Offset v prime focus chat

[radioagogo]

Admin edit - this topic has been split off the Astra 1N chat thread



Hola Chelsea Baz, I think it's been mentioned before but an offset dish of the same size performs better than a prime focus, with a 1.5 and good LNB you should be ok but some older sky boxes that I know of are deaf. The box Milestone11 has is as sensitive as my S10 which outperforms anything else I have tried so far. I have some friends with 1.35 metre dishes up here that I will be sperimenting with soon, will keep you posted.

 

[Huevos]

an offset dish of the same size performs better than a prime focus

That's pretty much a myth. On a 180cm dish the cross section of a Ku feed horn (8cm diameter) doesn't even block 0.25% of the total dish surface area. And in bad weather the PF works miles better as it hasn't got a wet LNB cap blocking the signal.

Anyway one thing I can say for certain, my 1.8m PF out performs my 1.5m offset by a huge margin, more than 2dB MER.

 

[radioagogo]

That's pretty much a myth. On a 180cm dish the cross section of a Ku feed horn (8cm diameter) doesn't even block 0.25% of the total dish surface area. And in bad weather the PF works miles better as it hasn't got a wet LNB cap blocking the signal.

Anyway one thing I can say for certain, my 1.8m PF out performs my 1.5m offset by a huge margin, more than 2dB MER.

Hola Huevos,
You may be right but with your sophisticated metering wouldn't it be interesting to compare a 1.8 pf with a 1.8 offset using the same LNB and it's not only the LNB shading the dish, don't forget the LNB support arms too. I do agree with the rain factor but that is something we very rarely have to worry about in Fuerteventura. Have a super Christmas and a Healthy prosperous New Year.

 

[Huevos]

Hola Huevos,
You may be right but with your sophisticated metering wouldn't it be interesting to compare a 1.8 pf with a 1.8 offset using the same LNB and it's not only the LNB shading the dish, don't forget the LNB support arms too. I do agree with the rain factor but that is something we very rarely have to worry about in Fuerteventura. Have a super Christmas and a Healthy prosperous New Year.

3 arms with a cross section 1.5cm by 50cm = 225cm. 8cm feed = 50cm cross section, total 275cm. Dish area = 25446cm. Percentage = 275/25446 = 1%. Now convert that to dB and you end up with about 0.01dB difference.

Anyway it's not the feed blocking the dish that lowers the efficiency of a PF dish, but the typically shorter focal length giving a lower f/d number. So if a PF and an offset had the same focal length (and f/d ratio) performance would be almost identical.

 

[PaulR]

3 arms with a cross section 1.5cm by 50cm = 225cm. 8cm feed = 50cm cross section, total 275cm. Dish area = 25446cm. Percentage = 275/25446 = 1%. Now convert that to dB and you end up with about 0.01dB difference.

Probably even less. That assumes that the arms and feed are lying against the dish. The further away from the dish they are; the less the shadowing effect.

I really don't understand why people get so uptight about the arms and feed being in front of the dish when, as you say, it's the difference in f/d that makes the real difference. Mind you, less gain means greater selectivity...

 

[ralphmagno]

hello,
using the same lnb and using 2 dishes prime and offset both being 1.8 meters in dia the offset will be better.
the efficency is higher.
it should be about 10% greater.
ralph

 

[Huevos]

using the same lnb and using 2 dishes prime and offset both being 1.8 meters in dia the offset will be better.

Based on what test? And even if that were true we are just talking about 0.1dB MER difference between the two. And the other thing is you can't use the same LNB because an offset uses an LNBf and a PF uses a custom feedhorn.

 

[ralphmagno]

ok my bad on the lnb.
you can not use the same lnb on a prime and offset.
but looking at winkapidia here is what it says.
4 types of dishes
prime focus has the least gain efficency at 55% to 60%, this is due to lnb shadowing the dish and support arms and blocking some signal.

next offset , this has 60% to 65% and is used in direct to home apps.

cassegrain is running about 65% to70% and this is used for up links and down links as well.

the last is gregorian this can have as much as 70%

if we are grabing as much gain in week areas we would want the last two.
SETI and deep space setups do not use the first two at all.
they want as much gain as possible.

judging by this a 2 meter offset would be better than a 2 meter prime. and a 2 meter gregorian would be the best.
now this is excluding the feed horn type and lnb gain and noise figure as well an the cross modulation of the lnb.
ralph:-righton

 

[Lazarus]

What is winkapidia ???

 

[ralphmagno]

ok spellling error,
wikipedia

go to ed.wikipedia.org/wiki/satellite_dish

there it explaines the thing about config and type of dish .
so the gregorian has the most surface area and has the highest gain.
ralph

 

[sonnetpete]

What is winkapidia ???

A spasmodic opening and closing of the upper eyelid, notably observed in human courting behaviour. Can be exaggerated by over consumption of alcohol and is often followed by temporary closure of the eyelid due to swelling and bruising.

 

[Huevos]

go to ed.wikipedia.org/wiki/satellite_dish

there it explaines the thing about config and type of dish .
so the gregorian has the most surface area and has the highest gain.
ralph

Seeing as wikipedia can be edited by any Tom, Dick or Harry you need to take what it says with a pinch of salt. According to the manufacturer of my PF dish it is 68% efficient and my offset is 70%.

 

[ralphmagno]

ok so what size are as talking about?
what model of dish and who makes it?
all the radar i worked on where cassegrain in design.
uplinks the military use are the same.
big radio telescopes have large arrays between 2 hills like in porto rico and a large sub reflictor .
if you have a 1.5 meter the difference in efficiency is almost no difference.
ralph

 

[Huevos]

in porto rico and a large sub reflictor

Arecibo is a completely different case. It has a spherical reflector and therefore secondary (and tertiary) reflectors are absolutely essential to correct spherical aberration.

if you have a 1.5 meter the difference in efficiency is almost no difference.

Compared to what?

 

[Channel Hopper]

For a given diameter a circle perpendicular to the line of site would have the greatest area, (and so the prime focus design would have the maximum area available), however with offset dishes the signal from the satellite is often improved with one dimension of the reflector that is larger than that specified as the actual size.

The only design around which conforms to true area specified and lack of shadow formed by feed arms/subreflector assembly/waveguide and electronics is the horn antenna, but the efficiency over the whole of the Ku band frequencies (and the cost) makes these a rarity.

For example a 1.8m offset dish could have the vertical dimension as large as 2m.

 

[radioagogo]

Oh Dear, Oh Dear, Mi Amigos, what have I started. It's nice to see all the comments though. I was only telling what I was taught many many years ago when $ky was just a twinkle and didn't intend to start an argument Happy new Year to all.

 

[PaulR]

Seeing as wikipedia can be edited by any Tom, Dick or Harry you need to take what it says with a pinch of salt.

I remember a report about a year ago which compared the number of errors in Wikipedia agains the number of errors in (I think) the on-line Encyclopaedia Brittanica and it came out very well.

 

[Vipersan]

..Imo a correctly shaped and set up pf should be as good if not better than an offset of similar size ie 1.8 pf as opposed to 1.8 offset ..
..and no have no stats to back this claim ...but certainly this appears to be the case in C band.
Where the offset scores over the PF is its ability to add extra offset lnbs without masking any illumination of the dish..
Just wanted to join in ;) ...and probably talking garbage except for the last bit..
all the best m8's
VS

My assumption for making the above claim is that ..since the offset is basicly eliptical ...and the feed is not ..then I would assume that a feed fully illuminating the dish to the top extremity ..is also seeing over the edges to the left and right of the dish surface ..thus picking up background noise ..
So I assume the feed is set to maximum illumination avoiding seeing over the edges of the dish ..for maximum signal ..and thus doesn't use the entire dish surface ..
..and thus ..a 1.8 offset sees less than a correctly set up 1.8 pf..
doh ...my brain hurts...

 

[PaulR]

..since the offset is basicly eliptical ...and the feed is not ..then I would assume that a feed fully illuminating the dish to the top extremity ..is also seeing over the edges to the left and right of the dish surface ..thus picking up background noise ..

But if you look upwards at the dish from the point of view of the LNB then the foreshortening effect will then make the dish look round - if the dish has been designed correctly that is.

 

[Vipersan]

But if you look upwards at the dish from the point of view of the LNB then the foreshortening effect will then make the dish look round - if the dish has been designed correctly that is.

You may well be right Paul ..
..I'll just sit back and enjoy the discussion ..
..I still think PFs are 'sexier'...lol
VS