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Stereocraig 14-Sep-2018 12:23 AM

Omni directioal FM Antenna
 
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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!

rabbit73 14-Sep-2018 1:30 AM

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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?

Stereocraig 14-Sep-2018 9:43 AM

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?

rabbit73 14-Sep-2018 10:41 PM

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Quote:

Originally Posted by Stereocraig (Post 60294)
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.

Stereocraig 15-Sep-2018 12:59 PM

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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