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  1. #1
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    Default Question about offset and Prime focus dishes


    Hi,

    I tried to search this through the forum, could not get anything concrete.

    My question is about Prime focus and Offset dish antennae, consider two dishes of the same size and diameter etc, if one is a Prime focus and other is offset, what would be difference in the signal strength? or is there no difference?
    -------------------------------------------------------
    Sky is an Experimental Lab In the World Of Satellites.
    ::\Dish TV India::90Cms MCBS Dish Hooked to ABS-1 and Edusat and May be Insat 4CR in the Near Future::

  2. #2
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    Quote Originally Posted by Mangalore View Post
    Hi,

    I tried to search this through the forum, could not get anything concrete.

    My question is about Prime focus and Offset dish antennae, consider two dishes of the same size and diameter etc, if one is a Prime focus and other is offset, what would be difference in the signal strength? or is there no difference?
    The signal strength would be more at its highest point with a prime focus dish
    with a offset dish it would be lower as you would only be using part of the dish .

  3. #3
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    Quote Originally Posted by EDIRECT View Post
    The signal strength would be more at its highest point with a prime focus dish
    with a offset dish it would be lower as you would only be using part of the dish .
    In fact not. If you ever look thru the LNB holder at the offset dish you will see the LNB only sees a fully circular dish. Optical illusions.

    What affects the signal quality if the type of LNB used. Many just buy cheap LNB's which offer a poor performance. Some LNB's give over 2db better gain than cheaper lnb's on the same dish size ( eg 1.8m) Quality of cable from dish to receiver is also an important factor, as is the antenna unit in the receiver. You can lose a lot of signals in coaxial cable.

    Dreamboxes are well known for their poor performing tuners. But never mind just install a bigger dish.

    The probem on this forum has been many people using an LNB designed for an offset dish on a prime focus dish and complaining the signals are worse. Which of course if true are they are using the wrong LNB's for the wrong dish.
    If Evolution works, why so many idiots?


  4. #4
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    if you require lesser than 1M size, go for offset. more than 1M in footprint requirement and go for prime focus.
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  5. #5
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    Quote Originally Posted by irdeto2engineer View Post
    In fact not. If you ever look thru the LNB holder at the offset dish you will see the LNB only sees a fully circular dish. Optical illusions.

    What affects the signal quality if the type of LNB used. Many just buy cheap LNB's which offer a poor performance. Some LNB's give over 2db better gain than cheaper lnb's on the same dish size ( eg 1.8m) Quality of cable from dish to receiver is also an important factor, as is the antenna unit in the receiver. You can lose a lot of signals in coaxial cable.

    Dreamboxes are well known for their poor performing tuners. But never mind just install a bigger dish.

    The probem on this forum has been many people using an LNB designed for an offset dish on a prime focus dish and complaining the signals are worse. Which of course if true are they are using the wrong LNB's for the wrong dish.
    Oh Well DrDish and all them Makers are wrong - hehehe

    So what is the best option

    Dish ? what make
    Receiver ? what make
    LNB ? what make
    Coaxial Cable ? what make

    Please answer this so we all know what we are doing wrong ?

    I have 92% signal on my prime focus with my dreambox and ALPS 0.2db LNB
    I have 85% signal on my offset dish with my dreambox and ALPS 0.2db LNB

    Let us all know whats wrong as we are all clowns !

    PS : Oh and why when installers fix a offset dish they turn the lnb sideways to try and get the best prime focus point as possible ?

  6. #6
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    a 1 m prime focus dish is 1 m wide and 1 m high
    as to a 1 m off set dish is 1 m wide and about 1.2 m high userly
    or even 1 m high by 1,2 m wide some but as many as the other way
    ABA

  7. #7
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    prime focus dishes have a little more gain than offset dishes as their low noise is higher cause the reflections around it.

    gnags

  8. #8
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    Quote Originally Posted by gnags View Post
    prime focus dishes have a little more gain than offset dishes as their low noise is higher cause the reflections around it.

    gnags
    They sure do thats why they are prime focus

    Offset does not use the complete reflection of the dish thats for sure

    Maybe some of you will know of the gregorion dish which also did give 30% more
    Prime focus with another small dish reverse at the lnb side which infact increased the reflections much much greater

    Here is a little video of a gregorian dish

    Best dish so far wish i could get some of these - I had many in UK

    http://www.youtube.com/watch?v=Y8cmjCcHw60

    VID is from a offset type - but we had prime focus


    AMAZING POWER

    Some of you might want to look this VID

    http://www.youtube.com/watch?v=J1EVmmzOMD4

    eNjOY

  9. #9
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    forget every thing manglore all are right except one thing why people buy offset if offset have less gain?
    simply look offset dishes used in those areas where u cannot install dishes on roof.so we used sidewalls for offset antenna.suppose a prime focus dish get a full signal on elevation of 20 degree then a offset dish get same satellite at 17 or 18 degree.because on side walls u have less area to elevate a dish.so if u use yur roof then buy prime focus or if u are living in appartment then look that if yur targeted satellite elevation is more then 20 degree if yes then buy offset

  10. #10
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    Quote Originally Posted by EDIRECT View Post

    Please answer this so we all know what we are doing wrong ?

    I have 92% signal on my prime focus with my dreambox and ALPS 0.2db LNB
    I have 85% signal on my offset dish with my dreambox and ALPS 0.2db LNB
    Imposable to answer due to the fact you have supplied no information on dish sizes.

    In actual fact set up correctly there should be no difference in signal strengths between Off-set and Prime focus dishes.
    Originally OFF-set dishes where designed for bad weather situations, where they would remain clear of obstructions [snow] due to the angle of the dish, This was further enhanced with dish heaters. Consider the weight facture of a 3 meter prime focus dish with 12 inches of snow, and the working capabilities , Whereas the same size dish in an Off-set configuration would not suffer the same problems. Due to the increased angles of the dishs

    It's really a case of horses for courses.

    With the home use, like all things it comes down to price and availability, the pay companies have nearly all adopted a small Off-set dish, as it fills there
    requirements, is cheap to produce in large numbers, and is easy to install.
    regards from OZ bassett

  11. #11
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    Quote Originally Posted by boknoy_ph View Post
    if you require lesser than 1M size, go for offset. more than 1M in footprint requirement and go for prime focus.
    Thank you all for your replies, this discussion has indeed elevated my knowledge.

    Thank You Boknoy for the suggestion above, but can you kindly explain why, do i need to go for a offset, for dish size less than 1M and Prime Focus for more than 1 M?

    No doubt this is a very sound advise, as i can see that this is the way majority of installations are done. But i am not sure of the scientific reason behind it?
    -------------------------------------------------------
    Sky is an Experimental Lab In the World Of Satellites.
    ::\Dish TV India::90Cms MCBS Dish Hooked to ABS-1 and Edusat and May be Insat 4CR in the Near Future::

  12. #12
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    Quote Originally Posted by bassett View Post
    Imposable to answer due to the fact you have supplied no information on dish sizes.

    In actual fact set up correctly there should be no difference in signal strengths between Off-set and Prime focus dishes.
    Originally OFF-set dishes where designed for bad weather situations, where they would remain clear of obstructions [snow] due to the angle of the dish, This was further enhanced with dish heaters. Consider the weight facture of a 3 meter prime focus dish with 12 inches of snow, and the working capabilities , Whereas the same size dish in an Off-set configuration would not suffer the same problems. Due to the increased angles of the dishs

    It's really a case of horses for courses.

    With the home use, like all things it comes down to price and availability, the pay companies have nearly all adopted a small Off-set dish, as it fills there
    requirements, is cheap to produce in large numbers, and is easy to install.
    Not withstanding the environmental issues and cost issues, your suggestion also looks logical, Thanks.

    I am looking foreward for a weekend project of my own this weekend, to experiment on this.
    -------------------------------------------------------
    Sky is an Experimental Lab In the World Of Satellites.
    ::\Dish TV India::90Cms MCBS Dish Hooked to ABS-1 and Edusat and May be Insat 4CR in the Near Future::

  13. #13
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    You can easily make a 60 cm C band dish using a wok or sand baandli (In India ask a sand lorry guy where to buy or hardware shop) If you are considering making one of your own, you will need some help in focal point calculation available on some sites.

    I have this project on soon & when I am done I will post pictures for all to see

  14. #14
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    consider it as this. you need a mirror. and dishes are mirrors for the satellite signals. with offset dish, you reflect only a part of that signal, whereas with primefocus, you see the whole of the mirror in front of you. the shape of a offset dish directs the signal to the lnb focal point just like the prime focus dish does. some will say that prime focus dish offers more signal reception per square inch of same sized offset.

    with offset dishes though, you get almost "solid" signal reception in that the lnb is focused on the dish area, whereas in prime focus dishes, you have the "earth noise" to consider since the lnb faces the dish and thus may have a tendency to pickup signals "noises" from outside the diameter of your dish.

    also, thing to consider if you are using less than 1M prime focus is that the lnb arm and mounts somehow 'block' a part of the signal, in offset, that does not happen.

    pricing also comes as a factor. bigger offset dishes cost more than same sized prime focus.

    anyway, all the factors have been given, the decision is yours. just don't forget to match the lnb to the type of dish you will install. offset lnb for offset dish, and prime focus for prime focus dish.
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  15. #15
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    Quote Originally Posted by boknoy_ph View Post
    consider it as this. you need a mirror. and dishes are mirrors for the satellite signals. with offset dish, you reflect only a part of that signal, whereas with primefocus, you see the whole of the mirror in front of you. the shape of a offset dish directs the signal to the lnb focal point just like the prime focus dish does. some will say that prime focus dish offers more signal reception per square inch of same sized offset.

    with offset dishes though, you get almost "solid" signal reception in that the lnb is focused on the dish area, whereas in prime focus dishes, you have the "earth noise" to consider since the lnb faces the dish and thus may have a tendency to pickup signals "noises" from outside the diameter of your dish.

    also, thing to consider if you are using less than 1M prime focus is that the lnb arm and mounts somehow 'block' a part of the signal, in offset, that does not happen.

    pricing also comes as a factor. bigger offset dishes cost more than same sized prime focus.

    anyway, all the factors have been given, the decision is yours. just don't forget to match the lnb to the type of dish you will install. offset lnb for offset dish, and prime focus for prime focus dish.
    Thanks for the explanation. This cleared a lot of my doubts.
    -------------------------------------------------------
    Sky is an Experimental Lab In the World Of Satellites.
    ::\Dish TV India::90Cms MCBS Dish Hooked to ABS-1 and Edusat and May be Insat 4CR in the Near Future::

 

 

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