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Author Topic: Non-symmetric propagation?  (Read 4463 times)
WX2S
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« on: February 06, 2014, 03:07:26 AM »

Why would propagation from A to B sometimes not be the same as propagation from B to A? I can think of some possibilities, but would like to hear from those who know.

73, Wx2s.
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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
KE7TMA
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« Reply #1 on: February 06, 2014, 05:37:07 PM »

Why would propagation from A to B sometimes not be the same as propagation from B to A? I can think of some possibilities, but would like to hear from those who know.

73, Wx2s.

Takeoff angle perhaps?  That is my first thought.  Here in my canyon I think diffraction does funny things as well.
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AA4PB
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« Reply #2 on: February 07, 2014, 05:16:45 AM »

It's possible that sometimes one station has a higher local noise level than the other. Receiving is all about signal-to-noise.
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AA6YQ
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« Reply #3 on: February 07, 2014, 01:37:45 PM »

Differences in antenna gain, TX power, and RX sensitivity can also contribute.
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KB4QAA
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« Reply #4 on: February 07, 2014, 05:53:08 PM »

Different antennas at each location, separate Tx/Rx antennas, different ground conditions...etc.
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WX7G
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« Reply #5 on: February 07, 2014, 06:38:05 PM »

Given a symmetrical ionospheric path the link is symmetrical regardless of the antennas. An antenna is is deficient in transmit is equally deficient in receive.

From what I read and have experienced the ionosphere is symmetrical most but not of the time. According to an article I read last year East-West ionospheric paths are the ones that are most likely to be non-symmetrical. The non-symmetry was said to be up to 4 dB.
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KE7TMA
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« Reply #6 on: February 10, 2014, 02:29:01 PM »

Given a symmetrical ionospheric path the link is symmetrical regardless of the antennas. An antenna is is deficient in transmit is equally deficient in receive.

Easy experiments at home can refute this idea.  Remember antennas have polarization as well as other characteristics aside from gain.
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K9AIM
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« Reply #7 on: February 10, 2014, 02:35:39 PM »

Why would propagation from A to B sometimes not be the same as propagation from B to A?
73, Wx2s.

good question!  it may be the same kind of thing where my wife has better reception about what i am up to than i do about her  Huh  Cheesy
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KI6LZ
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« Reply #8 on: February 10, 2014, 02:49:38 PM »

The ionosphere is not homogeneous. It's anything but the spherical representations in introductory articles. Consider it clouds of plasma, non uniform in density, varying in height by 10s of thousands kilometers, continually changing during the day and night. Travel of EM waves will never diffract from the same path, lots of times there a skew paths. This is a tough topic to condense, lots of info available on the web.
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W8JI
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« Reply #9 on: March 18, 2014, 03:38:30 PM »

Given a symmetrical ionospheric path the link is symmetrical regardless of the antennas. An antenna is is deficient in transmit is equally deficient in receive.


That is almost never true.

Receiving S/N ratio is a function of antenna directivity, local noise floor, propagated noise, receiver bandwidth, and the effective radiated power of the distant station at your direction, angle, and even the useful polarization for the band and path at that moment. Gain is largely meaningless unless gain is so low the system's internal noise starts to contribute significantly to S/N ratio.

Transmitting effectiveness only deals with absolute antenna gain at the desired angle and polarization, and the transmitter power, assuming bandwidth and audio has no issues hurting intelligibility. Directivity and noise floor at the TX site is meaningless when transmitting.

There are consistent HUGE differences in propagated and local noise floors, and this often causes very noticeable dissimilarities in reciprocity for the same antennas and power.

For example, in daylight my noise floor is nearly S1 on 40 meters. I can hear Europeans on 40 meters almost 24 hours a day in winter. I have no hope of working most of them no matter how much power I run, because it is dark over there and their noise floor is probably 20-30 dB higher than mine.

As sunset approaches, my noise floor on 160 through 40 increases 10-20 dB. My signal into Europe increases more than that, probably 70 dB or more, and their signals increase the same.

Let me do it with typical numbers for two locations with numbers instead of words:

Quiet location daylight noise floor = -125 dBm
Noisy location daylight noise floor = -100 dBm
Quiet location darkness noise floor = -110 dBm
Noisy location darkness noise floor = -100 dBm
Daylight path loss 180 dB
Night path loss 110 dB
Let's assume ERP is equal at +63 dBm

"S-0" no meter movement is -125 dBm in this example (no S meter system is really standard, so let's pretend like it is S-0)

daylight to dark 63 - 180 = -117 dBm (signal S1 or so) but the dark station has a noise floor -110 to - 100 dBm (S2 to S4). The dark station cannot copy the S1 signal.

dark to daylight 63 - 180 = -117 dBm daylight station location has -125 to -100 dBm noise floor, so the quiet location hears the signal very well but the noisy location does not.

dark to dark 63 - 110 = -47 dBm so any station clearly hears the signal. -47 dBm is well over S9.

Noise and ERP are much more pausable than any other explanation, and don't require any magical propagation.

This is why people with poor directivity antennas hear so much better at local sunrise and sunset, and even people with good directivity hear better at local sunrise or sunset.
 
73 Tom


 
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