Saturday, August 12, 2017

By What Path Do They Get Here

I was asked in one of  my Facebook groups how signals travel to their destination, or by what path do they come. The simple answer is " by a Great Circle path."

What does this mean?  Well if you think back to geometry class ( I know that is asking a lot!!) the definition of a great circle on a sphere is  " a line on the surface of the sphere defined by the intersection of a plane passing through the two points in question and through the center of the sphere"

OK, so that is a little esoteric. Think of it this way. Take the equator line and tilt it so that it passes through the two points, or cities, that you are looking at.

If you look down on the line while holding a globe,  this path looks like a straight line.  but if you look at it on a flat map it will look much different,  most likely as a curved line. If you are familiar with what is known as a polar projection, or a flat map made up as a circle with the center being the north or south pole you can get a better idea of what is going on. A really good way to look at it would be if you could find a polar projection based on your own location.  There are computer programs that can do this for you and I would be willing to bet with a little judicious searching on the internet you could find a website that can do it.  You would just need your latitude and longitude to enter.

Sometimes the direction from your place might be much different than you would intuitively think.
Like who would think that the beam heading for Alaska and Japan would almost be the same from Central Texas.  Of that the beam heading for Europe and East Africa would be almost the same.

You will notice if you do this mental exercise or even look at a globe with the two points actually on the equator that there are two paths between the points: the short one and the long one.  Thus the definition of " long path" and " short path".

Logically the shortest one is the one by which the signal you are hearing arrived,  and most of the time this is true. However, there are times and conditions under which the signal can arrive the other way round, or in some cases by both.

Which way it comes depends upon the time of day, frequency and general band conditions. If you are where I am, in Central Texas, the shortest distance to Australia is to the southwest over the Pacific Ocean.  If I want to hear or work an amateur station in Australia on forty meters, the best time for short path is early in the morning my time. That insures that the signal will have a darkness path most of the route.  Often the window is short depending on where in Australia the station is located, because the sun will be either just going down or has been down a little while  the sun is almost about to come up here. Propagation on forty meters ( 7 MHz) the other way around in the morning my time is simply not going to happen because that path is in full sunlight most of the way.

But let's take a look at " the other end of the day." Just before sunset in Texas, in some parts of Australia the sun is just rising, or just about to rise. The path the long way round is in darkness most of the way.  Prop on forty meters usually lasts a little beyond sunrise at most locations, sometimes by an hour or more.  If on that particular day, conditions are such that the Maximum Useable Frequency happens to be fairly low. it just might be that prop can begin a little before sunset and continue until a little after sunrise,  just enough to allow the signal to get through, and you get your signal via long path!

Now what direction is the signal going? Logic might try to tell you to the southeast from Texas, but that is not correct.  The signal still follows the Great Circle path,  which means it goes Northeast, up and over Europe, then down toward Australia.

Long path prop does not happen every day or on every frequency. There are a number of factors that might determine whether it will occur or not.

First, for the lower frequencies, there still must be darkness over much of the path. There might be some stretching at either end with some light if the first hop could still be in darkness. This can occur if the frequency involved is at the higher end of the range that can be propagated in darkness. For example, my most frequent success on the amateur bands for long path prop to Australia from Texas has been on the 30 meter band. There are times that rather long hauls can be made in the late daytime or near dusk that might make the first hop possible even a couple hours before sunset. The subsequent hops would then be in darkness.

For shortwave listeners,  the closest approximation might be the 31 meter broadcast band.  In the winter hemispheres, 25 meters might be too high for good prop after dark at the higher latitudes and would not make it,  but 31 meters might. We all know that 31 meters does well until a good ways after sunrise as evidenced by NHK from Japan and Radio New Zealand being listenable after sunrise in North America ( and of course, the now gone Radio Australia on 9580 was often listenable well after sunrise while it was still on the air)

There are times that the 40 meter amateur band and the 41 meter shortwave broadcast band can provide the same kind of performance.  The only difference is one must be much closer to darkness for it to work.

There may be exceptional times where prop can occur over both paths at the same time. I have noted this most often at higher levels of sunspot activity on the higher bands that can still sustain prop at night. This kind of prop is characterized by a pronounced echo on the signal brought about by the different transit times for the two paths. My earliest experience with this occurred listening to the VOA relay station in the Philippines back in the early sixties.

As a side note, echo does not always indicate both long and short path prop.  There are also times that I have noted Pacific or Japanese stations with pronounced echo in the mornings that is the result of something else altogether. It is a phenomenon known as " Backscatter". This occurs most often when a station in the Philippines or Japan is beaming a signal away from North America. Some of the signal arrives direct path off the back of the antenna. ( One must note with care when looking at beam headings listed for broadcasts.  Just because a certain beam heading is indicated does not mean that there is no signal sent in other directions.  Even if the front to back ratio of an antenna array is 20 db or even more, if the station transmitter is using high power in the 100-250 kw or more range and the effective radiated power in the main lobe is nearly a megawatt, 20 db down in the opposite direction is still a power level high enough to provide a fair signal. Twenty db down from an ERP of a megawatt is still 10 kw!)

In backscatter, signal that arrives at a distant point from a transmitter often has some bounced back toward the transmitter, and if the prop is good, can hop again toward a listener off the back of the antenna, too.  The difference in transit time can give the echo effect. Some times because the backscatter signal has originally come off the front of the antenna with much greater power and the "direct" signal was radiated with less power,  sometimes the echoed signal can be noticeably stronger than the direct signal,  giving a really strange effect of the echo being louder than the first part you hear.  Sometimes this effect is so pronounced that it can be difficult or impossible to understand the words being spoken.  On a cw amateur band signal,  the delayed dits and dahs can fill in the spaces and make the signal absolutely impossible to copy.

This is a bit of a digression, but because backscatter is an interesting phenomenon perhaps it should be included here. And in a way it fits because it can provide an anomaly in the direction from which a signal appears to come.  This happens most noticeably on the ham bands, but conceivably could happen with broadcast stations, too.

There are times when a relatively nearby station--that is, one too far away to be heard via groundwave but within the "skip zone" or area where the signal is usually not heard because it quite literally "skips over" the receiving site--can be heard, usually with a fairly weak and often very fluttery sound.  What is happening in this case is that the signal is being heard totally via backscatter. That is, the signal is going from the originating signal through the first hop distance, then when it hits the ground or water, while a good bit of the signal goes forward for the next hop,  some of the signal sort of "splatters" and usually most of it goes back in the general direction from which it came, making it audible in the general region where it originated.

A good example of this would be a Texas station being heard on 10,15, or 20 meters by other stations in Texas and Louisiana perhaps well beyond groundwave range but " too close" to be heard by skip. It has been my general observation, by no means meaning that this is the only time it happens,  that this occurs usually right before the band goes out to the original target area.

When this happens, some of the backscattered signal might go off in a range of azimuths, allowing the station to be heard in other areas besides where it was originally targeted or where the "regular skip" path would have taken it.  This would lead a receiver of the Texas signal in, say, Mexico or Central America, to say it appeared to have come from the west or southwest, making it appear that the signal came by the odd path if the receiving station were using a rotary beam of some kind.  It would peak up as coming from the point of the back scatter rather than from whence it really came.

This is where I believe the fabled phenomenon some times called " one-way skip" comes from.  If the station hearing the Texas station via backscatter were to try call or answer the station,  the contact would never be made because the chances of the answering station making the trip back to the original station via backscatter from a different angle would be very small indeed.

Other echo effects can occur on signals whose paths take them through the auroral zone in the arctic.
Signals get tumbled and jumbled and can take on a watery or echoey sound.  This often happens in the amateur bands in the Central US when listening to signals from Sweden, Norway, Finland and north-central Russia. As in the case of backscatter,  this can make a phone signal hard to understand or cw all but impossible to copy. On the amateur bands, when trying to work a station in Scandanavia or northern Russia on cw under these conditions,  the old school guys know the thing to do is to SLOW DOWN and put more spaces between characters so the echo doesn't kill your intelligibility.  This is why I always keep a straight key at the ready along with my Vibroplex bug and electronic keyer!

Gray line prop is another condition that can often make signals appear to come from a different direction than they " should"  There are lots of theories about what brings about so-called gray line enhancement, in which signals seem particularly strong along the boundary between daylight and darkness.  I make no claims to understand with any certainty what actually occurs,  but have learned to take the " bonus " signals or signal strength and enjoy it. There are times when the signals seem to come from other directions than they should.  Sometimes the best thing to do is just log 'em and don't worry about it.  If using a directional beam antenna,  just rotate for best signal strength and don't look at the azimuth indicator.  The bottom line: get them in the log!

One other time in which signals can appear to come from a different direction than they should is during Sporadic E propogation on VHF frequencies.  I have noted times when it almost appears a signal has taken a right or left 90-degree turn when arriving.

This happens when the signal arrives by multiple hops.  Sporadic E occurs when highly ionized patches or clouds form in the mid level of the ionosphere with the ionization levels high enough to provide reflection of signals well into the VHF region.  Sporadic E can also extend into lower frequencies, but usually only down to 25 MHz or so.

If there happens to be two different sets of E layer clouds and IF a signal bounces off one, strikes the round in such a way as to scatter and perchance strike a second E layer cloud somewhat off azimuth from the original path,  the signal path could in effect, be bent.

Sometimes during a really wild sporadic E opening one can almost wear out a good antenna rotator trying to figure out where best to point the antenna. Again, its often best to just leave it where the signal is best readable and not worry about it.  Truth be told,  often Sporadic E signals come down at such a steep angle that they strike the antenna from high above and it probably matters not which direction its pointed.

How do we predict these things?  Well,  that could be a really tricky business.  One could spend so much time trying to do it that there would be no time left for DX-ing.  And perhaps this is where differing philosophies come in.  Whether it be double hop Sporadic E,  grey line prop, long path prop or whatever,  there are some who would try to assign numbers, develop models,  look at the physics of the thing and basically geek themselves almost into a coma trying to predict and figure them out or explain what's happening.

It is my personal feeling that while this is well and good and a great academic exercise,  for me THIS IS A HOBBY!  While there are some fishermen who do the same thing,  others just go out and fish.  I go out and listen.  For me, what is more fun is to turn on the radio,  tune around and get a feel for what's happening and then intuitively shop for the best DX.

I have often said that DX-ing is a lot like fishing.  One learns to read the signs and then just know where to look and how to look. Experience has taught me that some indicators indicate certain conditions and to let that guide where I look.  There are some tools that can help a bit, like the prop forecasts.  Though if I had paid attention to prop forecasts that called for poor conditions or some kind of disturbance, I might never have turned on the radio and missed some great stuff.  But I have learned that when there is supposed to be no prop,  often folks do just that and there is little activity.  But if there is a little atmospheric noise and one tunes around, someone might just find somebody rare and distant hopefully calling CQ.  I well remember stumbling across my first Guam, Johnson Island, Banaba, Christmas Island and a few others just that way!!

Sometimes another "new school" barometer to look at is DX Summit or some of the other DX reporting sites that post real time signal spots.  Take a look and just see where the stuff is coming into various areas and on what frequencies.  This is even useful to the non ham or listener who is not interested in ham stuff.  But what is happening on the ham bands can be a really good barometer on what is happening on the nearby shortwave broadcast bands.  Forty meter spots being a good barometer for 41 meters and to a lesser degree for 49 meters. Twenty meter and seventeen meter spots are good indicators for 19 and 16 meter shortwave stations.  Thirty meter spots are good for 31 meter shortwave conditions and to a lesser degree 25 meter shortwave broadcast spots. Sixty meter spots are good for, well, the 60 meter shortwave band. And so on.....

But I am wandering a bit off track here.  Hopefully this will help some get a feeling about how things appear to me to work. This is based not on textbook theory, but over fifty years of observing what really happens out there.  Sometimes it seems better not to worry so much about the why's of something and to just jump in and take advantage of it.

Good DX-ing to all, and as always, I am happy to hear from anyone with their own observations and ideas through the comments section of this blog or through the Facebook groups you can find me on.
Don't forget,  if the bands sound dead or the signals sound weak,  don't turn the radio off-get in there and dig around.  You never know what might be lurking in the radio shadows.


1 comment:

  1. A great article, Roland. Very well explained and an enjoyable read! 73, Rob VK3BVW