HF Reciprocity?

[Steve - WB3HUZ]

Interesting article at the link below. Over the years I've noticed many hams think there is reciprocal propagation in all instances at HF. Some years ago, I even had someone with a very poor antenna (crappy dipole about 15 feet off the ground) claim that something was amiss since he was hearing one of the big guns of 75 meter AM at 40 db over 9 but was getting a PW report in return.

http://hfradio.org/ace-hf/ace-hf-reciprocal.html

[The Slab Bacon]

they definately dont always hear you the same way you hear them, or visa versa. Over the years I have notnced many anomalies in propagation. Many times different one way than the other.

One of the more unusual things I have heard many times is one-way propagation. I sometimes joke about it being "diodic" propagation!!
Someone will hear you pounding in at 40 or 50 ova, and you can barely hear them or not hear them at all, or the otha way around. This is not at all uncommon on HF. that is some of the fun and intrigue of it all. Never ever feel confident that the other station is hearing you the same way that you are hearing them.

You just nevva know, it can go either way!!

                                                 The Slab Bacon

[AF9J]

Nah, it doesn't surprise me a bit.  I've known & experienced it gfor decades, one-way & non-reciprocal propogation.  As the article mentioned several things can cause this:  1.) differences in noise levels on each end; 2.) different antennas with different radiation patterns & gain characteristics; 3.) even the non-homogenous nature of the ionosphere, and the fact that both stations in the path, could be refracting through different spots and/or layers of the ionosphere. 

73,
Ellen - AF9J
Trying to decide if she should use the Globe Scout 680, or the Heathkit Cheyenne

[AB2EZ]

Steve, et. al.

The "law of reciprocity" ...like any physical law... reads as follows: "If the following experiment is performed, and if the following experimental conditions are satisfied, then one will observe the following reciprocal behavior".

The problem is that most people who refer to the law of reciprocity are not aware of the experimental conditions under which it applies.

For example, a microwave "circulator", with three ports, does not obey the law of reciprocity because it does not present a time-invariant path to the signals that propagate around it. The fixed magnetic field applied to the circulator's material causes molecular-scale magnetic dipoles within the material to precess (like a top in the presence of gravity)... which produces Faraday rotation.... which corresponds to a time-varying medium... which violates the time-invariant path conditions which must be satisfied for the law of reciprocity to hold. Likewise, a satellite-to-ground  path that experiences Faraday rotation will not, in general, obey the law of reciprocity.

Another problem is that sometimes people forget that the law of reciprocity requires that, at each end of a communication path, the antenna system used for transmitting is the same as the antenna system used for receiving (although the antennas systems at each end can be vastly different). A subtle point here is that "the antenna system", in this context, refers to everything including the input circuit of the receiver and the output circuit of the transmitter. So, if, at one end, the receiver has a 50 ohm input impedance, and the transmitter (for the particular output tank settings being used) has a 100 ohm output impedance... then that does not satisfy the conditions needed for the law of reciprocity to hold.

Finally, as pointed out in the referenced article... reciprocity refers to the transmission loss between the transmitter at one end and the receiver at the other end... being the same in both directions, if the conditions that are associated with the law of reciprocity are satisfied. It does not guarantee that the noise will be the same at both ends. Thus, for equal transmitter power at both ends, the signal strength will be the same at both ends, but one station may have a much higher signal-to-noise ratio. This will usually occur if one end has a much better antenna (higher radiation efficiency) than the other end. The end with the "crappy" antenna will hear the signal of the station that has the good antenna, because the crappy antenna   will pick up much less atmospheric noise. The station with the good antenna will not hear the signal of the station that has the crappy antenna, because his station receives much more atmospheric noise.

Stu
AB2EZ

[WA1GFZ]

AH the magic of converting 377 ohms to 50 ohms.....

I built a run of #8 with Johnson 4 inch spreaders Saturday.
It really makes you feel good to make nice feed line...but my fingers still hurt after using $14 tie wire. 

[The Slab Bacon]

When Steve (HUZ) lived here in B-more we were only 10 or 12 air miles apart. (if that) There were many nights on 75 when we could not hear each other that well, or at least as well as one would have thought. I have noticed the same thing with Paul (VJB) who is prolly only 25-30 air miles away. I just think we were both too close and in the "skip zone" to each other. Too far for true "line of sight" ground wave with horizontal antenners, and too close for "sky wave" bounce off of the ionisphere. Itz just that simple.

                                                          The Slab Bacon

[W9GT]

Stu certainly summed it up very well.
 
Just my own observations: It seems to me that in order to have true reciprocity...you would have to have identical equipment and antennas on each end, the same ground conductivity and antenna height, and the same near-field conditions around the antennas.  Even, if you had all of that....the propagation could still vary in each direction due to variances in the layers of the atmosphere.  That variation can also occur over very short intervals of time.  The ambient noise level at each location is also a major factor, with urban areas typically being much noisier than rural sites.  Signal to noise ratio on each end has to be a major contributor to the overall path performance equation.
 
Most of us who have been active over a long period of time have experienced the "one-way propagation" phenomenon.  It can be frustrating, as well as interesting to observe.

73,  Jack, W9GT

[The Slab Bacon]

Another interesting anamolie that I remember is a few years ago one night late on 75. We were on 3.880 or 85 IIRC. We had at least or more callsign in this huge roundtable from every call district. NOONE could hear another within 5 or 600 miles. Basically seeming like the Mississippi was the divider. We would pass it back and forth from one side of the country to the other. We had to do it that way so you could tell that someone was talking. The signals from one side of the country to the other were huge and full quieting, but you absolutely could not hear anyone on your side of the river AT ALL! It was really strange, but a lot of fun! Everyone that could hear each other was pretty much passing back and forth identical signal reports. We had someone in there from every call district. It went on well into the wee hours of the morning. It was just kinda strange listening to someone on the other side of the country working one of the "big guns" on this side and you couldnt hear them at all.

                                          The Slab Bacon

[Todd, KA1KAQ]

Similar conditions for me Sunday on 40. Heard a WA8 going at it with Robt. W0VMC on 7296 +/-. Heard them quite well, initially. Tried jumping in twice, no reply from either. Moved down to 7289/7290, called CQ. Russ WB3FAU responded, loud and clear. We carried on for a couple hours or so with other stations popping in and out. Barely heard W9GOB in Chicago on his Beastly-610H but he was hearing me fine.

I've worked W0VMC several times with no issues. Sunday, no joy. And I was running the big rig, too. Ya just never know.

[Glenn NY4NC]

Progressive propagation?

Over the years I've noticed many hams think there is reciprocal propagation in all instances at HF.

[AB2EZ]

Hi!

I have been thinking about this for several days now. The question in my mind is: what necessary condition that underlies the law of reciprocity is not being met when people observe the "diode" effect?

Here is a possible explanation.

We observe reciprocity on long distance paths... which involves reflections off of the ionosphere, caused by the physical phenomenon of refraction of HF radio waves passing through the ionosphere. Whenever I try to contact a DX station that is coming in "loud" at my location, I can get through at his location... even in a pile-up, if I am patient.

However, for paths between (for example) NJ and Vermont, we are depending on Near Vertical Incidence Skywave (NVIS) propagation, which also involves a reflection off of the ionosphere; but which is a different physical phenomenon.

I searched the Web, and I read the articles I could find... but none of them had enough details about the physics of NVIS to help explain the cause of "one way" propagation. But from what I did find, NVIS appears to be associated with Faraday rotation... and a channel that includes Faraday rotation does not satisfy the necessary conditions for reciprocity to hold (e.g., the example of the microwave circulator in my earlier post, or, for another example, a microwave or optical isolator).

Stu
AB2EZ

[Steve - WB3HUZ]

It's not always reciprocal on long paths, especially on the lower frequencies. Do some reading on the the auroral oval, polarization effects on 160 meters and the day-night terminator.

[steve_qix]

There is something to this.  Stu and other stations in NJ and in that part of NY regularly experience the so-called "North-South Diode" effect (or should I say the stations in Vermont, New Hampshire, etc experience the effect).  The Northern hams can hear the operators from the NJ/NY area very well (to the point where the stations from the South are covering up the stations local to each other in the North) yet the reverse is not true - the Northern operators cannot be heard at all.

This has happened so many times (sometimes a daily event during the daytime on 75 meters) and with so many stations with differing levels of power, antennas, etc.  that, although hard to prove from a scientific standpoint, does appear to be an actual condition.

Sounds like a good doctoral thesis to me:  "A scientific basis for the ionispheric condition known in popular lingo as the "North-South Propagation Diode" on the 75 meter amateur band.

Regards,

Steve :-)

[Steve - WB3HUZ]

Possibly due to auroral effects/auroral oval. Stations further north are more likely to have their signal attenuated.