Difference between Prime Focus and Offset

[ssjenkins]

Hi ,

Does the prime focus offer any advantages over the offset ? My reason for asking is that , where I live SLY is available with a 1.8 prime focus but not with the 1.8 offset . Why would that be ?

Thanks

Sammy

[chrisspain]

There is no reason, the offset is simply put a slice (like from a cake) of the prime focus, so a 1.80 offset is a part of a 3.6m Prime focus, however, on the prime focus the lnb is shadowing part of the dish but at the size of these things, it makes no difference. Also, the offset is a better choice in snowy conditions since the snow is basically sliding off and not collecting inside the dish. (I did have to shovel snow in the 80s at my work inside the big 5.8 and 11m dishes,no heating, those cheapskates)

So basically, if in your area 1.80 receives Sly, you get it with 1.80 offset BUT! it will be difficult to find a good one, most dishes sold as 1.80 perform only slightly better then a good 1.20, these things cost money and a prime focus is much cheaper, also usually available used.

Chris

[Detlef]

We've had this discussion before over on Dudez.

A 180cm prime presents the whole of its area to the sat (less a tiny bit for the LNB shadow).

An offset is predominately, at an angle to the sat and thus as viewed from the sat is less that 180cm high but is still 180cm wide (ie it looks elliptical to the sat) thus it presents a far smaller area.

I've not actually seen an offset above 120cm though.

Also offsets are frequently designed for a specific latitude such that they appear vertical. Remember a dish on the equator pointing at a sat on the same longitude would be pointing straight up - fortunately there is not to much snow on the equator :D

[satwyn]

I use both p/f `1.8 precision and andrews 1.8 o/s the andrews performs better on weak signals using ivacom .3 lnb the precision is more versatile c band mostly

[ekon]

slightly off topic...but just replaced an old Strong 0.6 P/F lnb with an Invacom (single) 0.3 with Invacom (adjustable) feedhorn on my 10.8m P/F dish..not a lot of difference disappointingly! I spent an hour with the set-up..& I may play around with it a bit more.. but not sure I should have bothered!

[Fil]

my 10.8m P/F dish!


10.8M!!! pictures of your setup please!

[Detlef]

. . . on my 10.8m P/F dish . .
Do you live at Jodrell Bank or Goonhilly Down by any chance :D:D:D:D

[ssjenkins]

Thanks for the replies everyone . I must have missed the thread on Dudez .

Sammy

[satwyn]

10M dish tell us more did you buy from nasa

[ekon]

only just seen my typo...yes I meant 1.8m P/F dish..10.8m..I wish!

[TonyO]

P/F are ok for static positions or motorised but useles for more than one lnb, any gains the P/F shape have are lost on small dishes as the 3 feed arm supports and the lnb obstruction bring them back to about the same.

[Detlef]

. . . . . I must have missed the thread on Dudez .

Sammy
It wasn't meant as a criticism but I had hoped Kink would have put his site up again so you could have had a read there.

Det