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Old 13-Sep-2018, 11:23 PM   #1
Stereocraig
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Omni directioal FM Antenna

I know this isn't OTA related, but hoping somebody can give me an answer, since I'm already registered here.

I'd just like to know if this Rat Shack has a sweet spot when aiming.

Thanks guys!
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File Type: jpg radio shack omni FM antenna.jpg (186.1 KB, 35 views)
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Old 14-Sep-2018, 12:30 AM   #2
rabbit73
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Crossed dipoles form a turnstile antenna. In theory, it's a pretty good Omni with peaks at 0, 90, 180, and 270 degrees. There are slight nulls between those directions.
https://www.google.com/search?q=turn...iw=911&bih=363

But holl_ands modeled it, showing that it is a little less Omni that theory indicates.

https://imageevent.com/holl_ands/omni/fmturnstileacfmss

I don't do modeling, but that doesn't look like a 90 degree phase shift line between the crossed dipoles, so I don't have confidence in his model. How long is the feed line between your dipoles?
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File Type: jpg Antennacraft FM Omni_1.jpg (150.5 KB, 23 views)
File Type: jpg Antennacraft FMSS_1_zoom.jpg (12.5 KB, 20 views)
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Last edited by rabbit73; 14-Sep-2018 at 12:59 AM.
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Old 14-Sep-2018, 8:43 AM   #3
Stereocraig
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Thanks, Rabbit!

The feed line is about 8", but the output is not centered between the two.
It is connected to one dipole, which is jumpered to the other dipole.
Not sure if this would affect the phasing though, at this low freq.

So, zero degrees is off the end of the loop, or perpendicular to it?
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Old 14-Sep-2018, 9:41 PM   #4
rabbit73
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Quote:
Originally Posted by Stereocraig View Post
Thanks, Rabbit!

The feed line is about 8", but the output is not centered between the two.
It is connected to one dipole, which is jumpered to the other dipole.
Not sure if this would affect the phasing though, at this low freq.
That is the correct way to wire a turnstile antenna.

Quote:
So, zero degrees is off the end of the loop, or perpendicular to it?
In theory, perpendicular to either one of the dipoles, but I think you might need to make some aiming tests for your favorite stations.
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File Type: jpg Turnstile Antennna2.jpg (76.0 KB, 45 views)
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Last edited by rabbit73; 14-Sep-2018 at 10:39 PM.
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Old 15-Sep-2018, 11:59 AM   #5
Stereocraig
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MKE and Chi are almost a perfect 180 apart from me, so adjustments should be a snap. I'll probably just clamp a mast to the gable end of my garage, since I don't really care to climb the towers.

Thank you, Rabbit!
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