Just what is the difference from an offset feed reflector and a direct feed reflector.

Satellite "dishes"  are reflectors that focuses all of the energy coming from the satellite to a feed horn or antenna. An antenna all by its self with nodesign considerations is what we call unity or no gain (A unity gain is the wire sticking out of your car for your AM/FM radio). We add gain to a satellite signal be cause the signal coming 23,000 miles away at about 1 - 10 watts (Your cell phone is about half a watt) is to weak for a simple antenna to pick up. So we take a big metal reflector and catch thousands more in signal than just a wire, and reflect it to that wire, the average satellite antenna is only about 1/4" to an inch long, so that big reflector gathers all the signal it picks up and reflects it to a single point. That's where we really put the 1/4" antenna at.

If you ever took a geometry class and stayed awake long enough to learn about Parabolas and Ellipses, you will remember that they have one or two focus points.

Fig 1

Here I drew two ellipses. On the ellipse to right I sliced it vertically and created the old fashioned satellite dish, on the ellipse to the left, I sliced it diagonally and created the newer (Offset Feed) or your new classic Direct TV or DISH antenna.

Fig2

To create a multi sat dish we just increase the size of the dish a bit to account for the slight loss of signal from the sides and add as many more antennas as we want spaced as far apart in degrees (Most satellites arc 2.5 degrees on the arc) as the intended satellites are spaced. So if you took a protractor and measure the angle of the LNBs you would see that they are the same angle as the satellite companies satellites are from each other. You can almost always substitute any off set antenna for another offset antenna, but the placement of the LNBs (Antenna part) must be spaced exactly as far apart from each other as the satellites are that you intend to receive.

So why off set feed over the old direct feed type, well there are a couple of factors. An offset feed the focal point is out of line of site with the intended source, so you don't have the LNB or whatever blocking some of the signal. Also an offset feed reflector will have less of the bottom acting as a water/snow/ice collector which of course interferes with the reception of the signal.

Why cant you use a direct feed for an offset feed, well you can but you have to change the feed horn focus to a direct feed. Most of the new direct to home (DirecTV, DISH, SKY, ect) use a feed horn that is elliptical designed to only catch signals reflected in an oval pattern, using the wrong feed type will cause the antenna inside to pick up adjacent satellite signals and cause interference to the reception circuits trying to pick out the baseball game on the satellite at 101 degrees from the satellite at 102.5 show a rerun of Laugh in on the same transponder.

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