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ghz24 22-Nov-2011 1:10 AM

Secrets of reception, polarization
 
I'm starting this thread to discuss polarization and circular polarizations usefulness in reducing multipath interference if you live close enough to one of these stations that use circular polarization.

http://www.rabbitears.info/oddsanden...ization&type=C

If you do you can rotate your antenna from laying flat to standing straight up (and any angle in between with no loss in signal (equal signal in vertical and horizontal planes.
And since the reflected signals aren't likely to be in the same polarization as your antenna (at some angle you pick to optimize this effect) reflected multipath signals can be attenuated 20 Db or more.

GroundUrMast 2-Dec-2011 11:56 PM

A useful idea if the station uses elliptical polarization also.

http://www.rabbitears.info/oddsanden...ization&type=E

With circular polarization, equal power is available in the horizontal and vertical plane. With elliptical polarization, the amount of power radiated in the horizontal plane is usually greater than in the vertical. rabbitears is kind enough to publish the values for each station in their database.

be236 3-Dec-2011 5:15 AM

I find rabbitears site to be a little complicated and confusing to use.

Dave Loudin 3-Dec-2011 5:59 AM

Start by clicking on the listings tab. The rest of the way is like peeling an onion. Open a market to get a station list, click on a station to see programming details, etc.

be236 3-Dec-2011 7:24 AM

Ok, I went there, picked on Vancouver and it showed their market cities... so, what's next? Still confused.. too many menu options at once on the same page...

What does Rabbit Ears gives me that I can't get in Tvfool?

Dave Loudin 3-Dec-2011 1:10 PM

Oh, my gosh! There is so much detail if you keep clicking!

Click on any station in the list to get the list of subchannels with details on audio format and network. Click on the technical details link to reveal a detailed description of the transmission facility and subchannel characteristics. Clicking on the "Channel nn" link pops up a coverage map that shows both the allocation contours and a Longley-Rice coverage plot. Clicking on the coordinates pops up a GeoHack page that gives you links to information tied to that location. Clicking on the TX site name will show you what other stations broadcast from there.

I could go on, as there is a lot more. The site is set up to explore. Don't be intimidated, click on what're looks interesting and you'll discover more.

be236 3-Dec-2011 4:00 PM

I guess I'm too old and lazy (busy with other stuff). :)

If I get more time to play around I might.. but that site to me is too much "information overload." heh... so it does feel intimidating... alas...

ghz24 11-Dec-2011 3:15 AM

helical antennas
 
I have to agree with be236 at least somewhat I found rabbit ears site confusing at first to.
Quote:

Start by clicking on the listings tab
Thanks for the tip there Dave I didn't realize that was a link I thought it was just a drop down. But like be236 I gave up pretty easily even though I'd seen very informative pages from links, my curiosity took me elsewhere.
What about helical antennas:
I know a helical will get a gain boost of 3 Db if it's receiving a circular polarized signal (if both match direction of rotation wise).
And a helical is equally sensitive to vertical and horizontal polarized signals (and all angles in between)
But will a helical reject multipath or bounced or out of phase signals?

The equal sensitivity to all angles of linear polarization seems to suggest that it would be more prone to multipath when receiving from a linear polarized transmitter.
But what about when receiving from a circular polarized source?

found out bounced CP signals off large objects are reversed in direction and attenuated ~20 db

Would the 3 dB extra gain make up for the multipath interference.
And finally while only some stations use circular polarization what about using this to null out a stronger circular signal by using a left hand helix instead of the standard right hand the desired linear polarized signal would suffer a 3 dB loss but the circular signal (being right handed) would suffer a 20-25 dB loss?

Thoughts, comments, corrections?

ghz24 16-Jan-2012 1:32 AM

little update
 
Quote:

I know a helical will get a gain boost of 3 Db if it's receiving a circular polarized signal (if both match direction of rotation wise).
Actually the boost is 3 dB only if both ends have an "axial ratio" (the ratio of horizontal to vertical signals) of 1 equal horizontal and vertical signals.

Quote:

And a helical is equally sensitive to vertical and horizontal polarized signals (and all angles in between)
again the axial ratio affects how much.
Quote:

But will a helical reject multipath or bounced or out of phase signals?
odd numbered reflections are blocked (attenuated) even numbered are not effected.
So single bounces are blocked and twice bounced are received.

edit: Found this recently http://www.attplus.eu/hamradio/proje...cle/cppl_b.pdf


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