On (B) and (C), helices are tapered to broadband their frequency response. Usually the pitch changes along with the diameter.
73, geo - n4ua On Tue, Jun 5, 2012 at 7:47 AM, Michael Baker <[email protected]> wrote: > 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<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<http://dl.dropbox.com/u/60102282/Web%20Terra-Aqua%20Sample.jpg>> > > Mike Baker > ------------------------------**--- > > ______________________________**_________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/** > mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
