Channel Master 1,2m issue

[RimaNTSS]

And this is my Channel Master 1,2m Frankensteined to Gregory-type

 

[danristheman]

Rima whats your results from this and whats your dimensions on the sub reflector?

 

[RimaNTSS]

Results with this modification are better than in CM1,2m original setup. How much better.... perhaps up to 10%. Working area of the SUB is 239mm (horizontal) and 145mm (vertical).

 

[RimaNTSS]

Working area of the SUB is 239mm (horizontal) and 145mm (vertical).

Sorry, it was typo: SUB is 239mm x 245mm

 

[Titanium]

Doesn't the feed shadow a significant amount of the reflector from the sub-reflector? The feed would appear to block 20%+ of the reflector surface.

Wouldn't it make more sense to design the sub-reflector to focus to the feed positioned below the reflector illumination angle?

 

[Trust]

Doesn't the feed shadow a significant amount of the reflector from the sub-reflector? The feed would appear to block 20%+ of the reflector surface.

Wouldn't it make more sense to design the sub-reflector to focus to the feed positioned below the reflector illumination angle?

That crossed my mind some years ago to , but I never tried it .
I think it be worse , just like bringing the lnb in a much lower position on a offset dish .
I once tried to receive C-band on a original Fibo gregorian dish with conical scalaring (TRF) ( 160 mm dia ) in front of that subreflector .
Got the same signal with or without the TRF but it workt .
Later I used a reduced ( 90 mm dia ) scalaring , same dia as the original Fibo Ku feedhorn , with a little signal progress .
I still use that C lnb at my 6 lnb revolver .

 

[RimaNTSS]

Doesn't the feed shadow a significant amount of the reflector from the sub-reflector? The feed would appear to block 20%+ of the reflector surface.
Wouldn't it make more sense to design the sub-reflector to focus to the feed positioned below the reflector illumination angle?

That is the beauty of offset Gregorian antenna design- feed cannot block any main mirror of subreflector surface. Attached picture explains that (I hope).

 

[Titanium]

Thanks for the excellent photo demonstrating the Gregorian design, RimaNTSS! I hadn't considered the placement of the sub was beyond the convergence point.

Do you form your own sub reflectors or are they repurposed?

 

[RimaNTSS]

I hadn't considered the placement of the sub was beyond the convergence point

Right, Gregory type sub (ellipse) is located after Focal point of the main mirror. Cassegrain reflector (hyperbola) is located in front of the Focal point.

Do you form your own sub reflectors or are they repurposed?

I was thinking about that a lot. It is possible to calculate it's shape, but, unfortunately not so easy to make/reproduce it. So I am using subs from dishes made many years ago, it is still possible to buy them (not easy, but possible).

 

[Trust]

@ Rima , I assume you draw lines from top and bottum rim of the CM to the oposit rims of the subreflector , but are they realy the reflection of the incomming signal ?
I once made a drawing of my 120 Fibo pointed to 4.8ºE (elev 30.7º) reflected the lines of the incomming signal .
Shows a complete other picture .

 

[RimaNTSS]

Main reflector of the Gregory dish is nothing but simple offset dish and it has one single focal point. I do not clearly see that focal point on your drawing.

 

[RimaNTSS]

Some people like multifeeds ( I am not one of them ), but, anyway, knowing that those great Channel Master dishes are not very good for multifeeds, decided to make one. Made a new bottom-feedarm and extended little bit side-arms. Now, multifeeds are more possible. That does not mean I will be using this dish with multifeed..... did it for research purposes

 

[danristheman]

Rima you are starting to look like my setup now that does look nice on that dish.

 

[RimaNTSS]

danristheman I will not be using multifeeds on CM dishes, at least in the nearest future. Created this just for experiment and for other users to have an idea how that could be done. There are couple of more pictures- working multifeed for 4,8*E-13*E-19,2*E. Main issue when making multifeeds on CM dish is the fact that feedarms are to close to each other and not allowing to place additional LNBs. But if LNB is added than feedarm is blocking part of the signal.

 

[danristheman]

Rima it does still nice the way you have used those certain parts like that.

 

[RimaNTSS]

Even if CM 120cm has some issues (as from start of this topic), it is still nice dish. One of my favorite dishes.
I am in process of USALS motorizing it. Made this drawing and already nave some progress.

 

[Cham]

Was thinking of doing that with my 1.2m channel master. Transferring the weight load to the two bearings would make quite a difference to the stress on the positioner. Trick is getting the offset angle exact... Nicely done so far!

 

[RimaNTSS]

Main idea of this design is that turning axis goes through center of gravity (big red dot on the drawing). That means that antenna will go out of end-positions without any difficulties. I do not see offset angle trick, as mount has 2 turnbuckles for elevation and also declination adjustment (circled yellow).

 

[Titanium]

RimaNTSS , As I have mentioned before, You are a mechanical artist! Beautiful fabrication!

 

[RimaNTSS]

I am finishing with the USALS mount for Channel Master 1,2m. Need to wait till the good weather to go outside and install it. Mount allows to move antenna H-H depending on the motor used. My motor only able to go 160*, which is more then enough for my Lat to still have H-H. And VIDEO.