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13-Sep-2018, 11:23 PM
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#1
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Senior Member
Join Date: Sep 2012
Posts: 472
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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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14-Sep-2018, 12:30 AM
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#2
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Retired A/V Tech
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,750
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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?
Last edited by rabbit73; 14-Sep-2018 at 12:59 AM.
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14-Sep-2018, 8:43 AM
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#3
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Senior Member
Join Date: Sep 2012
Posts: 472
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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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14-Sep-2018, 9:41 PM
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#4
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Retired A/V Tech
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,750
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Quote:
Originally Posted by Stereocraig
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.
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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?
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In theory, perpendicular to either one of the dipoles, but I think you might need to make some aiming tests for your favorite stations.
Last edited by rabbit73; 14-Sep-2018 at 10:39 PM.
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15-Sep-2018, 11:59 AM
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#5
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Senior Member
Join Date: Sep 2012
Posts: 472
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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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