your satellite.

_http://www.satellite-calculations.com

According to this calculation if I make the adjustment on the 10th at 11.47am then the sun and the satellite are in the same position in the sky for me. Perfect. A small hole drilled into the center plate on the dish and the LNB removed, all I have to do it line up the dish so I see the sun (wearing sunglasses I think) through the holes.

Brett

 

[Vipersan]

Glad the info was useful Brett ..
There's a nice picture or 2 of one of these dishes ...complete with petal extensions in one of the thread sections I posted here >

http://www.satellites.co.uk/satelli...7-pictures-members-systems-26.html#post870667

..and its where I found out about Alcoa..
rgds
VS