[time-nuts] Those helix antennas in the photos...

[Michael Baker]

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
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