Anyone tried to make BIG dish at home?

[franksanderdo]

Hi fta-elite,

even though it does not look like in first view:
The results are very good!!

Testing on Ku is hard stuff for a home made dish as Ku needs accuracy down to a millimeter.
There are a lot of 180cm factory made (segmented) dishes which can not get close to a 90cm dish.

Your results show me that you have been very careful during production.
Good Job!!

 

[lnb15k]

Hi, fta-elite

Looks impressive your dish,but from the picture you make on the moonlight the focal point of the dish it s very large,beyound the feedhorn.20% of signal are lost.
You need to make this dish a cassegrain one,install the second reflector on the top for a better reception.Lots of professional dishes used for cassegrain one „V ” shape reflector.

How much increase in signal is expected if we assume to put a very good secondary reflector and a matching lnb for it?

 

[T_G]

Just a quick note to all you big dish builders: This thread is simply fantastic! I love the home made dishes and the idea that someone can design and build his own dish rather than go out and buy it.
There was a time where people would do that more often, and also repair broken equipment rather than simply buy a new one when all that was broken was a fuse.

So keep it up, I will continue to read this thread with interest.
Cheers, TG

 

[franksanderdo]

How much increase in signal is expected if we assume to put a very good secondary reflector and a matching lnb for it?

Hi lnb15k

I have done some trials with secondary refeloctors on one of my dishes and found the following:

1. You can improve a little a poorly focussing large dish.
2. It is some effort as you have to find the best "focus" of the large dish, and than produce a secondary fitting to that.
3. producing the secondary is far easier than the primary as it is much smaller
4. It helps a lot if you can adjust lnb distance to secondary dish for refocusing.

As my primary dish (1.4m) was prodicung in best position a 8cm diameter and 3 cm long focuspoint I did the trials with a 12cm secondary.
As the secondary has to have opposite shape I one of those wide range mirrors you can stick to you mirror on the car. With that I improved the signal appr. 5%.

See you around
Frank

 

[molloyp9]

Hi

As I worked in fibre glass many moons ago you coud buy one dosh then use it as a mould for making as many as you like fire glass and the resiin used is pretty cheap. It will work plus you have have all the holes marked for you.

Regards

 

[mouse3]

Secondary subreflector picture.

 

[mouse3]

How much increase in signal is expected if we assume to put a very good secondary reflector and a matching lnb for it?

Very good secondary subreflector are hard to be found.I think this items are not sold like spare parts.
The results depends only the patience to make the fine adjustment,move the tv set ant the receiver near to the dish and watch on screen the signal level on the tv when you make the adjustments.

 

[boyet05]

_http://boyet05.blogspot.com/2009/03/my-homebrew-satellite-antenna.html boyet05: My Homebrew Satellite Antenna

 

[JanH]

Hi,

Frank can you post some picture? :-)

What about subreflector and multifocus?

 

[franksanderdo]

Hi Jan,

sorry, picture (at this moment) is not possible as the 1.4m had to give place for a 2.4m.
Even though I am planning to give the 2.4m a secondary reflector I simply did not have any time to do so during the last months.

About your second question I am not sure what you are asking for.
I guess you ask for send lnb on different Satellite?
If so here what I believe (not tested):
You can point secondary lnbs the normal way using only the primary reflector. Using the subreflector will be difficult because you have to have the place for the lnb.
During my tests with the 1.4m dish the lnb was placed in the center of my dish using the the dish mount as a support. Seeing that I believe you might have to cut holes into your dih if you want second lnb using the subreflector. That would be counter productive ;-)

See you around
Frank

 

[fta-elite]

In a Cassegrain antenna the feed is located on or behind the dish, and radiates forward, illuminating a convex hyperboloidal secondary reflector at the focus of the dish.

Gregorian antenna is similar to the Cassegrain design except that the secondary reflector is concave (ellipsoidal) in shape.

Some questions:

Can a Cassegrain subreflector be used as a gregorian subreflector or viceversa?

If I want made a Cassegrain subreflector, how to calculate the size and the shape for my antenna (or any antenna)?

How to calculate the focus in a Cassegrain antenna?

regards
fta-elite

 

[JanH]

Hi Jan,

sorry, picture (at this moment) is not possible as the 1.4m had to give place for a 2.4m.

Frank

I see. It do not looks too practicle. I saw subreflector on offset multifeed feed antena, but it is other issue.

 

[franksanderdo]

Hi JanH

Subreflectors on in offset Multifeed are not that much another issue, but thos are very hard to kalkulate (at least for me ;-) )
As I said it is a bit tricky to modify a prime focus dish for holding the LNB. I was lucky with my 1.4m as there was the structure of the disk and the mount helping a lot. I only needed some hose clamps to get the LNB fixed. Not nice, but working :-D

Hi fta-elite

The calculation for the sub reflector is very similar to the main reflector.
The definition of focus length is again the distance to the lnb. The diameter and the depth I have seen some where can be calculated for optimum gain. I figured it more like:
Diameter as small as possible (used the feed horn diameter) depth was then calculated out of focus and diameter.

I only tried the cassegrain configuration, but I believe if you reflector follows the same shape on both sides (mine did not, it was flat on the back side) you easily can try both.

See you around
Frank

 

[JanH]

One of my curious fancy.
I am thinking about configuration with long focus subreflector and LNB behind dish looking throw hole. If possible it may help to do multifocus on round dish.
:-)

 

[franksanderdo]

Hi JanH

you want to do multi focus with 1 LNB?
That wil be difficult as I believe you will end up with some disturbance.

See you aorund
Frank

 

[JanH]

No, I can imagine two or three LNBs looking throw eliptic hole. It could be still in subreflector shade area. Of course depends on subreflector size.
For fun... Motorized subreflector??

Next one - I know, today is not joking day :-)
What about two offset antennas in mirror position with one LNB. Two 80 cm offset dishes like 160 cm round dish. Surface looks same.
Some dishes have LNB not too much inside, maybe it is possible to change configuration with LNB outside shape and a bit different antenna angle to sat. Looks very easy to do. It could be fasten in one pylon vertically. Only difference is LNB holder.

 

[fta-elite]

"What about two offset antennas in mirror position with one LNB. Two 80 cm offset dishes like 160 cm round dish."


Hi janH

It must work because an offset dish antenna has a reflector which is a section of a normal parabolic reflector.


source: _ttp://oz2oe.dk/radio/10ghzparabol/offset_geometry.jpg

What about six offset dishes to get a prime focus antenna, but with a 80 cm offset dishes you get a 140cm round dish.

 

[franksanderdo]

Hi folks,

as fta-elite said:
Offset antennas are just a section of normal parabolics.
The trouble you get is the exact positioning of those antennas.
As you can see in fta-elite pictures you end up with a certain overlapping which makes the positioning almost impossible.

But writing this I hav another Idea which might be worse looking at:
If you separate the dishes a bit more then in the picture. Use a sub reflector to optimize the poor focus (6 focuses) and place the LNB as you would on a prime focus sub reflector dish.
With such a configuration you might be even able to use all the 80cm of each dish "segment".

would that work?

See you around
Frank

 

[JanH]

I understood that six antenas are in plain configuration with deflector in focus point each all pointed to one LNB in the middle of system.
I am not shure if possible to send signal to LNB using marginal angle like this. Feedhorn is antanna as ordinary one with ordinary pattern. Second problem which make gain losses is that feedhorn can "see" open space in middle direct and main course. It made additional source of noise.
I think it is possible solution to use further deflector instead LNB and point it to LNB in centre of the system looking down to hub of antennas. Loks a bit complicated and series of deflectors make efficiency of system worse regardless system adjustment problems.
I am not shure that offset antenas are pure parabolic shape. LNB is not in right position and looks to midle of offset dish. If parabolic it should be pointed to centre of virtual parabole out of offset dish. It is similar but modify.

 

[fta-elite]

Hi Franksanderdo
I don't believe it work, because you must keep the dish into its original parameters, if you move a bit each dish, maybe you can focus all at the same point, but each dish would be pointing to a different satellite.

To avoid overlapping the offset dishes, the dishes must be cut in triangular shape but my idea is to use the back of a offset dish as a mold to made six triangular segments and so get a prime focus dish.

Hi JanH
I made this graphic for you, to confirm that the offset antenna has a parabolic shape.