Offset dish vs prime focus dish

  • I have tried to make a circular feed based on the idea of DC2TH with a 2x 4el. PCB yagis from LZ5HP. I found that one yagi gives me good result so I decided to try this feed. Used 100cm offset dish.

    I make similar construction like this one:

    But I have make a small experiement with 1 yagi only. If the yagi was horizontal (above or bellow LNB), the signal was noticable stronger than I try vertical (left or right). The difference was abt. 4dB.

    Question - it is due that the feed is not absolutely in the focus point or due that the offset dish push the energy not consequent?

    Did anybody observe similar results?

  • Your uplink should be circularly polarised so if you are using a linear antenna it's orientation would not matter though 3 dB down.

    The phase centre of the feed needs to be at the focus. The phase centre of two vertical antennas will be in the centre of that feed, the phase centre of just one will be where the antenna is, offset to the left or right. This means your dish pointing will be skewed. You can work out how much with simple trigonometry but why does this not impact the horizontal antenna? The same should apply in the vertical plane. I don't know. All I can think of is perhaps your dish is well aligned in azimuth but not well aligned in elevation?

    What about putting a single yagi in the middle and seeing what happens as it is rotated?