Newbie Here Prime focus vs offset dish

[MeanMachine]

Hello all. I've been a member of this forum for a couple of years now, still learning new things about satellite technology.

apologies if this is a very basic question: why are prime focus dish antennas preferred for C-band reception? whereas offset dish antennas for ku-band reception ?

are there significant differences in signal quality if they are interchanged?

thank you

 

[Channel Hopper]

Though your suggestion is not strictly true, it usually boils down to price.

Keeping a C band antenna on station requires less accuracy, so a cheaper parabola can often cope.
Additionally a C band signal usually requires a larger reflector.

If one can make all pieces the same of a segmented dish then axysymetric is the way to go.

 

[MeanMachine]

I was finally able to do a test. Ku band signal quality with 1.2m offset dish is significantly better compared to same signals with 1.2m prime focus dish. Tried to keep everything else as same as possible. Is this conclusion in-line with the theory? Or should I give another go to the dish alignment for better results with prime focus?

 

[RimaNTSS]

Is this conclusion in-line with the theory? Or should I give another go to the dish alignment for better results with prime focus?

Theoretically, performance PFA and Offset should be more or less same (if you neglect shadow effect in PFA case). Could you show some pictures of your antennas? I hope you used PFA LNB on PFA dish.

 

[PaulR]

... Tried to keep everything else as same as possible.

Does that extend to the LNB? Using an LNB designed for a prime focus dish on an off-set dish (and vice-versa) will give sub-optimal results.

 

[Satfaca]

I was finally able to do a test. Ku band signal quality with 1.2m offset dish is significantly better compared to same signals with 1.2m prime focus dish. Tried to keep everything else as same as possible. Is this conclusion in-line with the theory? Or should I give another go to the dish alignment for better results with prime focus?

Absolutely not in line with the theory ... but in practice people get this wrong impression for various reasons, one being the fact that an offset antenna is easier to align due to its bigger beamwidth. And as chaps above mentioned, people simply use one and the same LNBF on both types of the antenna which is absolutely wrong; PFA requires a dedicated PFA feedhorn.
Also the exact focal distance is much more critical with PFA.
So shortly, offset is easier to align & has same gain as an PFA, on the other hand PFA is more accurate in pinpointing particular satellite, receives less sidelobes and through this less disturbances from neighbouring satellites.

 

[MeanMachine]

Using the LNBF in the picture below for prime-focus.

And Inverto Black Ultra for offset. Will try to play around again when the weather permits. Thank you

 

[Manikm909]

Using the LNBF in the picture below for prime-focus.

And Inverto Black Ultra for offset. Will try to play around again when the weather permits. Thank you

guess it should be one like this to minimise signal block

Invacom 0.3db C120 Twin Output LNB

Established supplier of quality satellite TV equipment and accessories shipped here in the UK and worldwide

www.pulsat.com

however, i have no experience with PF dishes

 

[RimaNTSS]

Using the LNBF in the picture below for prime-focus.

Could you post the pictures of your antennas?

 

[Channel Hopper]

Could you post the pictures of your antennas?

I would ask for overall size, focal distance, f/d ratio and any modifications with the existing feed setup. outside the manufacturer specification.

 

[RimaNTSS]

overall size, focal distance, f/d ratio and any modifications with the existing feed setup

I think is too much! A coupe of pictures would be more than enough.

 

[Channel Hopper]

I think is too much! A coupe of pictures would be more than enough.

If generic or clone, but a properly designed reflector means the manufacturer has the overall characteristics of performance if correctly distributed, delivered and with the matched feed up front.

 

[RimaNTSS]

I meant not asking too much from author till I see at least some evidence.

 

[MeanMachine]

Could you post the pictures of your antennas?

Had to move the setup due to weather. Here is a screen grab from a short video captured a couple of days ago. There are no modifications to the reflector, nor is it damaged in any way except some minor rusts at corners. However I couldn't find any focal distance or f/d ratio specifications. This is a rather cheap made-in-china dish antenna.

 

[ozumo]

Had to move the setup due to weather. Here is a screen grab from a short video captured a couple of days ago. There are no modifications to the reflector, nor is it damaged in any way except some minor rusts at corners. However I couldn't find any focal distance or f/d ratio specifications. This is a rather cheap made-in-china dish antenna.

Looks like one of those 1.8m Jonsa dishes which are known for poor performance in the Ku band. In the photo it looks like the LNB isn't pointing at the centre of the dish though it could just be the lens of the camera making it look that way.

PS Please upload your photos to the forum rather than linking to photo hosting sites.

 

[Satfaca]

Agree, that kind of antenna & setup would probably match performance of a premium brand offset dish of 1.2m.

 

[MeanMachine]

Dish is indeed similar to "Jonsa dish" mentioned by @ozumo. All three support bars are equidistant from the LNBF though. Assuming the reply from @Satfaca is sarcastic, it seems the quality of my prime-focus setup is not up to the mark.

Thank you for the replies

 

[Satfaca]

Don't get it wrong, no judgement, no sarcasm, just facts.

 

[RimaNTSS]

Ku band signal quality with 1.2m offset dish is significantly better compared to same signals with 1.2m prime focus dish.

Could you show the pictures of your 1.2m PFA and offset dishes? Somehow, I start to doubt you have them at all.

 

[MeanMachine]

It's interesting you say that. Prime-focus dish is dismantled at the moment, thanks to the weather. Here is a picture of the offset dish taken a short while ago. Its manufacturer produces commercial dish setups as well, so possibly is of better quality as suggested earlier.