Cassegrain Vs Gregorian

[koansrc]

If the subreflector was at the same offset angle as the dish, (as is the case with a Prime dish) it would be round, but the LNB would then shadow some of the dish, thus negating the advantage of the offset.

Agreed johnsatuk. The question is how much would you gain by using a subreflector and how much would you loose by creating a shadow.

I'm tempted and thus considering this because on my "clunky" 3m Channel Master there is already a considerable shadow from the struts and the lnb holder etc and in the center of the dish there is a hole about 30cm wide, which I reckon corresponds to the shadow created by the original C-band LNBF and protective "bucket" (which is about 45cm round).

I *think* the attached excel attempts to answer the question "is it worth it" .. but it's over my head.

 

[johnsattuk]

I'm tempted .

Go on, you know you want to :-righton

Should give some interesting results

 

[johnsattuk]

This might fit in with your experiment

Ebay 270198870234

 

[koansrc]

This might fit in with your experiment

Ebay 270198870234

HEHE! No, that's old news. I've researched those LNBFs and came across the china factory that makes 'em :

http://www.satellites.co.uk/satellite/dish-setup-single-sat-multi-sat-motorised-systems/109857-prime-focus-c120-lnbs.html#post678806

I have now moved on and considering a subreflector from something that looks more like this:

http://cgi.ebay.co.uk/14inch-Professional-Non-Stick-6-Piece-Wok-Set_W0QQitemZ190185582689QQihZ009QQcategoryZ109065QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

I'll keep the lid and give the rest as a late Xmas present to the wife!

 

[Channel Hopper]

A dish that has not been designed as a Cassegrain/Gregorian is unlikely to be improved by fitting a secondary reflector, even if you got the measurements right.

 

[koansrc]

A dish that has not been designed as a Cassegrain/Gregorian is unlikely to be improved by fitting a secondary reflector, even if you got the measurements right.

Well I was impressed with the tecatel 1.35 which I originally bought as a standard offset. I was losing channels in bad weather and so bought the gregorian kit for it and it made a difference. +0.7dB according to the manufacturer. So this is an example of a dish that can work both ways.

Would you believe I actually spent an entire day looking for a suitable subreflector ... 30cm to 40cm diameter, 4cm to 6cm deep, but no luck. If and when I eventually come across one I'll give it a try.

 

[Channel Hopper]

Well I was impressed with the tecatel 1.35 which I originally bought as a standard offset. I was losing channels in bad weather and so bought the gregorian kit for it and it made a difference. +0.7dB according to the manufacturer. So this is an example of a dish that can work both ways.

Would you believe I actually spent an entire day looking for a suitable subreflector ... 30cm to 40cm diameter, 4cm to 6cm deep, but no luck. If and when I eventually come across one I'll give it a try.

Almost any mod will improve a bog standard Tecatel dish. :-googly

 

[koansrc]

Oh what the heck I'll post the photos ... of my gregorian experiment. I had to try and now I feel better for it

Can't find a suitable subreflector yet, so I used the offset(!) subreflector from a tecatel dish and a 0.3 lnb, adjusted the focal length, wired everything together and ended up (believe it or not) almost at where I started. Almost.

mti 0.6 with adjustable feedhorn : BER 1.6E-7, C/N 13.1dB
maximum 0.3 with gregorian : BER 1.6E-7 to 3.8E-6 (unstable), C/N 12.7dB

I think it actually went well considering :
- flimsy construction with wire
- unable to try subreflector at a longer focal distance due to the poles and lnb holder not being removed
- using an OFFSET subreflector !!

I actually tried it as a cassegrain too. But I ran out of daylight and couldn't get a stable result.Ccouldn't keep the thing in place. Although at one point I was again getting about the same readings.