the idea to build this feed was triggered by Peter, DB2OS in January 20, when he sent me some papers and calculations from Karl, DJ4ZC, to construct a QO-100 feed with 3 helix antennas around the LNB. Additional information came from Achim, DH2VA and Michael, DD5ER, who sent me dimensions for the antenne coupler and Mario who did final adjustments on the Amsat Helix antennas in Bochum.
Building a feed with three helix antennas is a complex project, so it took three month, until April 20, to get the first feed up and running.
The goal was to get best reception and not to shade the LNB, and to get the same good TX performance of a single helix feed.
To measure the shading of the LNB I built the reflector and LNB only, without helix, measured the transponder noise and DATV-MER and then glued three dummy helix on the reflector and repeated the measurement.
this is the reflector + LNB, with three dummy helix.
Resulting transponder noise: a single helix costs about 1.5 - 2 dB. The three helix antennas cost only 0.5 dB. Also MER was about 1dB better.
My first idea to simply paralleling three helix antennas was impossible, because I never measured the 140 ohms as stated in Wikipedia or Rothammel. Therefore I tuned each helix to 50 ohms and built an antenna coupler.
2.4 GHz coupler made from brass pipes, inner diameter 20mm.
Today the new 3-helix feed went into operation. It give the same good TX performance as my single helix feed, and the good RX signal of an unshaded patch feed. The only downside is that it's a lot of work, and a good workshop is needed to build all the components.
the 3-helix feed in operation
50 ohms match and holder made from polypropylen
tuning at the the coupler input with all three helix antennas connected
vy 73, Kurt, DJ0ABR