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

[George Dubovsky]

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 <mpb45 at clanbaker.org> 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
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