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Author Topic: Antenna Opinion?  (Read 4937 times)
KA1OWC
Member

Posts: 96




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« on: August 11, 2017, 05:51:25 AM »

Hi: If anyone is familiar with this antenna could you give me your opinion? Thanks

http://www.antennas.us/store/p/404-UC-1374-531-4-dBic-VHF-APT-Weather-Satellite-Antenna-WXSAT.html

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Steve, KA1OWC
Retired Lieutenant Colonel, US Army
Army Nurse Corps
VE3WGO
Member

Posts: 153




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« Reply #1 on: August 11, 2017, 09:31:10 AM »

Hi Steve, that looks like a very well built antenna, and weatherproofed.  Reasonably priced for all that.

It is called a Quadrifilar Helix Antenna (QFA), which is a circularly polarized antenna. That means it can receive a signal that is vertically, horizontally, or circularly polarized (if it's in the proper direction), so that if a satellite's antenna elements are not perfectly aligned with your ground station antenna elements, you can still receive the signal well.  The QFA is a very good antenna for immunity to fading signals and propagation variations, and has been used quite a lot in the aerospace industry.

For some descriptive info at a basic level, visit Amsat's web page at https://www.amsat.org/#  and look in the "Education" pull-down menu, the first two menu items.  You will find in that section under "For Beginners" there is a short discussion in part 3 of an article (see page 2 left side) plus 3 links for more info on that type of antenna at http://www.amsat.org/wordpress/xtra/Getting%20Started%203.pdf  

There is more info scattered around the web on this antenna, and a Google search will find what you want.  If you are interested, there is a more in-depth discussion on the QFA and circular polarized antennas in general here https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjvvLfey8_VAhUE5IMKHWefCNEQFggqMAA&url=http%3A%2F%2Forbanmicrowave.com%2Fwp-content%2Fuploads%2F2015%2F03%2FTheBasicsofQuadrifilarHelixAntennas.pdf&usg=AFQjCNHgj5YY6QH7Zb0D_SW6YXw2jE1r5w

73, Ed VE3WGO
« Last Edit: August 11, 2017, 09:35:12 AM by VE3WGO » Logged
KA1OWC
Member

Posts: 96




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« Reply #2 on: August 11, 2017, 09:35:06 AM »

Ed: Thanks for the reply and the resources...I have tried building an antenna myself without much success...Should have just worn an aluminum foil hat...I need something idiot and senior-proof...Thanks again...

Steve
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Steve, KA1OWC
Retired Lieutenant Colonel, US Army
Army Nurse Corps
VE3WGO
Member

Posts: 153




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« Reply #3 on: August 11, 2017, 09:40:41 AM »

Hi Steve, I re-edited my reply while you already answered.  I have built many types of circular polarized antennas, including crossed yagis, and helical antennas.  The helix is the most forgiving for home construction, but those types all need to be steered.

The QFA and crossed dipole over a reflector (a "Turnstile Reflector array")  is another type that works well, and does not need to be steered.

The Lindenblad is one more that is a bit tricky to make, but works very well, does not need steering..  http://www.amsat.org/wordpress/wp-content/uploads/2014/01/70ParaLindy.pdf     and http://www.amsat.org/wordpress/wp-content/uploads/2015/08/An-EZ-Lindenblad-Antenna-for-2-Meters2.pdf

Those are in the "Satellite Info" menu...  "Station and Operating Hints" item.

73, Ed VE3WGO
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K6LCS
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Posts: 1992


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« Reply #4 on: August 15, 2017, 07:57:29 PM »

"4db gain" from a 24" antenna for 137-138mHz?

Interesting claim ... I cannot find much published antenna theory to back it up, though ... (grin)

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Clint Bradford, K6LCS
http://www.work-sat.com
WD9EWK
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Posts: 502


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« Reply #5 on: August 16, 2017, 08:56:32 AM »

"4db gain" from a 24" antenna for 137-138mHz?

Interesting claim ... I cannot find much published antenna theory to back it up, though ... (grin)

The web page for that antenna lists 4 dBic gain. When converted to dBd (how the gain of most ham antennas are rated, compared to a dipole) using a simple formula listed at:

http://knowledge.digi.com/articles/Knowledge_Base_Article/dBi-vs-dBd-summary

4 dBi becomes 1.85 dBd gain. Not a lot of gain, but some. This antenna also has circular polarization (the "c" in the "4 dBic gain" listed on the antenna's web page), not linear polarization like a simple dipole or vertical.


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Patrick WD9EWK/VA7EWK
http://www.wd9ewk.net/ - Twitter: @WD9EWK
KA4NMA
Member

Posts: 514




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« Reply #6 on: September 10, 2017, 07:52:01 PM »

Wish I had 2m and 70cm versions of these antennas. Do you have any or can you make me one?

Randy ka4nma
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K5TED
Member

Posts: 96




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« Reply #7 on: October 16, 2017, 07:03:42 PM »

for under $20 you can get a 15dB or better LNA that would do wonders for the quadrifilar helix antenna, or the turnstile.
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