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Re: Helix antennas,



Zach,
 
> Thank you all for your input. It is nice to have so many helpful
> people give antenna advice.
 
One of the things AMSAT members are best known for is giving people advice :-)  Seriously, Steve's reference to Kent's Cheap Yagi is a good choice for the current fleet of LEO's.  They are easy to build and tune and have the simplicity of a linear design.  Any pair of Yagi's in the 3-5 elements range are more than good enough for all the LEOs.  Adding at least a 70 cm preamp (at the antenna) is always the best money you will spend.  
 
Save the helix effort for 1.2 GHz and up:  I no longer have the page on web, but I built and used a 70 cm helix based on the AF9Y design (I used an aluminum pan for the reflector :-)) and found it fine for transmit, but not a good receive antenna for that band.  The reason?  By it's nature the helix is very widebanded (thank you Dr. Kraus), a real convenience when building a transmit antenna, but not so good for receive selectivity.  I found the wideband UHF noise allowed into the receiver (amplified by a wideband UHF preamp) to greatly degrade receive perfomance.
 
Many people enjoyed success with helices on AO-40, including me, but that was a few years ago now and things have changed considerably since then.  The explosion of WiFi sources very near the 2.1 GHz band may very well have changed the spectrum profile enough to make a helix+preamp unusable in any densely poplulated area below 15 or 20 degrees elevation.  For S-band downlink on P3E and Eagle, I would suggest a narrow-banded antenna/feed such as a patch or Yagi is a better choice.  For uplink on 70 cm or L-band, a helix is still a great choice.  FWIW, I employed a pair of concentric helixes for L/S on a 4' dish for about 6 months and they worked fine.  I replaced the helices with the dual patch feed now shown on the web page and was much happier with that arrangement (better s/n, the real difference).

73,
Jerry, M0MOE
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