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Author Topic: Law of Reciprocity  (Read 2308 times)
EI2GLB
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Posts: 484




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« Reply #15 on: January 13, 2013, 09:34:09 PM »

Jonathan no I can't agree with your first point the whole point of using the best antenna is to increase your stations abaility to recieve better, maybe I'm missing your point sorry if I am.

And remember your ERP will be a huge deal more running 1kw into a 3 el yagi than the guy running 1kw into a dipole or something worse like a windom or G5,

Trevor
EI2GLB
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W6GX
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« Reply #16 on: January 13, 2013, 10:30:38 PM »

Jonathan no I can't agree with your first point the whole point of using the best antenna is to increase your stations abaility to recieve better, maybe I'm missing your point sorry if I am.

And remember your ERP will be a huge deal more running 1kw into a 3 el yagi than the guy running 1kw into a dipole or something worse like a windom or G5,

Trevor
EI2GLB

Hi Trevor,

You are missing my point but apologies are not needed.  You are correct in that I want the better antenna.  I'm not debating that.  A better antenna could RX and TX better.  If I only have a G5RV it would be challenging to work another station who is also running a G5RV.  So the big station wins since he could work both big and small stations.

This is my problem.  Let's say I have a G5RV mounted 10' above ground.  It's omnidirectional with no gain.  I call CQ with 100 watts.  Another station with a triple stacked yagi answers my CQ with also 100 watts.  I give him a 59 report but the super station only hears me as a 55.  The super station has gain on both RX and TX.  He puts out a tremendous ERP but my crappy antenna only produced a 59.  When I transmit I put out a very low ERP but the RF reaches his antenna with a very high gain.  The law of reciprocity says he should copy me with a 59 but he only sees a 55.  In my case I'm the large station that cannot hear as well as the little station.  I'm trying to find out why I don't hear them as well as they hear me when we run similar power levels.

73,
Jonathan W6GX
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N6DZR
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« Reply #17 on: January 14, 2013, 11:50:47 AM »

I am about the opposite of a propagation expert, but here's what I have gleaned from reading (and experience):

The ionosphere is not a smooth sphere-shaped mirror around the earth. It's lumpy, misshapen, and has varying levels of refraction/absorption. Given this, two identical stations could receive each other with different signal strengths. Sometimes the difference is great enough I call it "one-way propagation".

This unequal propagation can be either way, however. Sometimes I can hear them just fine but they cannot hear me. Other times I can barely hear them and they can easily hear me. I have been working on improving my skills to take advantage of the later situation.

-Jeff
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W1VT
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« Reply #18 on: January 14, 2013, 12:46:21 PM »

The key to the weirdness is the splitting of the radio waves into two separate waves of opposite circular polarity--depending on conditions, they can cancel each other out to produce fading, or unequal signal strengths at each of the path.  This is the result of interaction with the ionosphere, as opposed to a reflection off the surface of the Earth, which doesn't do that.

You could either take it on faith that the ionosphere does that, or build a circularly polarized antenna with polarity switching to see for yourself.

A circularly polarized antenna will allow you to see either a fading signal, or a signal of either polarization.  When signals are steady, one expects to see one polarization to be significantly stronger than the other.

Zack Lau W1VT
« Last Edit: January 14, 2013, 12:54:22 PM by W1VT » Logged
NU1O
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« Reply #19 on: January 14, 2013, 03:18:10 PM »

The key to the weirdness is the splitting of the radio waves into two separate waves of opposite circular polarity--depending on conditions, they can cancel each other out to produce fading, or unequal signal strengths at each of the path.  This is the result of interaction with the ionosphere, as opposed to a reflection off the surface of the Earth, which doesn't do that.

Zack Lau W1VT

Is that a theory or widely accepted by scientists as fact?

73,

Chris/NU1O

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W1VT
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« Reply #20 on: January 14, 2013, 03:37:51 PM »

I can't speak for the scientific community, but

http://en.wikipedia.org/wiki/Appleton-Hartree_equation

A bunch of scientists independently claimed to have discovered the theory of ordinary/extraordinary propagation resulting from electromagnetic wave interaction with cold plasma.

There are tons of references on the web if you search for ordinary/extraordinary propagation.  But, perhaps the best thing to do is to build a circularly polarized antenna with switchable polarization and see for yourself.  I used to do such experiments all the time, when I wasn't so busy answering questions and finding answers for other people.  I found it really made a difference when I wasn't sure about something.

Zack Lau W1VT

I might add that I could have been a scientist--having done exceptionally well in school, but like many other bright students, correctly calculated that the prospects for employment in the USA were dismal--so I picked engineering instead.  Job prospects for scientists are no better today--unless you count jobs that aren't really about science, but making money for large companies.  This is actually on-topic--the scientific expertise many folks assume exists in the USA just isn't there, because folks that can ignore employment prospects are few and far between.
« Last Edit: January 14, 2013, 04:03:50 PM by W1VT » Logged
W1VT
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« Reply #21 on: January 15, 2013, 06:01:14 AM »

http://umlcar.uml.edu/DPS.htm

Here is a description of an ionospheric sounder.  If the scientific community didn't believe in ordinary/extraordinary propagation, why would they make scientific instruments that measure both polarizations?

Zack Lau W1VT
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NU1O
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« Reply #22 on: January 15, 2013, 07:27:19 AM »

I had thought Jonathan was talking in a more general sense. IOW, when conditions are good, signals are good on both ends, when poor, they are poor on both ends. After reading this over it seems Jonthan is looking for virtually the exact signal strengths once the power on both ends has been equalized. I sent Jonathan an email asking for clarification late last night but I haven't heard back from him.

If I had picked that up right from the beginning I would've told Jonathan based on my 35 years of observing HF signals one is not going to see similar signal strengths even after the power on both ends has been equalized.

According to Wikipedia the Law of Reciprocity with respect to HF signals does not exist due to the complexity and ever changing nature of the ionosphere. I'm surprised Zack did not point that out early on.

Here is the part of the Wikipedia article which applies to this discussion:

"Although the claim is commonly made that two-way HF propagation along a given path is reciprocal, that is, if the signal from location A reaches location B at a good strength, the signal from location B will be similar at station A because the same path is traversed in both directions. However, the ionosphere is far too complex and constantly changing to support the reciprocity theorem. The path is never exactly the same in both directions.[7] In brief, conditions at the two terminii of a path generally cause dissimilar polarization shifts, dissimilar splits into ordinary rays and extraordinary or Pedersen rays which are erratic and impossibly identical or similar due to variations in ionization density, shifting zenith angles, effects of the earth's magnetic DIPOLE contours, antenna radiation patterns, ground conditions and other variables."




I have personally seen the difference in signal strength between antennas which are oppositely polarized, one vertical and one horizontal and it's a lot.  20 db I believe. However, I have never built a circularly polarized antenna. Too many other things to do and insufficient interest. As a teen I had a long wire but I spent a lot more time on baseball and football fields. 

73,

Chris/NU1O



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W1VT
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« Reply #23 on: January 15, 2013, 08:54:59 AM »

Saying that the ionosphere is complex and constantly changing doesn't explain why we don't have reciprocity.

But, explaining that that ionosphere generates two signals out of one that can cancel each other out does provide a useful theory that can be tested experimentally.

http://en.wikipedia.org/wiki/Richard_Feynman

Richard Feynman was a great believer in making experiments to prove or disprove theories.

Perhaps the biggest challenge in understanding theoretical physics, as is commonly understood by scientists today, is that much of it makes no sense to the average person, yet is the simplest way of explaining what we consider to be unusual phenomena.

Zack Lau W1VT

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W6GX
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« Reply #24 on: January 15, 2013, 09:20:37 AM »

Hi All,

Thank you for all the replies.  I'm still catching up on my e-mails so please be patient if you've sent me a private email.

I have two basic questions that I'm seeking answers for.

1) For the moment let's ignore the changing ionospheric conditions.  Station A has a big yagi and station B has a dipole only 10' off the ground.  Both will transmit at identical power level.  Should both see identical signal reports provided that two identical rigs are used to measure signal strengths?  Some have said that station A will sound louder.  However if station A has a great antenna shouldn't station A also RX as well as TX?

This question may not be related to law of reciprocity, which says the HF signal is propagated equally in both directions.  It doesn't take the antennas into account.

2) If the answer to #1 is yes, then why do I consistently get better signal reports?  If the answer to #1 is no, what is the reasoning behind it?  Jack W1VT has provided some theories behind non-reciprocal propagation.  However the theory doesn't explain why I consistently get better signal reports.

73,
Jonathan W6GX
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WS3N
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« Reply #25 on: January 15, 2013, 10:53:11 AM »

The key to the weirdness is the splitting of the radio waves into two separate waves of opposite circular polarity--depending on conditions, they can cancel each other out to produce fading, or unequal signal strengths at each of the path.  This is the result of interaction with the ionosphere, as opposed to a reflection off the surface of the Earth, which doesn't do that.

Zack Lau W1VT

Is that a theory or widely accepted by scientists as fact?

73,

Chris/NU1O



It is a theory, in the scientific sense, and the theory is widely accepted because it has been well established by experiment.

This is standard physics. Charged particles spiral around magnetic field lines because the magnetic force is perpendicular to both the particle velocity and the field direction. There is no force if the particle moves along the field line, so a wave moving normal to a field line and with its linear polarization along the field line (so that it pushes the particle back and forth along the line) will propagate differently than one with a linear polarization perpendicular to the field line (which drives the particle perpendicular to the field, and the magnetic force pushes it perpendicular to both). Waves traveling along the field lines have their linear polarizations rotated, which can best be understood by saying the two circular polarizations travel at different speeds. This causes an increasing phase difference between them, which results in an increasing rotation when expressed in terms of linear polarizations. This effect is known as Faraday rotation.
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WS3N
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« Reply #26 on: January 15, 2013, 10:58:31 AM »

Saying that the ionosphere is complex and constantly changing doesn't explain why we don't have reciprocity.

But, explaining that that ionosphere generates two signals out of one that can cancel each other out does provide a useful theory that can be tested experimentally.

Propagation depends on polarization. There's nothing about cancellation.
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W1VT
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« Reply #27 on: January 15, 2013, 12:54:18 PM »

Here are two web pages by hams on the issue of fading and circular polarization.

http://www.bruhns.us/CP_on_HF/CP_on_HF.html

http://www.pa0sim.nl/XOpropagation.htm

Zack Lau W1VT
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