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Author Topic: The 'Impossibility' of One-Way Propagation  (Read 2137 times)
MW1CFN
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Posts: 31




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« on: December 11, 2012, 12:44:04 AM »

I was curious to read some typically vehement exchanges, dating back to about 2005, on the asserted impossibility of one-way propagation on HF.

I'm never one to be quite so ready to draw conclusions, though the whole issue of propagation is very complex and straight-line paths we imagine to take place are frequently much more convoluted.

This morning, we have a grey line situation between myself and the east coast of South America.  Not much, other than clear Atlantic sea water, between us.

On WSPR, two stations lying in line with one another, separated by what is likely to be (so far as anyone can actually tell) less than a skip distance.  They are in southern Brazil and Uruguay.  They can hear my 5W, I can't hear theirs.  Yes, they are transmitting!  Typically, my WSPR set-up compares very favourably with any UK WSPR station, and on an assessment of other data, I'd say that often, it does better than most.  An unbuilt, elevated site, very little or no noise and a copper mine for ground does, I expect, help the situation.

WSPR tends to involve people using simple verticals or omnidirectional wires.  Beams are not common and of course defeat the whole object of a beacon mode like this.  That goes a long way to providing a more uniform system of set-ups, although the quality of receivers can of course vary considerably and may well fully explain apparent one-way propagation.

So, whilst I have no proper assessment to say apparent one-way HF propagation is in fact real, I think WSPR does allow a much more nuanced, evidence-based approach to the question where, previously, it was just down to who could shout loudest.

I wonder if anyone has done proper analysis of WSPR signals with this question in mind?

No doubt I should not worry about the strength of responses about to come this way!

J.



« Last Edit: December 11, 2012, 12:50:31 AM by MW1CFN » Logged
KA4POL
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Posts: 1862




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« Reply #1 on: December 11, 2012, 01:51:34 AM »

See http://www.eham.net/ehamforum/smf/index.php?topic=26665.0
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STAYVERTICAL
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Posts: 854




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« Reply #2 on: December 11, 2012, 02:36:48 AM »


From the book "Practical antenna handbook" by Joseph Carr, fourth edition:-

QUOTE:
Nonreciprocal direction -  If you listen to an amateur band receiver on the East
Coast of the United States, you will sometimes hear European stations—especially
in the late afternoon. But when you try to work those stations there is no reply whatsoever.
They simply don’t hear you! This propagation anomaly causes the radio wave
to travel different paths dependent on which direction it travels; i.e., an east→west
signal is not necessarily the reciprocal of a west→east signal. This anomaly can occur
when a radio signal travels through a heavily ionized medium in the presence of a
magnetic field, which is exactly the situation when the signal travels through the
ionosphere in the presence of the earth’s magnetic field.

Another anomaly seen in the radio literature of the 1930s is the Radio Luxembourg
effect. It is named after the radio station where it was first noticed. In a
nonlinear ionosphere, it is sometimes noted that the modulation of superpower
(i.e., > 500,000 W) shortwave broadcasters will be transferred to the carrier of a
weaker signal in the same or nearby band. The interchange noted in the 1930s
when this phenomenon was first discovered was between Radio Luxembourg and
Britain’s British Broadcasting Corporation (BBC) overseas outlets.

END-QUOTE

FROM WIKI:

" 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."


Common sense dictates that a chaotic place like the ionosphere with hams using different antenna types, different incident angles of reflection, and so on, reciprocity would be approximate at best.
For example, if one ham is using a vertical with low angle radiation and only uses two skips, when the other station uses four, the signal strengths should not be the same, other equipment being equal.
The angle of incidence even changes the penetration depth of your signal, and so its skip distance - there are just so many factors.
The ionosphere is not a shiny static metallic mirror in space, but a squishy sponge of swirling plasma in a strong magnetic field.

I really don't understand why people would argue about something that is so easy to find, understand, and appreciate on the internet.
There are even peer reviewed articles from researchers on the subject, so it is hardly hidden science.
The jury has handed in its verdict - yes it is possible to have non-reciprocal propagation at times.
Any argument to the contrary is really just being argumentative for no good reason.

I would not use the term "impossible" in almost anything these days.
Improbable perhaps, but never impossible.

73 - Rob
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WA3SKN
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Posts: 5420




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« Reply #3 on: December 11, 2012, 05:42:00 AM »

"asserted impossibility of one-way propagation on HF."
I have no idea why you would think this way, as one way propagation is not only possible--- it is quite common... and documented!  What you read is simply wrong.
73s.

-Mike.

(It's gotta be true, I read it on the internet!)
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N6AJR
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Posts: 9879




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« Reply #4 on: December 11, 2012, 02:14:19 PM »

look at it this way, if you have a good bounce on your end to get it there, it does not mean theirs will be strong enough for you on the reverse. 


take a flash light and hold a flash light on it at and angle and bounce a  beam onto the nearest wall.   this is you bouncing your signal off a good local  reflection source. 


no bounce the light off the wall and see how much bounces back through the mirror to you.  it does not have a good reflection off the wall and only a tiny part of the light bounces back to the mirror.

even though the reflection is still there, there is not enough returning signal to bounce back with any amount of usefullness.  and remember light is a wave just like radios waves.
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WX7G
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« Reply #5 on: December 11, 2012, 03:29:08 PM »

From what I have read and experienced, non-reciprocal propagation is the exception rather than the rule.
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WB6BYU
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Posts: 12980




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« Reply #6 on: December 11, 2012, 03:30:13 PM »

I don't think it has to be that complicated.  I had a classic example one day on
path from Oregon to Hawaii, with less than 100km of land between us.  Over the
course of close to an hour there were times when one of us could hear the other
but not the other way around.  Then I'd switch to a different antenna and the
situation would change.  Basically we were experiencing slow fading - signals
would go up and down in about a 5 minute cycle, but the peaks and nulls did
not coincide at each end.

A large part of the explanation, based on the results of switching antennas, appeared
to be polarization rotation.  One antenna was a 2-element yagi and the other an 80m
long end-fed wire.  The polarization of the wire will depend on the angle (elevation and
azimuth) of arrival, as is also the case with a simple dipole at a low height above ground
(which is primarily vertically polarized off the ends.)

If two stations have a 45 degree polarization difference and the ionosphere contributes
45 degrees of shift, then one station may be rotated to match the other in one direction,
but the second gets rotated to be cross-polarized with the first in the other direction.


Not that this is the only possibly explanation, of course, but it is a fairly simple example
of one possible mechanism.
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AB2RC
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Posts: 126


WWW

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« Reply #7 on: December 12, 2012, 07:33:41 AM »

They hear you, but you don't hear them.

Consider the takeoff angle of both antennas - you may have a lower takeoff angle, and they are getting the signal on the first hop, they may have a higher angle, and the signal take multiple hops to get to your qth. By the time the signal arrives at your end, it may be too weak to receive.

The reverse is possible also, a lower takeoff angle at their end, may cause the signal to skip right over you.
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K8GU
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Posts: 716


WWW

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« Reply #8 on: December 12, 2012, 09:44:33 AM »

I've lifted much of this text from a posting to the CQ-Contest e-mail list that I did in 2007.  I think everyone is familiar situations of differing local noise floor, e.g., Europeans are commonly audible on 160/80/40 on the East Coast of the U.S. long before they are workable.  This is, at least in part, due to the fact that they are receiving noise (unwanted signals of all kinds) from 360 degrees instead of just to the East where the band is open in the U.S.  These SNR differences tend to explain a lot of "one-way propagation" reports.  Signal-to-noise ratio equivalence at either end of a path does not constitute "reciprocity" in the electrical network sense as the following paragraph uses it.  But, in order to prove the theory outlined below, the much higher standard of reciprocity is required.

Reciprocity of skywave links is a very old topic indeed.  The underlying principle we need is Goubou's theorem (1942).  I now reach for a 2007 Hamvention flea market treasure (tnx N8TR and N8DMM): "Radio Waves in the Ionosphere: The Mathematical Theory of the Reflection of Radio Waves from Stratified Ionised Layers" by K. G. Budden (Cambridge 1966).  According to Budden, Goubou's theorem may be concisely stated the following way:  "For any aerial system at A it is always possible to construct at least one aerial system at B so that there is reciprocity between the terminals of the two aerials."  In other words, reciprocity between two points is always possible, but not guaranteed for any given pair of antennas at those points.  Budden continues with the proof and a discussion including magnetoionic effects.  I invite you to look it up if you're really interested in sausage production and swine-rearing.

I think our collective experience bears the theory out in practice.  That is, many combinations of practical amateur antenna systems have a high probability of reciprocity.  However, we do get surprises now and then, especially on the low bands and near the MUF.
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K6AER
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« Reply #9 on: December 12, 2012, 11:48:44 AM »

There is no one way propagation. What you are experiencing is most likely different noise levels at the two locations. I live out in the country and my noise level is typically 0 dB on the S meter. I have found I have use my amplifier for stations in the city to hear my signal when I have no problem hearing their 100 watts or less.

Also all antenna gains are additive no matter the direction. Many times I have heard ham say because I have a beam up high that is why my signal is at a certain level. If the two ham communicating are running the same power the signal strength will be the same at both ends.

S meters are calibrated all over the map and in most cases are not linear at all. Just because you are a S7 at one end and the other station is a S4 does not mean the signals are different, it is the S meters that are different.
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KD8MJR
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Posts: 1984




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« Reply #10 on: December 12, 2012, 02:17:01 PM »

Call it what you want but I have had many occasions when I am talking to a very good friend in another country and he is 59+10 and I am 59+10 on his side.  Then the QSB kicks in and sometimes we both drop to 57 or below and in 90% of cases pretty much have matched drops.

Now the weird part is on some occasions (the 10%) he drops to 56-57 yet tells me that my signal is still 59+10!
He will sometimes go right back up and drop down again while telling me my signal is constant. (and yes both of us are using fast agc setting and similar equipment.

So is this one way QSB or a small glimpse of WSPR at work?

« Last Edit: December 12, 2012, 02:21:33 PM by KD8MJR » Logged
STAYVERTICAL
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Posts: 854




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« Reply #11 on: December 12, 2012, 10:54:48 PM »

Imagine you are on the bridge of a ship at sea in a large swell.
Another guy is a on a similar ship on the horizon at the same height above the sea.

Both of you are rolling around in the swell.

Sometimes you are both vertical and at the top of a wave, other times you are both in troughs.
But most of the time you are somewhere in between.

If you were signaling each other by lamp, then you would get intermittent copy as you "faded" in and out of each others view.
But if the sea were calm you would have consistent results, without the extreme fading.

The ionosphere is sometimes calm, and results could be consistent, but not necessarily identical like line of sight operation.
All of the aforementioned properties of radio waves: angle of incidence, skip distance, local changes in ionospheric height, polarisation mismatches and local multipath reception will affect the skywave link.

When the ionosphere is in turmoil, you are refracting from a moving target in both space and time parameters.
Unless you are both in NVIS skywave mode, the ionosphere is extremely variable over any DX paths.
The gray line is one example of why good propagation is possible, since the terminator produces similar ionization for the path.

Sure local noise conditions will have an effect, but it is just one aspect of a complex issue.
Like being on the deck of a rolling ship, sometimes we see water, sometimes sky, and sometimes the guy on the next deck hurling over the side.
No one is right or wrong, they all have pieces of the puzzle, which if everyone puts in their piece gives a complete picture.

73 - Rob
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K6AER
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« Reply #12 on: December 13, 2012, 08:17:14 PM »

QSB fading effects both ends of the path at the same time. The problem is you are not duplex on HF. You only know of the signal level when you are receiving.
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