Technology
The basic technology behind the Duo LNB is not new. It takes advantage of the fact that signals hitting a dish off-axis will be focused (albeit with some diffusion) off axis in the opposite direction. So, with the dish aligned so that the central LNB is receiving one satellite, a secondary offset LNB can be aligned on the focus of a second satellite spaced away from the first.
This effect has been exploited for many years to receive signals from two satellites at once with a single dish, and two LNBs have been most commonly arranged on a dish in this way for reception of Astra 19.2°E and the Hot Bird satellites at 13° east, primarily for the abundance of TV channels from 19.2° east, and some additional channels (especially adult channels) from 13° east.
A monoblock LNB provide a convenient alternative to fixing and aligning two LNBs to a dish independently. The two feedhorns are positioned at the correct spacing for reception from the two satellites required and the DiSEqC switching system is used to select between the signals from the two satellites with commands from the connected receiver. In other respects, the monoblock LNB acts as a normal LNB to the connected receiver.
The required separation of the monoblock's feedhorns depends on the angular separation of the satellites to be received, the position of the receive site on the Earth's surface and the focal length of the dish. Fortunately, monoblock LNBs can be standardised for sites across Europe provided that a "standard" offset dish with a focal length/diameter (f/D) ratio of 0.6 is used.
Monoblock LNBs for 19.2° east and 13° east have been widely available for several years (indeed, the DiSEqC switching system was originally designed for just this setup). However, these do not function correctly for Astra 23.5°E and Astra19.2°E because these satellites are at a different angular separation.
In fact, it can be difficult to physically fit two separate LNBs onto a dish at the correct separation for Astra 23.5°E and Astra 19.2°E because their bulk may prevent the feedhorns sitting close enough together.
The Duo LNB is carefully designed with the correct spacing of the feedhorns, DiSEqC level 1.0 switching between the satellites and a low noise amplifier and conversion system.
Read more about this topic: Duo LNB
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