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

Rex rexa at sonic.net
Tue Jun 5 10:18:25 EDT 2012


On 6/5/2012 5:36 AM, George Dubovsky wrote:
> On (B) and (C), helices are tapered to broadband their frequency response.
> Usually the pitch changes along with the diameter.
>
> 73,
>
> geo - n4ua

I was just reading in the 3rd edition of Antenna Engineering Handbook, 
that the sharp taper at the end gives a better axial-ratio over a wider 
bandwidth (more circular as opposed to a bit of elliptical 
polarization). There is some trade-off as the gain is lowered a little too.


> 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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