Time-Nutters-- What I saw in the photos of the helix antennas that I found interesting was:
A) The tapered cone-shaped cup that some of the helix antennas sat in. What does this do? Most helix antennas seem to sit over a flat ground plane but these are different. B) Some of the helix antennas are tapered in diameter very gradually from the base to the top-end. Why? C) Some of the helix antennas that are tapered gradually along their entire length have an abrupt taper at the end. Why? And lastly; what is the material that is used for winding the helix elements onto? I have some large sheets of copper foil with an adhesive backing that would be ideal for fabricating helix antennas similar to the ones seen in the photos. I tried building a 3-turn helix to feed my 1.8 meter, 0.39 F/d dish on 1.7 GHz for downlinking the NOAA HRPT digital imagery. After several iterations I finally only got mediocre performance. I think this was due to poor illumination of the dish. I then tried to build a circular polarity patch feed by scaling the dimensions for a 2.4 GHz patch feed but this was a dismal failure as I never could get the circular polarity right. I finally wound up with a coffee-can style feed which works OK. Not wonderfully well, but just OK. Here is a DropBox link to a recent image: < http://dl.dropbox.com/u/60102282/FLA%201130-3May12.JPG > The HRPT imagery is pretty neat but now a group of us are working on figuring out how to demodulate/decode and display the much higher resolution (and far more natural looking) imagery from the AQUA and TERRA birds. These birds imaging telemetry comes down at 8.2 GHz at 15 Mbps in a Staggered Quadrature Phase Shift (SQPSK) format. I am currently trying to come up with an efficient 8.2 GHz feed and LNA or LNC for my dish. Should be a fun project! Here is a DropBox link to a sample TERRA image from the NASA archives: < http://dl.dropbox.com/u/60102282/Web%20Terra-Aqua%20Sample.jpg > Mike Baker --------------------------------- _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.