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Author Topic: Absolute Polairty in RF?  (Read 651 times)

Russell Ault

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Re: Absolute Polairty in RF?
« Reply #10 on: May 31, 2021, 09:20:38 PM »

I actually do this fairly regularly, and I've never had the ME or BE note the vocals or instruments on RF were out of polarity; I'll ask next time.

That might be two different things though, mightn't it? If the polarity of the demodulated signal is based off of the polarity of the carrier (which I believe it is), then inverting the polarity of the RF signal should have no impact on the polarity of the demodulated signal, right? If so, I would expect the audio polarity to be the same regardless of antenna orientation.

What I'm curious about is whether the electrical RF signal itself would be inverted by flipping an RX antenna (regardless of any practical impacts)?

-Russ
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Henry Cohen

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Re: Absolute Polairty in RF?
« Reply #11 on: May 31, 2021, 09:34:10 PM »

. . . If the polarity of the demodulated signal is not based off of the polarity of the carrier (which I believe it is), then inverting the polarity of the RF signal should have no impact on the polarity of the demodulated signal, right? If so, I would expect the audio polarity to be the same regardless of antenna orientation.

I think this is what you meant? If so, I agree.


Quote
What I'm curious about is whether the electrical RF signal itself would be inverted by flipping an RX antenna (regardless of any practical impacts)?

To my thinking, 0 and 180 physical polarization of antennas with respect to each other is still in like polarization; 0 and 90 (vertical vs horizontal) differential would be "flipped". As others have already pointed out, the RF carrier is just that, and has no bearing on the original audio signal polarity, if all is operating correctly.
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Henry Cohen

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Steve-White

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Re: Absolute Polairty in RF?
« Reply #12 on: May 31, 2021, 09:39:16 PM »

That might be two different things though, mightn't it? If the polarity of the demodulated signal is based off of the polarity of the carrier (which I believe it is), then inverting the polarity of the RF signal should have no impact on the polarity of the demodulated signal, right? If so, I would expect the audio polarity to be the same regardless of antenna orientation.

What I'm curious about is whether the electrical RF signal itself would be inverted by flipping an RX antenna (regardless of any practical impacts)?

-Russ

Yep, because there is a many to 1 relationship between the carrier and the data stream.

Been decades since I messed with antenna's - so, before spewing additional outdated blather, have a look at this.

https://www.electronics-notes.com/articles/antennas-propagation/antenna-theory/polarisation-polarization.php
« Last Edit: May 31, 2021, 09:41:18 PM by Steve-White »
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Russell Ault

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Re: Absolute Polairty in RF?
« Reply #13 on: June 01, 2021, 02:27:04 AM »

I think this is what you meant? If so, I agree. {...}

I actually meant the original, but the more that I think about it, the more I've come to realize that what I meant can't possibly be true, so let's go with your version. :P

{...} To my thinking, 0 and 180 physical polarization of antennas with respect to each other is still in like polarization; 0 and 90 (vertical vs horizontal) differential would be "flipped". {...}

I see what you're saying, but I'm not sure how it could be possible? For example, what happens if there's a 45 polarization difference? To me, it makes more sense if a 180 angular offset represents maximum transduction but opposite polarity, with efficiency dropping as the difference approaches 90, at which point the absolute polarity flips (at precisely the point where efficiency reaches -∞ dB), and then efficiency starts improving again towards maximum as you approach 0. In my mind, graphing efficiency against angular offset would produce some kind of figure-8 pattern with the bottom half (i.e. the half containing 180) being of inverted polarity.

Here's what just dawned on me, though: the ultimate answer to this question is one of modulation. If my understanding is correct, the demodulated audio from an AM signal that had undergone polarity inversion would, itself, be polarity-inverted. However, an FM receiver will always apply a positive voltage to its audio output when the carrier deviates above its nominal frequency (and a negative voltage when the carrier deviates below) regardless of the polarity of the modulated signal. Because of this, for FM, the message signal's absolute polarity is encoded in a way that is entirely independent of the absolute polarity of the modulated signal. I'm sure the same is effectively true for many digital modes as well.

I believe, then, that the answer to my original question (and, five years later, this one) is (still) a resounding "no".

Thanks all! :)

-Russ
« Last Edit: June 01, 2021, 02:34:07 AM by Russell Ault »
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Re: Absolute Polairty in RF?
« Reply #13 on: June 01, 2021, 02:27:04 AM »


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